Zooplankton

{{short description| Heterotrophic protistan or metazoan members of the plankton ecosystem }} [[File:Mixed zooplankton sample.jpg|thumb|upright=1.8| Zooplankton sample including several species of copepods (1–5), gastropod larva (6) doliolids (7), fish eggs (8), and decapod larva (9) (Photo by Iole Di Capua)]]

'''Zooplankton''' are the heterotrophic component of the planktonic community, having to consume other organisms to thrive. The name comes from Ancient Greek ζῷον (''zōîon''), meaning "animal", and πλαγκτός (''planktós''), meaning "drifter, wanderer, roamer", and thus, "animal drifter". Plankton are aquatic organisms that are unable to swim effectively against currents. Consequently, they drift or are carried along by currents in the ocean, or by currents in seas, lakes or rivers.

Zooplankton can be contrasted with phytoplankton (cyanobacteria and microalgae), which are the plant-like component of the plankton community (the "phyto-" prefix comes from {{Langx|grc|φῠτόν|translit=phutón|lit=plant|link=no}}, although taxonomically ''not'' plants). Zooplankton are heterotrophic (other-feeding), whereas phytoplankton are autotrophic (self-feeding), often generating biological energy and macromolecules through chlorophyllic carbon fixation using sunlight{{snd}}in other words, zooplankton cannot manufacture their own food, while phytoplankton can. As a result, zooplankton must acquire nutrients by feeding on other organisms such as phytoplankton, which are generally smaller than zooplankton. Most zooplankton are microscopic but some (such as jellyfish) are macroscopic, meaning they can be seen with the naked eye.<ref>{{Cite book |last=Sardet |first=Christian |url=https://books.google.com/books?id=7JnMCQAAQBAJ&dq=Zooplankton+are+generally+larger+than+phytoplankton,+mostly+still+microscopic+but+some+can+be+seen+with+the+naked+eye.&pg=PA2 |title=Plankton: Wonders of the Drifting World |date=2015-06-05 |publisher=University of Chicago Press |isbn=978-0-226-26534-6 |language=en}}</ref>

Many protozoans (single-celled protists that prey on other microscopic life) are zooplankton, including zooflagellates, foraminiferans, radiolarians, some dinoflagellates and marine microanimals. Macroscopic zooplankton include pelagic cnidarians, ctenophores, molluscs, arthropods and tunicates, as well as planktonic arrow worms and bristle worms.

The distinction between autotrophy and heterotrophy often breaks down in very small organisms. Recent studies of marine microplankton have indicated over half of microscopic plankton are mixotrophs, which can obtain energy and carbon from a mix of internal plastids and external sources. Many marine microzooplankton are mixotrophic, which means they could also be classified as phytoplankton.

==Overview== {{plankton sidebar|trophic}}

'''Zooplankton''' ({{IPAc-en|ˈ|z|oʊ|.|ə|p|l|æ|ŋ|k|t|ən}};{{refn|{{Cite dictionary |url=http://www.lexico.com/definition/zooplankton |archive-url=https://web.archive.org/web/20200301210443/https://www.lexico.com/definition/zooplankton |archive-date=March 1, 2020 |title=zooplankton |dictionary=Lexico UK English Dictionary |publisher=Oxford University Press}} }} {{IPAc-en|ˌ|z|oʊ|.|ə|ˈ|p|l|æ|ŋ|k|t|ən}}){{refn|{{cite Merriam-Webster|zooplankton}}}} are heterotrophic (sometimes detritivorous) plankton. The word ''zooplankton'' is derived from {{langx|grc|ζῷον|translit=zôion|lit=animal}}; and {{Langx|grc|πλᾰγκτός|translit=planktós|label=none|lit=wanderer; drifter}}.<ref>{{cite book |last=Thurman |first=H. V. |year=1997 |title=Introductory Oceanography |publisher=Prentice Hall College |location=New Jersey, USA |isbn=978-0-13-262072-7}}</ref>

Zooplankton is a categorization spanning a range of organism sizes including small protozoans and large metazoans. It includes holoplanktonic organisms whose complete life cycle lies within the plankton, as well as meroplanktonic organisms that spend part of their lives in the plankton before graduating to either the nekton or a sessile, benthic existence. Although zooplankton are primarily transported by ambient water currents, many have locomotion, used to avoid predators (as in diel vertical migration) or to increase prey encounter rate.{{cn|date=February 2026}}

Just as any species can be limited within a geographical region, so are zooplankton. However, species of zooplankton are not dispersed uniformly or randomly within a region of the ocean. As with phytoplankton, 'patches' of zooplankton species exist throughout the ocean. Though few physical barriers exist above the mesopelagic, specific species of zooplankton are strictly restricted by salinity and temperature gradients, while other species can withstand wide temperature and salinity gradients.<ref name="Lalli and Parsons">{{cite book |author1=Lalli, C.M. |author2=Parsons, T.R. |name-list-style=amp |title=Biological Oceanography An Introduction |year=1993 |publisher=Elsevier |location=Burlington, MA |isbn=978-0-7506-3384-0 |page=314}}</ref> Zooplankton patchiness can also be influenced by biological factors, as well as other physical factors. Biological factors include breeding, predation, concentration of phytoplankton, and vertical migration.<ref name="Lalli and Parsons"/> The physical factor that influences zooplankton distribution the most is mixing of the water column (upwelling and downwelling along the coast and in the open ocean) that affects nutrient availability and, in turn, phytoplankton production.<ref name="Lalli and Parsons"/>

Through their consumption and processing of phytoplankton and other food sources, zooplankton play a role in aquatic food webs, as a resource for consumers on higher trophic levels (including fish), and as a conduit for packaging the organic material in the biological pump. Since they are typically small, zooplankton can respond rapidly to increases in phytoplankton abundance,{{Clarify|date=March 2011}} for instance, during the spring bloom. Zooplankton are also a key link in the biomagnification of pollutants such as mercury.<ref>{{Cite web |date=2017-04-05 |title=How We Do Things at IISD-ELA: Researching Mercury |url=https://www.iisd.org/library/how-we-do-things-iisd-ela-researching-mercury |access-date=2020-07-06 |website=IISD |language=en}}</ref>

<gallery mode="packed" heights="360px" style="float:right;" caption="Typical models featuring zooplankton"> File:Typical ocean models featuring zooplankton 2.jpg|{{align|left|{{space|5}} Upper left: Biogeochemical models {{space|22}} Right: Ecosystem models}}<br />{{align|left|{{space|5}} Lower left: Size-spectra models}}<br />{{center|<small>These models also have temporal and spatial components.</small><ref>Everett, J.D., Baird, M.E., Buchanan, P., Bulman, C., Davies, C., Downie, R., Griffiths, C., Heneghan, R., Kloser, R.J., Laiolo, L. and Lara-Lopez, A. (2017) "Modeling what we sample and sampling what we model: challenges for zooplankton model assessment". ''Frontiers in Marine Science'', '''4''': 77. {{doi|10.3389/fmars.2017.00077|doi-access=free}}. 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>}}|alt=Upper left: Biogeochemical models Right: Ecosystem models Lower left: Size-spectra modelsThese models also have temporal and spatial components. </gallery>

Ecologically important protozoan zooplankton groups include the foraminiferans, radiolarians and dinoflagellates (the last of these are often mixotrophic). Important metazoan zooplankton include cnidarians such as jellyfish and the Portuguese Man o' War; crustaceans such as cladocerans, copepods, ostracods, isopods, amphipods, mysids and krill; chaetognaths (arrow worms); molluscs such as pteropods; and chordates such as salps and juvenile fish. This wide phylogenetic range includes a similarly wide range in feeding behavior: filter feeding, predation and symbiosis with autotrophic phytoplankton as seen in corals. Zooplankton feed on bacterioplankton, phytoplankton, other zooplankton (sometimes cannibalistically), detritus (or marine snow) and even nektonic organisms. As a result, zooplankton are primarily found in surface waters where food resources (phytoplankton or other zooplankton) are abundant.{{cn|date=February 2026}}

Zooplankton can also act as a disease reservoir. Crustacean zooplankton have been found to house the bacterium ''Vibrio cholerae'', which causes cholera, by allowing the cholera vibrios to attach to their chitinous exoskeletons. This symbiotic relationship enhances the bacterium's ability to survive in an aquatic environment, as the exoskeleton provides the bacterium with carbon and nitrogen.<ref> {{cite journal |author1=Jude, B.A. |author2=Kirn, T.J. |author3=Taylor R.K. |title=A colonization factor links Vibrio cholerae environmental survival and human infection |journal=Nature |volume=438 |issue=7069 |pages=863–6 |year=2005 |pmid=16341015 |doi=10.1038/nature04249 |bibcode=2005Natur.438..863K |s2cid=1964530}}</ref>

==Size classification== Body size has been defined as a "master trait" for plankton as it is a morphological characteristic shared by organisms across taxonomy that characterises the functions performed by organisms in ecosystems.<ref>{{cite journal |doi=10.1093/plankt/fbt019 |title=Trait-based approaches to zooplankton communities |year=2013 |last1=Litchman |first1=Elena |last2=Ohman |first2=Mark D. |last3=Kiørboe |first3=Thomas |journal=Journal of Plankton Research |volume=35 |issue=3 |pages=473–484 |doi-access=free}}</ref><ref>{{cite journal |doi=10.1086/675241 |title=Shifts in Mass Scaling of Respiration, Feeding, and Growth Rates across Life-Form Transitions in Marine Pelagic Organisms |year=2014 |last1=Kiørboe |first1=Thomas |last2=Hirst |first2=Andrew G. |journal=The American Naturalist |volume=183 |issue=4 |pages=E118–E130 |pmid=24642502 |bibcode=2014ANat..183E.118K |s2cid=15891709 |url=http://qmro.qmul.ac.uk/xmlui/handle/123456789/13552 }}</ref> It has a paramount effect on growth, reproduction, feeding strategies and mortality.<ref>{{cite journal |doi=10.1146/annurev-marine-122414-034144 |title=Characteristic Sizes of Life in the Oceans, from Bacteria to Whales |year=2016 |last1=Andersen |first1=K.H. |last2=Berge |first2=T. |last3=Gonçalves |first3=R.J. |last4=Hartvig |first4=M. |last5=Heuschele |first5=J. |last6=Hylander |first6=S. |last7=Jacobsen |first7=N.S. |last8=Lindemann |first8=C. |last9=Martens |first9=E.A. |last10=Neuheimer |first10=A.B. |last11=Olsson |first11=K. |last12=Palacz |first12=A. |last13=Prowe |first13=A.E.F. |last14=Sainmont |first14=J. |last15=Traving |first15=S.J. |last16=Visser |first16=A.W. |last17=Wadhwa |first17=N. |last18=Kiørboe |first18=T. |journal=Annual Review of Marine Science |volume=8 |pages=217–241 |pmid=26163011 |bibcode=2016ARMS....8..217A |hdl=11336/52445 |url=https://backend.orbit.dtu.dk/ws/files/139512184/Postprint.pdf}}</ref> One of the oldest manifestations of the biogeography of traits was proposed over 170 years ago, namely Bergmann's rule, in which field observations showed that larger species tend to be found at higher, colder latitudes.<ref>{{cite journal |last=Bergmann |first=Carl |title=Über die Verhältnisse der Wärmeökonomie der Thiere zu ihrer Grösse |journal=Göttinger Studien |year=1847 |volume=3 |issue=1 |pages=595–708 |url=https://books.google.com/books?id=EHo-AAAAcAAJ&pg=PA3}}</ref><ref name="Brandão2021">{{cite journal |doi=10.1038/s41598-021-94615-5 |title=Macroscale patterns of oceanic zooplankton composition and size structure |year=2021 |last1=Brandão |first1=Manoela C. |last2=Benedetti |first2=Fabio |last3=Martini |first3=Séverine |last4=Soviadan |first4=Yawouvi Dodji |last5=Irisson |first5=Jean-Olivier |last6=Romagnan |first6=Jean-Baptiste |last7=Elineau |first7=Amanda |last8=Desnos |first8=Corinne |last9=Jalabert |first9=Laëtitia |last10=Freire |first10=Andrea S. |last11=Picheral |first11=Marc |last12=Guidi |first12=Lionel |last13=Gorsky |first13=Gabriel |last14=Bowler |first14=Chris |last15=Karp-Boss |first15=Lee |last16 = Henry |first16=Nicolas |last17=De Vargas |first17=Colomban |last18=Sullivan |first18=Matthew B. |last19=Acinas |first19=Silvia G. |last20=Babin |first20=Marcel |last21=Bork |first21=Peer |last22=Boss |first22=Emmanuel |last23=Bowler |first23=Chris |last24=Cochrane |first24=Guy |last25=De Vargas |first25=Colomban |last26=Gorsky |first26=Gabriel |last27=Guidi |first27=Lionel |last28=Grimsley |first28=Nigel |last29=Hingamp |first29=Pascal |last30=Iudicone |first30=Daniele |journal=Scientific Reports |volume=11 |issue=1 |page=15714 |pmid=34344925 |pmc=8333327 |bibcode=2021NatSR..1115714B |display-authors=1}} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

In the oceans, size is critical in determining trophic links in planktonic ecosystems and is thus a critical factor in regulating the efficiency of the biological carbon pump.<ref>{{cite journal |doi=10.1038/s41467-017-02450-y |title=A unifying theory for top-heavy ecosystem structure in the ocean |year=2018 |last1=Woodson |first1=C. Brock |last2=Schramski |first2=John R. |last3=Joye |first3=Samantha B. |journal=Nature Communications |volume=9 |issue=1 |page=23 |pmid=29295998 |pmc=5750233 |bibcode=2018NatCo...9...23W}}</ref> Body size is sensitive to changes in temperature due to the thermal dependence of physiological processes.<ref>{{cite journal |doi=10.1890/03-9000 |title=Toward a Metabolic Theory of Ecology |year=2004 |last1=Brown |first1=James H. |last2=Gillooly |first2=James F. |last3=Allen |first3=Andrew P. |last4=Savage |first4=Van M. |last5=West |first5=Geoffrey B. |journal=Ecology |volume=85 |issue=7 |pages=1771–1789|bibcode=2004Ecol...85.1771B }}</ref> The plankton is mainly composed of ectotherms which are organisms that do not generate sufficient metabolic heat to elevate their body temperature, so their metabolic processes depends on external temperature.<ref>{{cite journal |doi=10.1016/j.tree.2011.03.005 |title=Declining body size: A third universal response to warming? |year=2011 |last1=Gardner |first1=Janet L. |last2=Peters |first2=Anne |last3=Kearney |first3=Michael R. |last4=Joseph |first4=Leo |last5=Heinsohn |first5=Robert |journal=Trends in Ecology & Evolution |volume=26 |issue=6 |pages=285–291 |pmid=21470708|bibcode=2011TEcoE..26..285G }}</ref> Consequently, ectotherms grow more slowly and reach maturity at a larger body size in colder environments, which has long puzzled biologists because classic theories of life-history evolution predict smaller adult sizes in environments delaying growth.<ref name="Angilletta2004">{{cite journal |doi=10.1093/icb/44.6.498 |title=Temperature, Growth Rate, and Body Size in Ectotherms: Fitting Pieces of a Life-History Puzzle |year=2004 |last1=Angilletta |first1=M. J. |last2=Steury |first2=T. D. |last3=Sears |first3=M. W. |journal=Integrative and Comparative Biology |volume=44 |issue=6 |pages=498–509 |pmid=21676736 |doi-access=free}}</ref> This pattern of body size variation, known as the temperature-size rule (TSR),<ref>{{cite book |doi=10.1016/S0065-2504(08)60212-3 |title=Temperature and Organism Size—A Biological Law for Ectotherms? |series=Advances in Ecological Research |year=1994 |last1=Atkinson |first1=D. |volume=25 |pages=1–58 |isbn=978-0-12-013925-5}}</ref> has been observed for a wide range of ectotherms, including single-celled and multicellular species, invertebrates and vertebrates.<ref name="Angilletta2004" /><ref>{{cite journal |doi=10.1016/S0169-5347(97)01058-6 |title=Why are organisms usually bigger in colder environments? Making sense of a life history puzzle |year=1997 |last1=Atkinson |first1=David |last2=Sibly |first2=Richard M. |journal=Trends in Ecology & Evolution |volume=12 |issue=6 |pages=235–239 |pmid=21238056|bibcode=1997TEcoE..12..235A }}</ref><ref name="Brandão2021" />

The processes underlying the inverse relationship between body size and temperature remain to be identified.<ref name="Angilletta2004" /> Despite temperature playing a major role in shaping latitudinal variations in organism size, these patterns may also rely on complex interactions between physical, chemical and biological factors. For instance, oxygen supply plays a central role in determining the magnitude of ectothermic temperature-size responses, but it is hard to disentangle the relative effects of oxygen and temperature from field data because these two variables are often strongly inter-related in the surface ocean.<ref>{{cite journal |doi = 10.1126/science.1261359|title = Structure and function of the global ocean microbiome|year = 2015|last1 = Sunagawa|first1 = S.|last2 = Coelho|first2 = L. P.|last3 = Chaffron|first3 = S.|last4 = Kultima|first4 = J. R.|last5 = Labadie|first5 = K.|last6 = Salazar|first6 = G.|last7 = Djahanschiri|first7 = B.|last8 = Zeller|first8 = G.|last9 = Mende|first9 = D. R.|last10 = Alberti|first10 = A.|last11 = Cornejo-Castillo|first11 = F. M.|last12 = Costea|first12 = P. I.|last13 = Cruaud|first13 = C.|last14 = d'Ovidio|first14 = F.|last15 = Engelen|first15 = S.|last16 = Ferrera|first16 = I.|last17 = Gasol|first17 = J. M.|last18 = Guidi|first18 = L.|last19 = Hildebrand|first19 = F.|last20 = Kokoszka|first20 = F.|last21 = Lepoivre|first21 = C.|last22 = Lima-Mendez|first22 = G.|last23 = Poulain|first23 = J.|last24 = Poulos|first24 = B. T.|last25 = Royo-Llonch|first25 = M.|last26 = Sarmento|first26 = H.|last27 = Vieira-Silva|first27 = S.|last28 = Dimier|first28 = C.|last29 = Picheral|first29 = M.|last30 = Searson|first30 = S.|journal = Science|volume = 348|issue = 6237|pmid = 25999513|s2cid = 206562917|display-authors = 1| article-number=1261359 |hdl = 10261/117712|url = https://hal.archives-ouvertes.fr/hal-01233742/file/Sunagawa_et_al_2015_preprint.pdf|hdl-access = free}}</ref><ref>{{cite journal |doi = 10.1111/geb.12847|title = Is oxygen limitation in warming waters a valid mechanism to explain decreased body sizes in aquatic ectotherms?|year = 2019|last1 = Audzijonyte|first1 = Asta|last2 = Barneche|first2 = Diego R.|last3 = Baudron|first3 = Alan R.|last4 = Belmaker|first4 = Jonathan|last5 = Clark|first5 = Timothy D.|last6 = Marshall|first6 = C. Tara|last7 = Morrongiello|first7 = John R.|last8 = Van Rijn|first8 = Itai|journal = Global Ecology and Biogeography|volume = 28|issue = 2|pages = 64–77|s2cid = 92601781|doi-access = free| bibcode=2019GloEB..28...64A |hdl = 10536/DRO/DU:30117155|hdl-access = free}}</ref><ref name="Brandão2021" />

Zooplankton can be broken down into size classes<ref>{{Cite journal |last1=Steinberg |first1=Deborah K. |last2=Landry |first2=Michael R. |date=2017-01-03 |title=Zooplankton and the Ocean Carbon Cycle|url=https://www.annualreviews.org/doi/10.1146/annurev-marine-010814-015924 |journal=Annual Review of Marine Science |language=en |volume=9 |issue=1 |pages=413–444 |doi=10.1146/annurev-marine-010814-015924 |pmid=27814033 |bibcode=2017ARMS....9..413S |issn=1941-1405|url-access=subscription }}</ref> which are diverse in their morphology, diet, feeding strategies, etc. both within classes and between classes: {| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;" !scope="col"|Type of zooplankton !scope="col"|Size range |- |picozooplankton |<math><</math>2μm |- |nanozooplankton |2–20μm |- |microzooplankton |20–200μm |- |mesozooplankton |0.2–20 millimeters |}

===Microzooplankton=== Microzooplankton are defined as heterotrophic and mixotrophic plankton. They primarily consist of phagotrophic protists, including ciliates, dinoflagellates, and mesozooplankton nauplii.<ref>{{cite journal |doi = 10.4319/lo.1978.23.6.1256|title = Pelagic ecosystem structure: Heterotrophic compartments of the plankton and their relationship to plankton size fractions 1|year = 1978|author1-link=John McNeill Sieburth|last1 = Sieburth|first1 = John McN.|last2 = Smetacek|first2 = Victor|last3 = Lenz|first3 = Jürgen|journal = Limnology and Oceanography|volume = 23|issue = 6|pages = 1256–1263|bibcode = 1978LimOc..23.1256S| s2cid=85568208 |doi-access = free}}</ref> Microzooplankton are major grazers of the plankton community. As the primary consumers of marine phytoplankton, microzooplankton consume ~ 59–75% daily of the marine primary production, much larger than mesozooplankton. That said, macrozooplankton can sometimes have greater consumption rates in eutrophic ecosystems because the larger phytoplankton can be dominant there.<ref>{{cite journal |doi = 10.4319/lo.2004.49.1.0051 |doi-access = free|title = Phytoplankton growth, microzooplankton grazing, and carbon cycling in marine systems|year = 2004|last1 = Calbet|first1 = Albert|last2 = Landry|first2 = Michael R.|journal = Limnology and Oceanography|volume = 49|issue = 1|pages = 51–57|bibcode = 2004LimOc..49...51C|hdl = 10261/134985|s2cid = 22995996|hdl-access = free}}</ref><ref name="Calbet2008">{{cite journal |doi = 10.1093/icesjms/fsn013|title = The trophic roles of microzooplankton in marine systems|year = 2008|last1 = Calbet|first1 = Albert|journal = ICES Journal of Marine Science|volume = 65|issue = 3|pages = 325–331|doi-access = free|hdl = 10261/16567|hdl-access = free}}</ref> Microzooplankton are also pivotal regenerators of nutrients which fuel primary production and food sources for metazoans.<ref name="Calbet2008" /><ref name="Liu2020">{{cite journal |last1=Liu |first1=Kailin |last2=Chen |first2=Bingzhang |last3=Zheng |first3=Liping |last4=Su |first4=Suhong |last5=Huang |first5=Bangqin |last6=Chen |first6=Mianrun |last7=Liu |first7=Hongbin |year=2021 |title=What controls microzooplankton biomass and herbivory rate across marginal seas of China?|journal=Limnology and Oceanography |volume=66 |issue=1 |pages=61–75 |bibcode=2021LimOc..66...61L |doi=10.1002/lno.11588 |issn=0024-3590 |s2cid=224916151 |doi-access=free}} 50x50px Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.</ref>

Despite their ecological importance, microzooplankton remain understudied. Routine oceanographic observations seldom monitor microzooplankton biomass or herbivory rate, although the dilution technique, an elegant method of measuring microzooplankton herbivory rate, has been developed for over four decades (Landry and Hassett 1982). The number of observations of microzooplankton herbivory rate is around 1600 globally,<ref>{{cite journal |doi = 10.4319/lo.2012.57.2.0519 |doi-access = free|title = Does warming enhance the effect of microzooplankton grazing on marine phytoplankton in the ocean?|year = 2012|last1 = Chen|first1 = Bingzhang|last2 = Landry|first2 = Michael R.|last3 = Huang|first3 = Bangqin|last4 = Liu|first4 = Hongbin|journal = Limnology and Oceanography|volume = 57|issue = 2|pages = 519–526|bibcode = 2012LimOc..57..519C}}</ref><ref>{{cite journal |doi = 10.1093/plankt/fbt023|title = Microzooplankton grazing in the oceans: Impacts, data variability, knowledge gaps and future directions|year = 2013|last1 = Schmoker|first1 = Claire|last2 = Hernández-León|first2 = Santiago|last3 = Calbet|first3 = Albert|journal = Journal of Plankton Research|volume = 35|issue = 4|pages = 691–706|doi-access = free}}</ref> far less than that of primary productivity (> 50,000).<ref>{{cite journal |doi = 10.1002/gbc.20074|title = Combined constraints on global ocean primary production using observations and models|year = 2013|last1 = Buitenhuis|first1 = Erik T.|last2 = Hashioka|first2 = Taketo|last3 = Quéré|first3 = Corinne Le|journal = Global Biogeochemical Cycles|volume = 27|issue = 3|pages = 847–858|bibcode = 2013GBioC..27..847B|s2cid = 140628035|doi-access = free}}</ref> This makes validating and optimizing the grazing function of microzooplankton difficult in ocean ecosystem models.<ref name="Liu2020" />

===Mesozooplankton=== Mesozooplankton are one of the larger size classes of zooplankton. In most regions, mesozooplankton are dominated by copepods, such as ''Calanus finmarchicus'' and ''Calanus helgolandicus''. Mesozooplankton are an important prey for fish.{{cn|date=February 2026}}

As plankton are rarely fished, it has been argued that mesoplankton abundance and species composition can be used to study marine ecosystems' response to climate change. This is because they have life cycles that generally last less than a year, meaning they respond to climate changes between years. Sparse, monthly sampling will still indicate vacillations.<ref>{{Cite journal |last1=Mackas |first1=David L. |last2=Beaugrand |first2=Gregory |date=2010-02-10 |title=Comparisons of zooplankton time series |url=https://www.sciencedirect.com/science/article/pii/S0924796309000852 |journal=Journal of Marine Systems |series=Impact of climate variability on marine ecosystems: A comparative approach |language=en |volume=79 |issue=3 |pages=286–304 |doi=10.1016/j.jmarsys.2008.11.030 |bibcode=2010JMS....79..286M |issn=0924-7963|url-access=subscription }}</ref>

==Sampling methods== thumb|PairoVET tow thumb|Bongo tow thumb|Retrieving a plankton sample thumb|EZ net (=BIONESS) night-time net retrieval {{see also|Plankton net}}

Research vessels collect zooplankton from the ocean using fine mesh nets. The vessels either tow the nets through the sea or pump sea water onboard and then pass it through the net.<ref name=NOAA2>[http://swfsc.noaa.gov/textblock.aspx?Division=FRD&ParentMenuId=436&id=6268 Ichthyoplankton sampling methods] Southwest Fisheries Science Center, ''NOAA''. Retrieved 11 July 2020.</ref>

thumb|upright=2| In addition to net tows, plankton is collected while the research vessel is moving using (e.g. for ichthyoplankton) a Continuous Underway Fish Egg Sampler, or CUFES. Water is pumped aboard the vessel from 3 m depth at 640 liters/min. The water is sent through a concentrator where it passes through a net, and the plankton is diverted to a collector. While CUFES is running, a data logger is recording the date, time, and position for each sample as well as other environmental data from the ship's sensors (e.g. wind speed, direction, SST).<ref name=NOAA2 />

There are many types of plankton tows:<ref name=NOAA2 /> * Neuston net tows are often made at or just below the surface using a nylon mesh net fitted to a rectangular frame * The PairoVET tow, used for collecting fish eggs, drops a net about 70 metres into the sea from a stationary research vessel and then drags it back to the vessel. * Ring net tows involve a nylon mesh net fitted to a circular frame. These have largely been replaced by bongo nets, which provide duplicate samples with their dual-net design. * The bongo tow drags nets shaped like bongo drums from a moving vessel. The net is often lowered to about 200 metres and then allowed to rise to the surface as it is towed. In this way, a sample can be collected across the whole photic zone where most ichthyoplankton is found. * MOCNESS and BIONESS tows and Tucker trawls utilize multiple nets that are mechanically opened and closed at discrete depths in order to provide insights into the vertical distribution of the plankton * The manta trawl tows a net from a moving vessel along the surface of the water, collecting larvae, such as grunion, mahi-mahi, and flying fish which live at the surface.

After the tow the plankton is flushed with a hose to the cod end (bottom) of the net for collection. The sample is then placed in preservative fluid prior to being sorted and identified in a laboratory.<ref name=NOAA2 />

Plankton pumps: Another method of collecting ichthyoplankton is to use a Continuous Underway Fish Egg Sampler (see illustration). Water from a depth of about three metres is pumped onto the vessel and filtered with a net. This method can be used while the vessel is underway.<ref name=NOAA2 />

==Taxonomic groups== ===Protozooplankton=== {{further|marine protists#Protozoans}}

'''Protozooplankton''' refers to protist zooplankton (planktonic protozoans).<ref name="Flynn2019">{{cite journal | last1=Flynn | first1=Kevin J | last2=Mitra | first2=Aditee | last3=Anestis | first3=Konstantinos | last4=Anschütz | first4=Anna A | last5=Calbet | first5=Albert | last6=Ferreira | first6=Guilherme Duarte | last7=Gypens | first7=Nathalie | last8=Hansen | first8=Per J | last9=John | first9=Uwe | last10=Martin | first10=Jon Lapeyra | last11=Mansour | first11=Joost S | last12=Maselli | first12=Maira | last13=Medić | first13=Nikola | last14=Norlin | first14=Andreas | last15=Not | first15=Fabrice | last16=Pitta | first16=Paraskevi | last17=Romano | first17=Filomena | last18=Saiz | first18=Enric | last19=Schneider | first19=Lisa K | last20=Stolte | first20=Willem | last21=Traboni | first21=Claudia | title=Mixotrophic protists and a new paradigm for marine ecology: where does plankton research go now? | journal=Journal of Plankton Research | volume=41 | issue=4 | date=2019-07-26 | issn=0142-7873 | doi=10.1093/plankt/fbz026 | doi-access=free | pages=375–391 | url=https://academic.oup.com/plankt/article-pdf/41/4/375/30279486/fbz026.pdf | access-date=2025-07-06}}</ref> All protozooplankton are protozoans, but not all protozoans are protozooplankton, since some live in environments like soil or as parasites. Marine planktonic protozoans include zooflagellates, foraminiferans, radiolarians and some dinoflagellates.{{cn|date=February 2026}}

Protozoans are protists that feed on organic matter such as other microorganisms or organic tissues and debris.<ref>{{Cite book |url=https://books.google.com/books?id=sYgKY6zz20YC&q=panno+the+cell&pg=PA130 |title=The Cell: Evolution of the First Organism |last=Panno |first=Joseph |date=14 May 2014 |publisher=Infobase Publishing |isbn=978-0-8160-6736-7 |language=en}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=2zVqBgAAQBAJ&q=endocytosis&pg=PA9 |title=Environmental Microbiology: Fundamentals and Applications: Microbial Ecology |last1=Bertrand |first1=Jean-Claude |last2=Caumette |first2=Pierre |last3=Lebaron |first3=Philippe |last4=Matheron |first4=Robert |last5=Normand |first5=Philippe |last6=Sime-Ngando |first6=Télesphore |date=2015-01-26 |publisher=Springer |isbn=978-94-017-9118-2 |language=en}}</ref> Historically, the protozoa were regarded as "one-celled animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.<ref>{{Cite book|url=https://books.google.com/books?id=RawZTwEACAAJ&q=brock+biology+of+microorganisms+13th|title=Brock Biology of Microorganisms |last=Madigan |first=Michael T. |date=2012 |publisher=Benjamin Cummings |isbn=978-0-321-64963-8}}</ref><ref>{{Cite book |url=https://www.ncbi.nlm.nih.gov/books/NBK8325/ |title=Protozoa: Structure, Classification, Growth, and Development |last=Yaeger |first=Robert G. |date=1996 |publisher=NCBI |pmid=21413323 |access-date=2018-03-23 |isbn=978-0-9631172-1-2 }}</ref> Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy.{{cn|date=February 2026}}

====Radiolarians==== {{multiple image | align = right | direction = horizontal | header = Radiolarian shapes | header_align = center | header_background = | footer = &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Drawings by Haeckel 1904 (click for details) | footer_align = center | footer_background = | background color = | image1 = Haeckel Phaeodaria 1.jpg | width1 = 160 | alt1 = | caption1 = | image2 = Haeckel Stephoidea edit.jpg | width2 = 160 | alt2 = | caption2 = }}

Radiolarians are unicellular predatory protists encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of the ocean sediment. These remains, as microfossils, provide valuable information about past oceanic conditions.<ref name=Wassilieff2006b>Wassilieff, Maggy (2006) [http://www.TeAra.govt.nz/en/photograph/5138/radiolarian-fossils "Plankton – Animal plankton"], ''Te Ara – the Encyclopedia of New Zealand''. Accessed: 2 November 2019.</ref>

<gallery mode="packed" heights="150px" style="float:left;"> File:Mikrofoto.de-Radiolarien 6.jpg|Like diatoms, radiolarians come in many shapes File:Podocyrtis papalis Ehrenberg - Radiolarian (30448963206).jpg|Also like diatoms, radiolarian shells are usually made of silicate File:Acantharian radiolarian Xiphacantha (Haeckel).jpg|However acantharian radiolarians have shells made from strontium sulfate crystals File:Spherical radiolarian 2.jpg|Cutaway schematic diagram of a spherical radiolarian shell </gallery>

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{{ external media | float = right | width = 280px | video1 = [https://www.youtube.com/watch?v=5rxwn6vT9JE Radiolarian geometry] | video2 = [https://www.youtube.com/watch?v=tl_onFMjJWA Ernst Haeckel's radiolarian engravings] }}

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====Foraminiferans==== Like radiolarians, foraminiferans (''forams'' for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often called tests, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made of agglutinated sediment particles or chiton, and (rarely) silica. Most forams are benthic, but about 40 species are planktic.<ref name=Hemleben>{{cite book |first1=C. |last1=Hemleben |first2=O.R. |last2=Anderson |first3=M. |last3=Spindler |title=Modern Planktonic Foraminifera |url=https://books.google.com/books?id=NaHOmAEACAAJ |year=1989 |publisher=Springer-Verlag |isbn=978-3-540-96815-3}}</ref> They are widely researched with well-established fossil records which allow scientists to infer a lot about past environments and climates.<ref name=Wassilieff2006b />

{{multiple image | align = left | direction = horizontal | header = Foraminiferans | header_align = center | header_background = | footer = Foraminiferans are important unicellular zooplankton protists, with calcium tests | footer_align = center | footer_background = | caption_align = center | background color = | image1 = Foram-globigerina hg.jpg | width1 = 136 | alt1 = | caption1 = ...can have more than one nucleus | image2 = G bulloides Brady 1884.jpg | width2 = 104 | alt2 = | caption2 = ...and defensive spines }}

<gallery mode="packed" heights="144px" style="float:left;"> File:EB1911 Foraminifera - Section of Rotalia beccarii.jpg|section showing chambers of a spiral foram File:Live Ammonia tepida.jpg|Live ''Ammonia tepida'' streaming granular ectoplasm for catching food File:Planktic Foraminifera of the northern Gulf of Mexico.jpg|Group of planktonic forams File:All Gizah Pyramids.jpg|The Egyptian pyramids were constructed from limestone that contained nummulites.<ref>[http://www.ucl.ac.uk/GeolSci/micropal/foram.html#histofstudy Foraminifera: History of Study], University College London. Retrieved: 18 November 2019.</ref> </gallery>

{{ external media | float = right | width = 280px | video1 = [https://www.youtube.com/watch?v=JLSa8cGJixQ foraminiferans] | video2 = [https://www.youtube.com/watch?v=q0WbN34Mh7k Foraminiferal networks and growth] }}

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====Amoeba==== {{multiple image | align = left | direction = horizontal | header = Shelled and naked amoeba | header_align = center | header_background = | footer = &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Amoeba can be shelled (testate) or naked | footer_align = center | footer_background = | background color = | width1 = 180 | image1 = Cyphoderia ampulla - Testate amoeba - 160x (14997391862).jpg | alt1 = | caption1 = Testate amoeba, ''Cyphoderia'' sp. | image2 = Chaos carolinense.jpg | width2 = 210 | alt2 = | caption2 = {{center|Naked amoeba, ''Chaos carolinensis''}} }}

<gallery mode="packed" heights="170px" style="float:left;"> File:Amoeba proteus 2.jpg|Naked amoeba sketch showing food vacuoles and ingested diatom File:Arcella sp.jpg|Shell or test of a testate amoeba, ''Arcella'' sp. File:Collection Penard MHNG Specimen 533-2-1 Pamphagus granulatus.tif|Xenogenic testate amoeba covered in diatoms </gallery>

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====Ciliates==== <gallery mode="packed" heights="150px" style="float:left;"> File:Stylonychia putrina - 160x - II (13215594964).jpg|''Stylonychia putrina'' File:Holophyra ovum - 400x (9836710085).jpg|''Holophyra'' ovum File:Mikrofoto.de-Blepharisma japonicum 15.jpg|''Blepharisma japonicum'' File:The ciliate Frontonia sp.jpg|This ciliate is digesting cyanobacteria. The mouth is at the bottom right. </gallery>

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====Dinoflagellates==== {{see also|Predatory dinoflagellate}}

Dinoflagellates are a phylum of unicellular flagellates with about 2,000 marine species.<ref name="Gómez12">{{cite journal|author=Gómez F |title=A checklist and classification of living dinoflagellates (Dinoflagellata, Alveolata) |journal=CICIMAR Oceánides |volume=27 |issue=1 |pages=65–140 |year=2012 |doi=10.37543/oceanides.v27i1.111 |doi-access=free }}</ref> Some dinoflagellates are predatory, and thus belong to the zooplankton community. Their name comes from the Greek "dinos" meaning ''whirling'' and the Latin "flagellum" meaning a ''whip'' or ''lash''. This refers to the two whip-like attachments (flagella) used for forward movement. Most dinoflagellates are protected with red-brown, cellulose armour. Excavates may be the most basal flagellate lineage.<ref name=Dawson2013>{{cite journal |last1=Dawson |first1=Scott C |last2=Paredez |first2=Alexander R |title=Alternative cytoskeletal landscapes: cytoskeletal novelty and evolution in basal excavate protists |journal=Current Opinion in Cell Biology |year=2013 |volume=25 |issue=1 |pages=134–141 |doi=10.1016/j.ceb.2012.11.005 |pmid=23312067 |pmc=4927265}}</ref>

{{multiple image | align = left | direction = horizontal | width = 120 | header = Dinoflagellates | header_align = center | header_background = | footer = Traditionally dinoflagellates have been presented as armoured or unarmoured | footer_align = center | footer_background = | background color = | image1 = Peridinium digitale.jpg | alt1 = | caption1 = &nbsp; &nbsp; &nbsp; &nbsp; Armoured | image2 = Gymnodinium agile sp.jpg | alt2 = | caption2 = &nbsp; &nbsp; &nbsp; &nbsp; Unarmoured }}

<gallery mode="packed" heights="144px" style="float:right;"> File:Gyrodinium dinoflagellate.jpg|''Gyrodinium'', one of the few naked dinoflagellates which lack armour File:Protoperidinium dinoflagellate.jpg|The dinoflagellate ''Protoperidinium'' extrudes a large feeding veil to capture prey File:Radiolarian - Podocyrtis (Lampterium) mitra Ehrenberg - 160x.jpg|Nassellarian radiolarians can be in symbiosis with dinoflagellates </gallery>

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Dinoflagellates often live in symbiosis with other organisms. Many nassellarian radiolarians house dinoflagellate symbionts within their tests.<ref>{{Cite book |title=Handbook of the Protists |last1=Boltovskoy |first1=Demetrio |last2=Anderson |first2=O. Roger |last3=Correa |first3=Nancy M. |date=2017 |publisher=Springer, Cham |isbn=978-3-319-28147-6 |pages=731–763 |language=en |doi=10.1007/978-3-319-28149-0_19}}</ref> The nassellarian provides ammonium and carbon dioxide for the dinoflagellate, while the dinoflagellate provides the nassellarian with a mucous membrane useful for hunting and protection against harmful invaders.<ref>{{Cite book |title=Radiolaria |last=Anderson |first=O. R. |publisher=Springer Science & Business Media |year=1983}}</ref> There is evidence from DNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as with foraminifera.<ref>{{Cite journal |last1=Gast |first1=R. J. |last2=Caron |first2=D. A. |date=1996-11-01 |title=Molecular phylogeny of symbiotic dinoflagellates from planktonic foraminifera and radiolaria |journal=Molecular Biology and Evolution |language=en |volume=13 |issue=9 |pages=1192–1197 |doi=10.1093/oxfordjournals.molbev.a025684 |pmid=8896371 |issn=0737-4038 |doi-access=}}</ref>

<gallery mode="packed" heights="144px" style="float:left;"> File:Ceratium tripos.jpg|''Tripos muelleri'' is recognisable by its U-shaped horns File:Archives de zoologie expérimentale et générale (1920) (20299351186).jpg|''Oodinium'', a genus of parasitic dinoflagellates, causes velvet disease in fish<ref>{{cite web|title=Protozoa Infecting Gills and Skin|url=http://www.merckvetmanual.com:80/mvm/index.jsp?cfile=htm/bc/170410.htm|publisher=The Merck Veterinary Manual|access-date= 4 November 2019|archive-url=https://web.archive.org/web/20160303221140/http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm%2Fbc%2F170410.htm|archive-date=3 March 2016|df=dmy-all}}</ref> File:Karenia brevis.jpg|''Karenia brevis'' produces red tides highly toxic to humans<ref>{{Cite journal|last1=Brand|first1=Larry E.|last2=Campbell|first2=Lisa|last3=Bresnan|first3=Eileen|title=''Karenia'': The biology and ecology of a toxic genus|journal=Harmful Algae|volume=14|pages=156–178|doi=10.1016/j.hal.2011.10.020|year=2012|pmid=36733478 |pmc=9891709 |bibcode=2012HAlga..14..156B }}</ref> File:Algal bloom(akasio) by Noctiluca in Nagasaki.jpg|Red tide </gallery>

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===Mixoplankton=== {{main|Mixoplankton}} {{see also|Mixotroph|Mixotrophic dinoflagellate}}

Mixoplankton are mixotrophic plankton, capable of both photosynthesis and predation. A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.<ref>{{Cite web |last=Collins |first=Richard |date=2016-11-14 |title=Beware the mixotrophs – they can destroy entire ecosystems 'in a matter of hours' |url=https://www.irishexaminer.com/opinion/columnists/arid-20430358.html |website=Irish Examiner |language=en}}</ref> There are two types of eukaryotic mixotrophs: those with their own chloroplasts, and those with endosymbionts—and others that acquire them through kleptoplasty or by enslaving the entire phototrophic cell.<ref>{{Cite web |last=University |first=Swansea |title=Microscopic body snatchers infest our oceans |url=https://phys.org/news/2017-08-microscopic-body-snatchers-infest-oceans.html |website=phys.org |language=en}}</ref>

The distinction between plants and animals often breaks down in very small organisms. Possible combinations are photo- and chemotrophy, litho- and organotrophy, auto- and heterotrophy or other combinations of these. Mixotrophs can be either eukaryotic or prokaryotic.<ref name='Eiler'>{{cite journal |author=Eiler A |title=Evidence for the Ubiquity of Mixotrophic Bacteria in the Upper Ocean: Implications and Consequences |journal=Appl Environ Microbiol |volume=72 |issue=12 |pages=7431–7 |date=December 2006 |pmid=17028233 |doi=10.1128/AEM.01559-06 |pmc=1694265 |bibcode=2006ApEnM..72.7431E }}</ref> They can take advantage of different environmental conditions.<ref>{{cite journal |vauthors=Katechakis A, Stibor H |title=The mixotroph ''Ochromonas tuberculata'' may invade and suppress specialist phago- and phototroph plankton communities depending on nutrient conditions |journal=Oecologia |volume=148 |issue=4 |pages=692–701 |date=July 2006 |pmid=16568278 |doi=10.1007/s00442-006-0413-4 |bibcode=2006Oecol.148..692K |s2cid=22837754 }}</ref>

Many marine microzooplankton are mixotrophic, which means they could also be classified as phytoplankton. Recent studies of marine microzooplankton found 30–45% of the ciliate abundance was mixotrophic, and up to 65% of the amoeboid, foram and radiolarian biomass was mixotrophic.<ref name=Leles2017>{{cite journal | last1 = Leles | first1 = S.G. | last2 = Mitra | first2 = A. | last3 = Flynn | first3 = K.J. | last4 = Stoecker | first4 = D.K. | last5 = Hansen | first5 = P.J. | last6 = Calbet | first6 = A. | last7 = McManus | first7 = G.B. | last8 = Sanders | first8 = R.W. | last9 = Caron | first9 = D.A. | last10 = Not | first10 = F. | last11 = Hallegraeff | first11 = G.M. | year = 2017 | title = Oceanic protists with different forms of acquired phototrophy display contrasting biogeographies and abundance | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 284 | issue = 1860| article-number = 20170664 | doi = 10.1098/rspb.2017.0664 | pmid = 28768886 | pmc = 5563798 | bibcode = 2017PBioS.28470664L }}</ref>

{|class="wikitable" ! colspan=7 |{{centre|Mixotrophic zooplankton that combine phototrophy and heterotrophy – table based on Stoecker et al., 2017 <ref name=Stoecker2017>{{cite journal | last1 = Stoecker | first1 = D.K. | last2 = Hansen | first2 = P.J. | last3 = Caron | first3 = D.A. | last4 = Mitra | first4 = A. | year = 2017 | title = Mixotrophy in the marine plankton | url = http://pdfs.semanticscholar.org/8492/ec7724a468af240e014aa539a8865568473d.pdf | archive-url = https://web.archive.org/web/20190227180157/http://pdfs.semanticscholar.org/8492/ec7724a468af240e014aa539a8865568473d.pdf | archive-date = 2019-02-27 | journal = Annual Review of Marine Science | volume = 9 | pages = 311–335 | doi = 10.1146/annurev-marine-010816-060617 | pmid = 27483121 | bibcode = 2017ARMS....9..311S | s2cid = 25579538 }}</ref>}} |- ! colspan=2 | Description ! colspan=2 | Example ! Further examples |- | colspan=5 | The term '''nonconstitutive mixotrophs''' was defined by Mitra et al. in 2016), and refers to zooplankton capable of photosynthesis.<ref name=Mitra2016>{{cite journal | last1 = Mitra | first1 = A | last2 = Flynn | first2 = KJ | last3 = Tillmann | first3 = U | last4 = Raven | first4 = J | last5 = Caron | first5 = D | display-authors = etal | year = 2016 | title = Defining planktonic protist functional groups on mechanisms for energy and nutrient acquisition; incorporation of diverse mixotrophic strategies | journal = Protist | volume = 167 | issue = 2| pages = 106–20 | doi = 10.1016/j.protis.2016.01.003 | pmid = 26927496 | doi-access = free | hdl = 10261/131722 | hdl-access = free }}</ref> These include microzooplankton or metazoan zooplankton that gain phototrophic abilities by retaining chloroplasts^∆ (often through kleptoplasty) acquired from ingested algal prey or by maintaining photosynthetic endosymbiotic algae within their cells. They are photosynthetic zooplankton that do not inherently possess any genetic machinery for photosynthesis but acquire it secondarily from their prey or symbionts. This contrasts with constitutive mixotrophs, which have their own built-in photosynthetic capabilities.<ref name=Mitra2016 /> |- | Generalists | Protists that sequester chloroplasts (occasionally alternate organelles) from a diverse array of algal species | 100px | | Most oligotrich ciliates with plastid retention ^∆ |- | rowspan=2 | Specialists | 1. Protists that sequester chloroplasts (occasionally alternate organelles) either from a single algal species or from several strongly related strains | 100px | ''Dinophysis acuminata'' | ''Dinophysis'' spp.<br />''Myrionecta rubra'' |- | 2. Protists or zooplankton that maintain endosymbiosis by hosting either a single algal species or several closely related strains as their internal photosynthetic partners | 100px | ''Noctiluca scintillans'' | Metazooplankton with algal endosymbionts<br />Most mixotrophic Rhizaria (Acantharea, Polycystinea, and Foraminifera)<br />Green ''Noctiluca scintillans'' |- | colspan=7 style="text-align:center;" | <small>^∆ Chloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm.</small> |}

''Phaeocystis'' species are endosymbionts to acantharian radiolarians.<ref name=":4">{{cite journal |last1=Decelle |first1=Johan |last2=Simó |first2=Rafel |last3=Galí |first3=Martí |last4=Vargas |first4=Colomban de |last5=Colin |first5=Sébastien |last6=Desdevises |first6=Yves |last7=Bittner |first7=Lucie |last8=Probert |first8=Ian |last9=Not |first9=Fabrice |date=2012-10-30 |title=An original mode of symbiosis in open ocean plankton |journal=Proceedings of the National Academy of Sciences |language=en |volume=109 |issue=44 |pages=18000–18005 |doi=10.1073/pnas.1212303109 |issn=0027-8424 |pmid=23071304 |pmc=3497740 |bibcode=2012PNAS..10918000D |doi-access=free}}</ref><ref name=":5">{{Cite journal |last1=Mars Brisbin |first1=Margaret |last2=Grossmann |first2=Mary M. |last3=Mesrop |first3=Lisa Y. |last4=Mitarai |first4=Satoshi |date=2018 |title=Intra-host Symbiont Diversity and Extended Symbiont Maintenance in Photosymbiotic Acantharea (Clade F) |journal=Frontiers in Microbiology |language=en |volume=9 |article-number=1998 |doi=10.3389/fmicb.2018.01998 |pmid=30210473 |pmc=6120437 |issn=1664-302X |doi-access=free}}</ref> ''Phaeocystis'' is an important algal genus found as part of the marine phytoplankton around the world. It has a polymorphic life cycle, ranging from free-living cells to large colonies.<ref name=":0">{{Cite journal|title = Phaeocystis blooms in the global ocean and their controlling mechanisms: a review|journal = Journal of Sea Research|date = 2005-01-01|pages = 43–66|volume = 53|series = Iron Resources and Oceanic Nutrients – Advancement of Global Environmental Simulations|issue = 1–2|doi = 10.1016/j.seares.2004.01.008|first1 = Véronique|last1 = Schoemann|first2 = Sylvie|last2 = Becquevort|first3 = Jacqueline|last3 = Stefels|first4 = Véronique|last4 = Rousseau|first5 = Christiane|last5 = Lancelot|citeseerx = 10.1.1.319.9563|bibcode = 2005JSR....53...43S}}</ref> It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size during blooms.<ref>{{cite web |url=http://www.phaeocystis.org/ |title=Welcome to the Phaeocystis antarctica genome sequencing project homepage |access-date=2020-08-23 |archive-date=2015-11-20 |archive-url=https://web.archive.org/web/20151120043948/http://www.phaeocystis.org/ }}</ref> As a result, ''Phaeocystis'' is an important contributor to the marine carbon<ref>{{cite journal |title=Rapid and early export of Phaeocystis antarctica blooms in the Ross Sea, Antarctica <!--http://www.nature.com/doifinder/10.1038/35007061-->|journal = Nature|pages = 595–598|volume = 404|issue = 6778|doi = 10.1038/35007061|pmid = 10766240|first1 = G. R.|last1 = DiTullio|first2 = J. M.|last2 = Grebmeier|author-link2=Jacqueline M. Grebmeier|first3 = K. R.|last3 = Arrigo|first4 = M. P.|last4 = Lizotte|first5 = D. H.|last5 = Robinson|first6 = A.|last6 = Leventer|first7 = J. P.|last7 = Barry|first8 = M. L.|last8 = VanWoert|first9 = R. B.|last9 = Dunbar|year = 2000|bibcode = 2000Natur.404..595D|s2cid = 4409009}}</ref> and sulfur cycles.<ref>{{Cite journal|title = DMSP-lyase activity in a spring phytoplankton bloom off the Dutch coast, related to Phaeocystis sp. abundance|journal = Marine Ecology Progress Series|date = 1995-07-20|pages = 235–243|volume = 123|doi = 10.3354/meps123235|first1 = Stefels|last1 = J|first2 = Dijkhuizen|last2 = L|first3 = Gieskes|last3 = WWC|url = https://pure.rug.nl/ws/files/62552225/DMSP_lyase_activity_in_a_spring_phytoplankton_bloom_off_the_Dutch_coast.pdf|bibcode = 1995MEPS..123..235S|doi-access = free}}</ref>

<gallery caption="Mixoplankton" mode="packed" heights="144px" style="float:left;"> File:Tintinnid ciliate Favella.jpg|Tintinnid ciliate ''Favella'' File:Euglena mutabilis - 400x - 1 (10388739803) (cropped).jpg|''Euglena mutabilis'', a photosynthetic flagellate File:Stichotricha secunda - 400x (14974779356).jpg|Zoochlorellae (green) living inside the ciliate ''Stichotricha secunda'' File:Dinophysis acuta.jpg| The dinoflagellate ''Dinophysis acuta'' </gallery>

{{multiple image | align = right | direction = horizontal | header = Mixotrophic radiolarians | header_align = center | header_background = | footer = | footer_align = center | footer_background = | background color = | width1 = 170 | image1 = Phaeocystis symbionts within an acantharian host.png | alt1 = | caption1 = Acantharian radiolarian hosts ''Phaeocystis'' symbionts | width2 = 200 | image2 = Ecomare - schuimalg strand (7037-schuimalg-phaeocystis-ogb).jpg | alt2 = | caption2 = White ''Phaeocystis'' algal foam washing up on a beach }}

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A number of forams are mixotrophic. These have unicellular algae as endosymbionts, from diverse lineages such as the green algae, red algae, golden algae, diatoms, and dinoflagellates.<ref name=Hemleben/> Mixotrophic foraminifers are particularly common in nutrient-poor oceanic waters.<ref>{{Cite book |last=Marshall |first=K. C. |url=https://books.google.com/books?id=QvvlBwAAQBAJ&dq=%22The+symbiont-bearing+foraminifera+are+particularly+common+in+nutrient-poor+oceanic+waters%22&pg=PA22 |title=Advances in Microbial Ecology |date=2013-11-11 |publisher=Springer Science & Business Media |isbn=978-1-4684-7612-5 |language=en}}</ref> Some forams are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.<ref>{{Cite journal|title = Benthic Foraminifera of dysoxic sediments: chloroplast sequestration and functional morphology|year = 1999|last= Bernhard|first=J. M.|author2=Bowser, S.M.|journal = Earth-Science Reviews|volume = 46|issue = 1|pages = 149–165|doi = 10.1016/S0012-8252(99)00017-3|bibcode=1999ESRv...46..149B}}</ref>

By trophic orientation, dinoflagellates are all over the place. Some dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy).<ref>{{Cite journal | last1 = Stoecker | first1 = D. K. | title = Mixotrophy among Dinoflagellates | doi = 10.1111/j.1550-7408.1999.tb04619.x | journal = The Journal of Eukaryotic Microbiology | volume = 46 | issue = 4 | pages = 397–401 | year = 1999 | s2cid = 83885629 | name-list-style = vanc}}</ref> Some species are endosymbionts of marine animals and other protists, and play an important part in the biology of coral reefs. Others predate other protozoa, and a few forms are parasitic. Many dinoflagellates are mixotrophic and could also be classified as phytoplankton. The toxic dinoflagellate ''Dinophysis acuta'' acquires cryptophyte chloroplasts from its ciliate prey<ref name=Stoecker2017 /> who in turn salvage chloroplasts from ingested cryptophytes.<ref>{{cite journal|last1=Dierssen|first1=Heidi|last2=McManus|first2=George B.|last3=Chlus|first3=Adam|last4=Qiu|first4=Dajun|last5=Gao|first5=Bo-Cai|last6=Lin|first6=Senjie|title=Space station image captures a red tide ciliate bloom at high spectral and spatial resolution|journal=PNAS|volume=112|issue=48|pages=14783–14787|year=2015|doi=10.1073/pnas.1512538112|doi-access=free}}</ref> Stoecker et al. (2017) state that "[''D. acuta''] cannot catch the cryptophytes by itself, and instead relies on ingesting ciliates (red ''Myrionecta'' spp.), which sequester their chloroplasts from a specific cryptophyte clade (''Geminigera''/''Plagioselmis''/''Teleaulax'')".<ref name=Stoecker2017>{{cite journal | last1 = Stoecker | first1 = D.K. | last2 = Hansen | first2 = P.J. | last3 = Caron | first3 = D.A. | last4 = Mitra | first4 = A. | year = 2017 | title = Mixotrophy in the marine plankton | url = http://pdfs.semanticscholar.org/8492/ec7724a468af240e014aa539a8865568473d.pdf | archive-url = https://web.archive.org/web/20190227180157/http://pdfs.semanticscholar.org/8492/ec7724a468af240e014aa539a8865568473d.pdf | archive-date = 2019-02-27 | journal = Annual Review of Marine Science | volume = 9 | pages = 311–335 | doi = 10.1146/annurev-marine-010816-060617 | pmid = 27483121 | bibcode = 2017ARMS....9..311S | s2cid = 25579538 }}</ref>{{rp|317}}

===Planktonic metazoa (animals)=== [[File:Fish3562 - Flickr - NOAA Photo Library.jpg|thumb|{{center|Octopus larva and pteropod}}]]

Free-living species in the crustacean class Copepoda are typically 1 to 2&nbsp;mm long with teardrop-shaped bodies. Like all crustaceans, their bodies are divided into three sections: head, thorax, and abdomen, with two pairs of antennae; the first pair is often long and prominent. They have a tough exoskeleton made of calcium carbonate and usually have a single red eye in the centre of their transparent head.<ref name=IZ>{{cite book |author=Robert D. Barnes |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location=Philadelphia, Pennsylvania |pages=683–692 |isbn=978-0-03-056747-6}}</ref> About 13,000 species of copepods are known, of which about 10,200 are marine.<ref>{{Cite web |url=http://www.marinespecies.org/aphia.php?p=taxdetails&id=1080 |title=WoRMS - World Register of Marine Species - Copepoda |website=www.marinespecies.org |access-date=2019-06-28 |archive-url=https://web.archive.org/web/20190630192104/http://www.marinespecies.org/aphia.php?p=taxdetails&id=1080 |archive-date=2019-06-30 |url-status=live}}</ref><ref>{{cite journal |author1=Geoff A. Boxhall |author2=Danielle Defaye |year=2008 |title=Global diversity of copepods (Crustacea: Copepoda) in freshwater |journal=Hydrobiologia |volume=595 |issue=1 |pages=195–207 |doi=10.1007/s10750-007-9014-4 |bibcode=2008HyBio.595..195B |s2cid=31727589 }}</ref> They are usually among the more dominant members of the zooplankton.<ref>{{cite web |author1=Johannes Dürbaum |author2=Thorsten Künnemann |date=November 5, 1997 |title=Biology of Copepods: An Introduction |url=http://www.uni-oldenburg.de/zoomorphology/Biologyintro.html |publisher=Carl von Ossietzky University of Oldenburg |access-date=December 8, 2009 |archive-url=https://web.archive.org/web/20100526164720/http://www.uni-oldenburg.de/zoomorphology/Biologyintro.html |archive-date=May 26, 2010 }}</ref>

In addition to copepods the crustacean classes ostracods, branchiopods and malacostracans also have planktonic members. Barnacles are planktonic only during the larval stage.<ref>[https://books.google.com/books?id=5kv3AwAAQBAJ&dq=pelagic+realm+significant+crustaceans+pelagic+fauna+Ostracoda+Copepoda&pg=PA25 Treatise on Zoology – Anatomy, Taxonomy, Biology. The Crustacea]</ref>

<gallery mode="packed" heights="120px" caption="Metazoan zooplankton" style="float:left"> File:Copepod 2 with eggs.jpg| Copepod with eggs File:Tomopteriskils.jpg|Segmented worm File:Hyperia.jpg| Amphipod File:Krill666.jpg| Krill File:Glaucus atlanticus 1 cropped.jpg| Blue ocean slug </gallery> {{clear}}

====Holoplankton and meroplankton==== ====Ichthyoplankton==== Ichthyoplankton are the eggs and larvae of fish ("ichthyo" comes from the Greek word for ''fish''). They are planktonic because they cannot swim effectively under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larvae swim better and cease to be planktonic as they grow into juvenile fish. Fish larvae are part of the zooplankton that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals.<ref name="NOAA">{{Cite web |date=2007-09-03 |title=What are Ichthyoplankton? |url=https://swfsc.noaa.gov/textblock.aspx?Division=FRD&id=6210 |archive-url=https://web.archive.org/web/20180218090243/https://swfsc.noaa.gov/textblock.aspx?Division=FRD&id=6210 |archive-date=2018-02-18 |access-date=2011-07-22 |website=Southwest Fisheries Science Center}}</ref><ref name=Moser2006>{{Cite book|url=https://books.google.com/books?id=Qdzg0Vfql2sC&pg=PA269|title = The Ecology of Marine Fishes: California and Adjacent Waters|pages = 269–319|isbn = 978-0-520-93247-0|last1 = Allen|first1 = Dr. Larry G.|last2 = Horn|first2 = Dr. Michael H.|date = 15 February 2006| publisher=University of California Press }}</ref>

<gallery mode="packed" heights="120px" style="float:left"> File:Squidu.jpg| Juvenile planktonic squid File:Molalavdj.jpg| Ocean sunfish larvae (2.7mm) File:FMIB 47039 Ostracion hoops.jpeg| Boxfish larva </gallery> {{clear}}

====Gelatinous zooplankton==== Gelatinous zooplankton include ctenophores, medusae, salps, and Chaetognatha in coastal waters. Jellyfish are slow swimmers, and most species form part of the plankton. Traditionally jellyfish have been viewed as trophic dead ends, minor players in the marine food web, gelatinous organisms with a body plan largely based on water that offers little nutritional value or interest for other organisms apart from a few specialised predators such as the ocean sunfish and the leatherback sea turtle.<ref name=Hamilton2016>Hamilton, G. (2016) [https://www.nature.com/news/polopoly_fs/1.19613!/menu/main/topColumns/topLeftColumn/pdf/531432a.pdf "The secret lives of jellyfish: long regarded as minor players in ocean ecology, jellyfish are actually important parts of the marine food web"]. ''Nature'', '''531'''(7595): 432–435. {{doi|10.1038/531432a}}</ref><ref name=Hays2018>Hays, G.C., Doyle, T.K. and Houghton, J.D. (2018) "A paradigm shift in the trophic importance of jellyfish?" ''Trends in ecology & evolution'', '''33'''(11): 874–884. {{doi|10.1016/j.tree.2018.09.001}}</ref>

That view has recently been challenged. Jellyfish, and more gelatinous zooplankton in general, which include salps and ctenophores, are very diverse, fragile with no hard parts, difficult to see and monitor, subject to rapid population swings and often live inconveniently far from shore or deep in the ocean. It is difficult for scientists to detect and analyse jellyfish in the guts of predators, since they turn to mush when eaten and are rapidly digested.<ref name=Hamilton2016/> But jellyfish bloom in vast numbers, and it has been shown they form major components in the diets of tuna, spearfish and swordfish as well as various birds and invertebrates such as octopus, sea cucumbers, crabs and amphipods.<ref>Cardona, L., De Quevedo, I.Á., Borrell, A. and Aguilar, A. (2012) "Massive consumption of gelatinous plankton by Mediterranean apex predators". ''PLOS ONE'', '''7'''(3): e31329. {{doi|10.1371/journal.pone.0031329|doi-access=free}}</ref><ref name=Hays2018 /> "Despite their low energy density, the contribution of jellyfish to the energy budgets of predators may be much greater than assumed because of rapid digestion, low capture costs, availability, and selective feeding on the more energy-rich components. Feeding on jellyfish may make marine predators susceptible to ingestion of plastics."<ref name=Hays2018 /> According to a 2017 study, narcomedusae consume the greatest diversity of mesopelagic prey, followed by physonect siphonophores, ctenophores and cephalopods.<ref name=Choy2017 />

<gallery mode="packed" heights="120px" style="float:left"> File:Parumbrosa polylobata 01.jpg| Jellyfish File:Tunicate off Atauro island.jpg| This free-floating pyrosome is made up of hundreds of individual bioluminescent tunicates File:23 salpchain frierson odfw (8253212250).jpg|Salp chain </gallery> {{clear}}

The importance of the so-called "jelly web" is only beginning to be understood, but it seems medusae, ctenophores and siphonophores can be key predators in deep pelagic food webs with ecological impacts similar to predator fish and squid. Traditionally gelatinous predators were thought ineffectual providers of marine trophic pathways, but they appear to have substantial and integral roles in deep pelagic food webs.<ref name=Choy2017>Choy, C.A., Haddock, S.H. and Robison, B.H. (2017) "Deep pelagic food web structure as revealed by ''in situ'' feeding observations". ''Proceedings of the Royal Society B: Biological Sciences'', '''284'''(1868): 20172116. {{doi|10.1098/rspb.2017.2116}}. 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

==Role in food webs== Grazing by single-celled zooplankton accounts for the majority of organic carbon loss from marine primary production.<ref name=MendenDeuer2021>{{cite journal |doi = 10.3389/fmars.2021.695938|doi-access = free|title = Promoting Instrument Development for New Research Avenues in Ocean Science: Opening the Black Box of Grazing|year = 2021|last1 = Menden-Deuer|first1 = Susanne|author-link1=Susanne Menden-Deuer|last2 = Slade|first2 = Wayne Homer|last3 = Dierssen|first3 = Heidi|journal = Frontiers in Marine Science|volume = 8 | article-number=695938 | bibcode=2021FrMaS...895938M }} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> However, zooplankton grazing remains one of the key unknowns in global predictive models of carbon flux, the marine food web structure and ecosystem characteristics, because empirical grazing measurements are sparse, resulting in poor parameterisation of grazing functions.<ref name=Stock2010>{{cite journal |doi = 10.1016/j.dsr.2009.10.006|title = Controls on the ratio of mesozooplankton production to primary production in marine ecosystems|year = 2010|last1 = Stock|first1 = Charles|last2 = Dunne|first2 = John|journal = Deep Sea Research Part I: Oceanographic Research Papers|volume = 57|issue = 1|pages = 95–112|bibcode = 2010DSRI...57...95S}}</ref><ref name=Bisson2020>{{cite journal |doi = 10.3389/fmars.2020.00505|doi-access = free|title = Diagnosing Mechanisms of Ocean Carbon Export in a Satellite-Based Food Web Model|year = 2020|last1 = Bisson|first1 = Kelsey|last2 = Siegel|first2 = David A.|last3 = Devries|first3 = Timothy|journal = Frontiers in Marine Science|volume = 7 | article-number=505 | bibcode=2020FrMaS...7..505B }}</ref> To overcome this critical knowledge gap, it has been suggested that a focused effort be placed on the development of instrumentation that can link changes in phytoplankton biomass or optical properties with grazing.<ref name=MendenDeuer2021 />

Grazing is a central, rate-setting process in ocean ecosystems and a driver of marine biogeochemical cycling.<ref>{{cite journal |doi = 10.1126/science.1257594|title = Rethinking the marine carbon cycle: Factoring in the multifarious lifestyles of microbes|year = 2015|last1 = Worden|first1 = A. Z.|last2 = Follows|first2 = M. J.|last3 = Giovannoni|first3 = S. J.|last4 = Wilken|first4 = S.|last5 = Zimmerman|first5 = A. E.|last6 = Keeling|first6 = P. J.|journal = Science|volume = 347|issue = 6223| article-number=1257594 |pmid = 25678667|s2cid = 206560125|doi-access = free}}</ref> In all ocean ecosystems, grazing by heterotrophic protists constitutes the single largest loss factor of marine primary production and alters particle size distributions.<ref name="Steinberg12017"/> Grazing affects all pathways of export production, rendering grazing important both for surface and deep carbon processes.<ref>{{cite journal |doi = 10.4319/lo.2013.58.1.0173|title = Control of plankton seasonal succession by adaptive grazing|year = 2013|last1 = Mariani|first1 = Patrizio|last2 = Andersen|first2 = Ken H.|last3 = Visser|first3 = André W.|last4 = Barton|first4 = Andrew D.|last5 = Kiørboe|first5 = Thomas|journal = Limnology and Oceanography|volume = 58|issue = 1|pages = 173–184|bibcode = 2013LimOc..58..173M|doi-access = free}}</ref> Predicting central paradigms of ocean ecosystem function, including responses to environmental change requires accurate representation of grazing in global biogeochemical, ecosystem and cross-biome-comparison models.<ref name=Stock2010 /> Several large-scale analyses have concluded that phytoplankton losses, which are dominated by grazing are the putative explanation for annual cycles in phytoplankton biomass, accumulation rates and export production.<ref>{{cite journal |doi = 10.1890/09-1207.1|title = Abandoning Sverdrup's Critical Depth Hypothesis on phytoplankton blooms|year = 2010|last1 = Behrenfeld|first1 = Michael J.|journal = Ecology|volume = 91|issue = 4|pages = 977–989|pmid = 20462113| bibcode=2010Ecol...91..977B }}</ref><ref>{{cite journal |doi = 10.1038/s41467-017-02143-6|title = Floats with bio-optical sensors reveal what processes trigger the North Atlantic bloom|year = 2018|last1 = Mignot|first1 = A.|last2 = Ferrari|first2 = R.|last3 = Claustre|first3 = H.|journal = Nature Communications|volume = 9|issue = 1|page = 190|pmid = 29335403|pmc = 5768750|bibcode = 2018NatCo...9..190M}}</ref><ref name=Bisson2020 /><ref name=MendenDeuer2021 />

<gallery mode="packed" heights="330px" style="float:left;" caption="Pelagic food web"> File:Export Processes in the Ocean from Remote Sensing.jpg| Pelagic food web and the biological pump. Links among the ocean's biological pump and pelagic food web and the ability to sample these components remotely from ships, satellites, and autonomous vehicles. Light blue waters are the euphotic zone, while the darker blue waters represent the twilight zone.<ref>{{cite journal |doi=10.3389/fmars.2016.00022|title=Prediction of the Export and Fate of Global Ocean Net Primary Production: The EXPORTS Science Plan|year=2016|last1=Siegel|first1=David A.|last2=Buesseler|first2=Ken O.|last3=Behrenfeld|first3=Michael J.|last4=Benitez-Nelson|first4=Claudia R.|last5=Boss|first5=Emmanuel|last6=Brzezinski|first6=Mark A.|last7=Burd|first7=Adrian|last8=Carlson|first8=Craig A.|last9=d'Asaro|first9=Eric A.|last10=Doney|first10=Scott C.|last11=Perry|first11=Mary J.|last12=Stanley|first12=Rachel H. R.|last13=Steinberg|first13=Deborah K.|journal=Frontiers in Marine Science|volume=3|page=22 |doi-access=free |bibcode=2016FrMaS...3...22S }} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> </gallery>

thumb|upright=2| {{center|Schematic of how common seawater constituents, including particulate and dissolved components, could both be generated and altered through the process of herbivorous zooplankton grazing{{hsp}}<ref name=MendenDeuer2021 />}}

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==Role in biogeochemistry== In addition to linking primary producers to higher trophic levels in marine food webs, zooplankton also play an important role as "recyclers" of carbon and other nutrients that significantly impact marine biogeochemical cycles, including the biological pump. This is particularly important in the oligotrophic waters of the open ocean. Through sloppy feeding, excretion, egestion, and leaching of fecal pellets, zooplankton release dissolved organic matter (DOM) which controls DOM cycling and supports the microbial loop. Absorption efficiency, respiration, and prey size all further complicate how zooplankton are able to transform and deliver carbon to the deep ocean.<ref name=Steinberg12017 />

=== Sloppy feeding and release of DOM === {{see also|Zooplankton grazing}} [[File:Sloppy feeding by zooplankton.jpg|thumb|right|upright=1.3| {{center|'''Sloppy feeding by zooplankton'''<br />DOC {{=}} dissolved organic carbon<br />POC {{=}} particulate organic carbon<br /><small>Adapted from Møller et al. (2005),<ref name="Møller2003">{{cite journal |doi = 10.3354/meps262185|title = Production of DOC by ''Calanus finmarchicus'', ''C. Glacialis'' and ''C. Hyperboreus'' through sloppy feeding and leakage from fecal pellets|year = 2003|last1 = Møller|first1 = EF|last2 = Thor|first2 = P.|last3 = Nielsen|first3 = TG|journal = Marine Ecology Progress Series|volume = 262|pages = 185–191|bibcode = 2003MEPS..262..185M|doi-access = free}}</ref><br />Saba et al. (2009)<ref name=Saba2009>{{cite journal |doi = 10.3354/meps08070|title = Effects of diet on release of dissolved organic and inorganic nutrients by the copepod ''Acartia tonsa''|year = 2009|last1 = Saba|first1 = GK|last2 = Steinberg|first2 = DK|last3 = Bronk|first3 = DA|journal = Marine Ecology Progress Series|volume = 386|pages = 147–161|bibcode = 2009MEPS..386..147S|doi-access = free}}</ref> and Steinberg et al. (2017).<ref name=Steinberg12017>{{cite journal |doi = 10.1146/annurev-marine-010814-015924|title = Zooplankton and the Ocean Carbon Cycle|year = 2017|last1 = Steinberg|first1 = Deborah K.|last2 = Landry|first2 = Michael R.|journal = Annual Review of Marine Science|volume = 9|pages = 413–444|pmid = 27814033|bibcode = 2017ARMS....9..413S}}</ref></small>}}]]

Excretion and sloppy feeding (the physical breakdown of food source) make up 80% and 20% of crustacean zooplankton-mediated DOM release respectively.<ref name=Saba2011>{{cite journal |doi = 10.1016/j.jembe.2011.04.013|title = The relative importance of sloppy feeding, excretion, and fecal pellet leaching in the release of dissolved carbon and nitrogen by Acartia tonsa copepods|year = 2011|last1 = Saba|first1 = Grace K.|last2 = Steinberg|first2 = Deborah K.|last3 = Bronk|first3 = Deborah A.|journal = Journal of Experimental Marine Biology and Ecology|volume = 404|issue = 1–2|pages = 47–56| bibcode=2011JEMBE.404...47S }}</ref> In the same study, fecal pellet leaching was found to be an insignificant contributor. For protozoan grazers, DOM is released primarily through excretion and egestion and gelatinous zooplankton can also release DOM through the production of mucus. Leaching of fecal pellets can extend from hours to days after initial egestion and its effects can vary depending on food concentration and quality.<ref name=Thor2003>{{cite journal |doi = 10.3354/ame033279|title = Fate of organic carbon released from decomposing copepod fecal pellets in relation to bacterial production and ectoenzymatic activity|year = 2003|last1 = Thor|first1 = P.|last2 = Dam|first2 = HG|last3 = Rogers|first3 = DR|journal = Aquatic Microbial Ecology|volume = 33|pages = 279–288|doi-access = free | bibcode=2003AqME...33..279T }}</ref><ref name=Carlso2014>{{Cite book|url=https://books.google.com/books?id=7iKOAwAAQBAJ&q=%22Biogeochemistry+of+marine+dissolved+organic+matter%22|title=Biogeochemistry of Marine Dissolved Organic Matter|isbn=978-0-12-407153-7|last1=Hansell|first1=Dennis A.|last2=Carlson|first2=Craig A.|date=2 October 2014|publisher=Academic Press }}</ref> Various factors can affect how much DOM is released from zooplankton individuals or populations. Absorption efficiency (AE) is the proportion of food absorbed by plankton that determines how available the consumed organic materials are in meeting the required physiological demands.<ref name=Steinberg12017 /> Depending on the feeding rate and prey composition, variations in AE may lead to variations in fecal pellet production, and thus regulates how much organic material is recycled back to the marine environment. Low feeding rates typically lead to high AE and small, dense pellets, while high feeding rates typically lead to low AE and larger pellets with more organic content. Another contributing factor to DOM release is respiration rate. Physical factors such as oxygen availability, pH, and light conditions may affect overall oxygen consumption and how much carbon is loss from zooplankton in the form of respired CO₂. The relative sizes of zooplankton and prey also mediate how much carbon is released via sloppy feeding. Smaller prey are ingested whole, whereas larger prey may be fed on more "sloppily", that is more biomatter is released through inefficient consumption.<ref name="Møller2005">{{cite journal |doi = 10.1093/plankt/fbh147|title = Sloppy feeding in marine copepods: Prey-size-dependent production of dissolved organic carbon|year = 2004|last1 = Moller|first1 = E. F.|journal = Journal of Plankton Research|volume = 27|pages = 27–35|doi-access = free}}</ref><ref name="Møller2007">{{cite journal |doi = 10.4319/lo.2007.52.1.0079|title = Production of dissolved organic carbon by sloppy feeding in the copepods Acartia tonsa, Centropages typicus, and Temora longicornis|year = 2007|last1 = Møller|first1 = Eva Friis|journal = Limnology and Oceanography|volume = 52|issue = 1|pages = 79–84|bibcode = 2007LimOc..52...79M|doi-access = free}}</ref> There is also evidence that diet composition can impact nutrient release, with carnivorous diets releasing more dissolved organic carbon (DOC) and ammonium than omnivorous diets.<ref name=Thor2003 />

{{multiple image | align = left | direction = horizontal | header = Comparison of zooplankton-mediated carbon cycles{{hsp}}<ref name=Halfter2020>{{cite journal |doi = 10.3389/fmars.2020.567917|title = The Role of Zooplankton in Establishing Carbon Export Regimes in the Southern Ocean – A Comparison of Two Representative Case Studies in the Subantarctic Region|year = 2020|last1 = Halfter|first1 = Svenja|last2 = Cavan|first2 = Emma L.|last3 = Swadling|first3 = Kerrie M.|last4 = Eriksen|first4 = Ruth S.|last5 = Boyd|first5 = Philip W.|journal = Frontiers in Marine Science|volume = 7| article-number=567917 |s2cid = 222003883|doi-access = free | bibcode=2020FrMaS...767917H }} 50px Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> | header_align = center | header_background = | footer = <small>Grazing and fragmentation of particles at both sites increases nutrient recycling in the upper water column</small> | footer_align = center | footer_background = | background color = | image1 = Zooplankton-mediated carbon cycle 1.jpg | width1 = 360 | alt1 = | caption1 = {{center|'''Kerguelen Plateau'''<br />Naturally iron-fertilized}} On the Kerguelen Plateau in summer, high iron levels lead to high chlorophyll a as a proxy for algae biomass at the surface. The diverse zooplankton community feeds on the sinking particle flux and acts as a gate-keeper to the deeper ocean by ingesting and fragmenting sinking particles and, consequently, significantly reducing the export flux out of the epipelagic. The main export particles are diatom resting spores, which bypass the intense grazing pressure, followed by fecal pellets.<ref name=Halfter2020 /> | image2 = Zooplankton-mediated carbon cycle 2.jpg | width2 = 364 | alt2 = | caption2 = {{center|'''Southern Ocean waters'''<br />High nutrient, low chlorophyll}} In Southern Ocean waters in summer, iron levels are relatively low and support a more diverse phytoplankton community, but with lower biomass, which, in turn, affects zooplankton community composition and biomass. The grazing pressure during summer is focused mostly on picoplankton, which leaves large particles for export.<ref name=Halfter2020 /> }}

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===Carbon export=== Zooplankton play a critical role in supporting the ocean's biological pump through various forms of carbon export, including the production of fecal pellets, mucous feeding webs, molts, and carcasses. Fecal pellets are estimated to be a large contributor to this export, with copepod size rather than abundance expected to determine how much carbon actually reaches the ocean floor. The importance of fecal pellets can vary both by time and location. For example, zooplankton bloom events can produce larger quantities of fecal pellets, resulting in greater measures of carbon export. Additionally, as fecal pellets sink, they are reworked by microbes in the water column, which can thus alter the carbon composition of the pellet. This affects how much carbon is recycled in the euphotic zone and how much reaches depth. Fecal pellet contribution to carbon export is likely underestimated; however, new advances in quantifying this production are currently being developed, including the use of isotopic signatures of amino acids to characterize how much carbon is being exported via zooplankton fecal pellet production.<ref>{{Cite conference |bibcode=2019AGUFMPP42C..12D |title=Compound-Specific Isotope Analysis of Zooplankton Fecal Pellets: Insights into Dietary and Trophic Processes and Characterization of Fecal Pellets as Organic Matter End-Member |last1=Doherty |first1=S.|last2 = Maas |first2=A. E. |last3=Steinberg |first3=D. K. |last4=Popp |first4=B. N. |last5=Close |first5=H. G. |book-title=American Geophysical Union, Fall Meeting 2019 Abstracts |date=2019 |at=abstract #PP42C–12}}</ref> Carcasses are also gaining recognition as being important contributors to carbon export. Jelly falls – the mass sinking of gelatinous zooplankton carcasses – occur across the world as a result of large blooms. Because of their large size, these gelatinous zooplankton are expected to hold a larger carbon content, making their sinking carcasses a potentially important source of food for benthic organisms.<ref name=Steinberg12017 />

==See also== * Census of Marine Zooplankton * Diel vertical migration * Ocean acidification * Primary production * Thin layers (oceanography)

==References== {{Reflist|30em}}

==External links== * [http://www.sahfos.ac.uk/ SAHFOS] Sir Alister Hardy Foundation for Ocean Science * [http://vimeo.com/84872751/ Ocean Drifters] Short film narrated by David Attenborough about the varied roles of plankton * [http://news.bbc.co.uk/1/hi/sci/tech/8498786.stm Sea Drifters] BBC Audio slideshow * [http://www.planktonchronicles.org/en Plankton Chronicles] Short documentary films & photos * [http://www.st.nmfs.noaa.gov/plankton/ COPEPOD: The global plankton database]. A global coverage database of zooplankton biomass and abundance data. * [https://web.archive.org/web/20080530005405/http://www.tafi.org.au/zooplankton/ Guide to the marine zooplankton of south eastern Australia], [http://www.tafi.org.au/ Tasmanian Aquaculture and Fisheries Institute] * [http://imos.org.au/auscpr.html Australian Continuous Plankton Recorder Project] {{Webarchive|url=https://web.archive.org/web/20081201125235/http://imos.org.au/auscpr.html |date=2008-12-01 }} * [http://cfb.unh.edu/cfbkey/html/groups.html An Image-Based Key to Zooplankton of North America]

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