Marine Plankton

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Definition of Plankton:
Aquatic organisms, usually microscopic, that are transported by water movements and have limited capabilities for autonomous movement. Plankton include bacteria and microalgae (phytoplankton), microscopic crustaceans, cnidarians, and the larvae of fish and other animals (zooplankton). They form the basis of aquatic food webs[1].
This is the common definition for Plankton, other definitions can be discussed in the article


Fig. 1. Principal groups of marine plankton, arranged approximately according to increasing organism size. The size indications are approximate, and some groups include species smaller or larger than the ranges shown. General size classes after Sieburth et al. (1978[2]).



Marine plankton comprise the great variety of organisms that live suspended in seawater and are transported mainly by currents. Some can swim or regulate their buoyancy to migrate vertically and can therefore influence their distribution over small spatial and temporal scales. They range from viruses and microscopic bacteria and protists to comparatively large crustaceans and gelatinous animals. This article introduces the principal plankton groups, their characteristic forms, sizes, feeding modes and life histories, and the methods used to observe and sample them. It does not attempt to provide a comprehensive account of plankton ecology. Several other Coastal Wiki articles introduce related aspects of plankton ecology, including:



Introduction

Fig. 2. The ‘Sieburth-scale’, redrawn from Sieburth et al. (1978[2]).

Plankton consists of a diverse range of living organisms that spend at least a part of their life cycle suspended in water. The term plankton is actually a Greek word, meaning that which is made to wander or drift. This term is further divided into the phytoplankton and zooplankton, meaning plant- (Gk. phyto) and animal- (Gk. zoön) drifters respectively.

Holoplankton refers to those organisms that spend their entire life in the plankton, as opposed to the meroplankton, which are only planktonic for a part of their lives. Organisms that are capable of resisting the powers of currents, such as fish and squid, are referred to as nekton.

Planktonic organisms are typically classified into broad size categories according to the 'Sieburth-scale' (Fig. 2), originally proposed in 1978. Viruses and jellyfish sit at opposite ends of this scale, which runs from fractions of a millimeter to meters. Most planktonic species are microscopic in size. Picoplankton is on average ten times more abundant than nanoplankton in the ocean and a thousand times more abundant than microplankton. However, the size classes of picoplankton and nanoplankton represent similar particle surface concentrations, and thus contribute similarly to the light scattering and absorption properties of seawater. The volume concentration of nanoplankton in the ocean is on average significantly greater than the volume concentration of microplankton and picoplankton, except in Arctic waters where the volume concentrations of nanoplankton and microplankton are comparable[3].

Fig. 3. Average sea surface chlorophyll for the period January 1998 to December 2006 from the SeaWIFS satellite. The average is composed from 8 day composites with a spatial resolution of 0.5° in latitude and longitude. Chlorophyll is in mg chl m-3 (note that the colour scale is logarithmic). It is plotted here using a Mollweide projection (using MATLAB and the M_Map package). Image provided courtesy of Dr A. Yool.

Besides this size-based classification marine plankton, other classifications are also relevant (see Marine microorganisms):

  • Taxonomic classes: viruses, bacteria, archaea, protists, fungi, animals
  • Trophic classes: phototrophs, heterotrophs, mixotrophs
  • Functional classes: e.g. primary producers, grazers, decomposers, parasites, calcifiers


Phytoplankton

Phytoplankton support most pelagic marine food webs and account for roughly half of global photosynthetic carbon fixation. They provide the basic food in the food web/chain and play an important role in nutrient recycling and gaseous exchange.

Marine phytoplankton are taxonomically diverse and comprise various groups, including diatoms, coccolithophores, chlorophytes, flagellate protists (dinoflagellates, raphidophytes) and cyanobacteria. Several thousand marine phytoplankton species have been described, while molecular studies reveal considerable additional diversity. Photosynthesis is the process by which phytoplankton combines inorganic building blocks such as carbon dioxide (CO2) and water (H2O) to produce organic compounds, using energy from the Sun. Organisms that are capable of this process are referred to as photoautotrophs (Gk. Photon, auto, troph; light-self-nutrition) and primary producers. In eukaryotic phytoplankton, photosynthetic pigments are generally contained in plastids or chloroplasts. The photosynthetic machinery of cyanobacteria is located in internal membranes. Organic matter formed by these organisms supports most marine food webs. Approximately 50% of global primary production occurs in the oceans. When the water temperature is favorable and light and nutrients are sufficiently available, algal colonies can grow exponentially and form huge blooms that collapse after depletion of the nutrient pool, see Plankton bloom. Physico-chemical parameters including light, nutrients, density stratification and turbulence are major factors influencing spatial and temporal variations of phytoplankton.

On land, plants are typically large, conspicuous organisms; trees, herbs, bushes and grasses etc. They have root structures to take up water and nutrients from the soils beneath them, and also to provide anchorage. Phytoplankton are fundamentally different. Most of them are microscopic, single-celled organisms. Filamentous cyanobacteria grow as integrated chains of cells and some plankton species form colonies (e.g. Phaeocystis, Trichodesmium). Smaller objects have a greater surface area relative to their volume and mass. Because of their small size, phytoplankton have very low settling velocities: viscous resistance is large relative to their weight. Being small in the oceans confers several advantages: Because of their small size, phytoplankton settle very slowly: even a low settling velocity generates sufficient viscous drag to balance their small excess weight over buoyancy. Thus, they typically sink very slowly, enabling them to stay within the surface, sunlit waters. Phytoplankton species often have ornate spines and appendages which increase drag, and may serve as defense mechanisms. Many plankton species also use a chemical defense against their predators, either through toxin production or feeding deterrence, see Chemical ecology. Phytoplankton cells do not have roots, and must take up nutrients from their surrounding environment. Having a large surface area relative to their volume ensures that they maximize their chances of attaining enough resources for growth.

Pelagic environments are usually classified according to their levels of plankton chlorophyll-a (chl-a) as oligotrophic (chl-a < 0.1 mg/m3), mesotrophic (0.1 < chl-a < 1 mg/m3), and eutrophic (chl-a > 1 mg/m3), which is approximately equivalent in terms of primary production to < 50, 50—200, and > 200 gC/m2 per annum. In addition, most pelagic phytoplankton belong to one of the three major size classes: picoplankton (0.2—2.0 μm), nanoplankton (2—20 μm), and microplankton (20—200 μm) [4]. Large parts of the subtropical open ocean are oligotrophic, whereas coastal and shelf seas generally have higher and more variable chlorophyll concentrations. Some coastal waters are eutrophic because of natural nutrient inputs or anthropogenic nutrient enrichment (Fig. 3).

Fig. 4. Cyanobacteria Prochlorococcus spp. These tiny cyanobacteria (approximately 0.5–0.7 μm) are highly efficient at harvesting light and nutrients. Prochlorococcus is probably the most abundant photosynthetic organism on Earth and makes a major contribution to primary production in oligotrophic oceans[5]. They are the most abundant (~1027) cells on Earth. Photo credit Derek Tan http://www3.botany.ubc.ca/derek/ Creative commons noncommercial licence

Picophytoplankton accounts for a significant fraction of primary production in seawater, especially in oligotrophic environments. This group includes the cyanobacteria Prochlorococcus and Synechococcus, which are the most productive photosynthetic organisms on Earth[6]. Prochlorococcus (Fig. 4) are extremely well adapted to oligotrophic conditions and dominate the tropical oceans throughout the 200 m thick photic layer[7]. Synechococcus (Fig. 10) and larger-sized phytoplankton tend to be more abundant under mesotrophic conditions in the subtropical-temperate oceans. Many heterotrophic bacteria and some protists also fall in the picoplankton size class. Heterotrophic bacteria take up dissolved organic compounds from their surroundings and are major contributors to the cycling of carbon and nutrients (see Nutrient conversion in the marine environment). Other bacteria are capable of both autotrophic and heterotrophic nutrition: they are mixotrophic.

Microphytoplankton comprise the larger photosynthetic plankton, especially single cells and chains of diatoms and larger photosynthetic or mixotrophic dinoflagellates. The broader microplankton size class also includes heterotrophic dinoflagellates, ciliates and other phagotrophic protists.

Nanoplankton include most species of flagellates, autotrophic, heterotrophic, and mixotrophic, along with some smaller-sized non-flagellated green algae and diatoms, and the smallest species of dinoflagellates and ciliates. Heterotrophic nanoflagellates are dominant consumers of picophytoplankton and important remineralizers of organic matter and nutrients[4].

Fig. 5. The diatom Coscinodiscus wailesii. The two ‘halves’ of the cell can be seen in the top left image. Image taken by M. Hoppenrath, courtesy of Plankton Net, Alfred Wegener Institute for Polar and Marine Research.

Diatoms

Perhaps the most conspicuous group of phytoplankton are the diatoms (Fig. 5). Their name is derived from the Greek words dia and temnein, literally cut in half. This seemingly bizarre name arose because of the nature of their cell wall, or frustule, which is made up of two halves or valves like that of a laboratory Petri dish. The silicon-rich frustule is perforated by numerous pore-like structures which can give the cell a beautiful appearance when viewed under high magnification. These connect the cell to the outside seawater. They also help strengthen the cell wall whilst reducing its mass. Diatoms can regulate their buoyancy via the biosynthesis of lipids (triacylglycerols), which are less dense than other chemical constituents of these organisms. As these lipids are rich in energy, diatoms have great potential as a biofuel source[8].

Diatoms tend to be dominant in eutrophic environments, often referred to as 'classical' marine food webs. It has been shown that their cell cycle is interrupted under silicon-limited conditions. In temperate latitudes, diatoms are well known for their capacity to form immense blooms in spring as the sun’s energy infiltrates nutrient-rich surface waters. Diatoms require silicon for cell division because of the unique structure and morphogenesis of their frustule. Their rapid growth rates are thought to confer them a competitive advantage during these times, only to be thwarted as silicate concentrations become depleted. During the decline of some blooms, nutrient stress and the production of extracellular polymeric substances promote cell aggregation. The resulting aggregates can sink rapidly and contribute to the downward transport of organic carbon. Many planktonic diatoms, especially bloom-forming centric species, can form resting spores or resting cells that survive in bottom sediments. When favorable conditions return, viable resting stages near the sediment surface may germinate and, after resuspension in shallow waters, contribute to the development of a new plankton population[9][10]. Sinking diatoms also contribute to the downward transport of organic carbon. Diatoms are one of the major producers of polyunsaturated fatty acids, which are now widely reputed as being beneficial for human health. Diatoms are a good food source for herbivores, but some species produce chemicals (oxylipins) as a defence against being grazed, see Functional metabolites in phytoplankton. Some species and strains of the diatom genus "Pseudo-nitzschia" produce domoic acid, which can accumulate in shellfish and other food-web organisms and cause amnesic shellfish poisoning, see Harmful algal bloom.

Fig. 6. The flagellate Leucocryptos marina. Image taken by Wiktor, courtesy of Plankton Net, Alfred Wegener Institute for Polar and Marine Research.

Flagellates

Many marine nano- and microplankton are unicellular flagellates (Fig. 6). These cells possess a whip-like flagellum. It is at this end of the plankton size spectrum that the distinction between plant and animal becomes blurred. Flagella enable these organisms to move through the surrounding water and respond to environmental gradients, acquire food and sometimes reduce exposure to predators. Some flagellates, although mobile, contain photosynthetic pigments and are thus autotrophic. Others are devoid of pigments and hence are heterotrophic. Phytoflagellates and dinoflagellates associated with seasonal stratification, fronts, and upwelling zones are often mixotrophic. Marine mixotrophy is the ability of certain microorganisms to combine photosynthesis with ingesting prey or absorbing organic matter within a single cell. The overall effect of mixotrophy is to increase trophic transfer to higher levels in the food web. Modeling has indicated that mixotrophy has a profound impact on marine planktonic ecosystems and may enhance primary production and the functioning of the biological carbon pump[11]. Forms of mixotrophy are also common in surface waters among bacteria and archaea.

Fig. 7. The thecate dinoflagellate Tripos pentagonus. Image taken by Fatima Santos, courtesy of Plankton Net, Alfred Wegener Institute for Polar and Marine Research.
Dinoflagellates

Dinoflagellates are comparatively large flagellates. They are so-called because of the way that they ‘whirl’ or corkscrew through the water (Gk. Dinos; whirling). Dinoflagellates include photosynthetic, heterotrophic and mixotrophic species, and many photosynthetic species can also ingest prey (see below). Many armor their cell wall with cellulose plates, forming the theca (Fig. 7). In some species of thecate dinoflagellates the plates form long spines which serve to increase the cell’s frictional drag and also act as a grazing deterrent. The ‘naked’ dinoflagellates lack any such extracellular armor.

Several species of dinoflagellates are bioluminescent: They produce a flicker of light when disturbed. This is frequently observed by SCUBA divers at night when they turn off their torches and disturb the water with their hands. Some species of dinoflagellates can produce virulent neurotoxins such as saxitoxin and brevetoxin. These can cause serious harm if they enter the human food chain. For this reason, monitoring programs exist to ensure that the fish and shellfish that we consume are safe, see Harmful algal bloom.

Fig. 8. The flagellate Phaeocystis globosa. Image taken by Fjouenne, courtesy of Plankton Net, Alfred Wegener Institute for Polar and Marine Research.
Phaeocystis

Phaeocystis (Fig. 8) is a particularly interesting genus of flagellate. Phaeocystis can occur as small solitary cells or as cells embedded in large gelatinous colonies. Colonial forms can develop extensive blooms, particularly in temperate and polar shelf seas. These blooms can alter food-web structure and, after colony breakdown, produce large quantities of foam along some coasts. Phaeocystis produces a strong-smelling compound called dimethyl sulfide (DMS) which is thought to serve as a grazing deterrent. The contribution of Phaeocystis-derived DMS to climate regulation through cloud formation remains debated. Nutrient enrichment can favour blooms in some coastal regions, but bloom occurrence also depends on grazing, water-column structure, temperature and species-specific life cycles.


It can exist as small, individual cells or large gelatinous colonies. The latter are long known for fouling fishermen’s nets. Large blooms of these organisms can occur throughout the world, particularly in the temperate and polar seas. These are problematic for fish as the gelatinous colonies can clog their gills. Indeed, blooms of Phaeocystis can trigger dramatic changes in the structure of marine ecosystems owing to its seemingly unpleasant nature. Phaeocystis produces dimethylsulfoniopropionate (DMSP), which contributes to osmotic regulation and protection against oxidative stress. Enzymatic cleavage of DMSP produces dimethyl sulfide (DMS) and acrylate, which may also influence grazing interactions and microbial processes. The role of Phaeocystis' DMS in climate regulation through cloud formation remains debated[12]. There is growing concern that the frequency and magnitude of Phaeocystis blooms in coastal waters are increasing as a result of eutrophication. See also Foam beach, Sydney.

Fig. 9. The coccolithophorid Emiliania huxleyi var. corona. Image taken by Claudia Sprengel & Jeremy Young, courtesy of Plankton Net, Alfred Wegener Institute for Polar and Marine Research.
Coccolithophores

Coccolithophores (or coccolithophorids, Fig. 9) are another important group of flagellates. Each cell is covered by an array of intricate plates or coccoliths (the coccosphere) made from calcium carbonate. Coccolithophores are the most prolific calcifying organisms on Earth. The exact function of the coccoliths is unknown. The coccoliths have been suggested to serve as a grazing deterrent, to help maintain buoyancy and to act as an ultra-violet radiation filter. Certain species produce enormous blooms that are clearly visible from space, causing the water to have a milky-white appearance that fishermen refer to as ‘white water’.

Coccolithophores influence the marine carbon cycle through both photosynthetic organic-carbon production and calcium-carbonate formation. Calcification changes seawater alkalinity and tends to increase surface-water CO2, whereas coccoliths may contribute to particle aggregation, sinking and long-term carbonate burial. The net effect depends on production, dissolution, remineralization and ocean circulation (see also Ocean acidification).

Fig. 10. The cyanobacterium Synechococcus sp. Image taken by Chantal Billard, courtesy of Plankton Net, Alfred Wegener Institute for Polar and Marine Research.

Cyanobacteria

Cyanobacteria, or the blue-green algae (Fig. 10), play a unique role in the nutrient-depleted waters of the open oceans. Some marine cyanobacteria, including Trichodesmium and several unicellular or symbiotic groups, are diazotrophs: they convert dissolved N2, which is subsequently converted to ammonium or organic nitrogen. This supplies biologically available nitrogen to nutrient-poor waters, see Nutrient conversion in the marine environment.

Zooplankton

The extremely diverse collection of organisms referred to as zooplankton is all but hidden from the public eye. It is presumably for this reason that some of the most numerous animals on our planet remain without common names. Most major animal phyla are represented in the zooplankton, either throughout their life cycle or during a larval stage. Zooplankton play crucial roles in pelagic food webs as links between primary producers, microbial consumers and higher trophic levels.

Microzooplankton and heterotrophic protists

Microzooplankton comprise a diverse assemblage of mostly heterotrophic or mixotrophic protists in the approximate size range 20–200 μm. Important groups include heterotrophic dinoflagellates, ciliates, foraminiferans and radiolarians. Many smaller heterotrophic protists belong to the nanoplankton. Foraminifera and radiolarians are mostly heterotrophic planktonic protists that capture small prey or organic particles. Their mineral skeletons contribute to marine sediments after death and are widely used to reconstruct past ocean conditions. Parasitic protists infect many phytoplankton and zooplankton species and can strongly influence plankton mortality, bloom termination and food-web structure.

Microzooplankton can consume a large fraction of daily phytoplankton production and therefore exert strong control over phytoplankton communities. In addition, the high lipid content of microzooplankton and their ability to buffer nutritional imbalances renders them high quality food items for mesozooplankton and newly hatched fish larvae[13]. In fact, microzooplankton is often a primary food source for mesozooplankton, bypassing the direct link between phytoplankton and copepods[14]. Marine food webs are typically complex because many organisms feed at several trophic levels, switch diet during their life cycle, or combine photosynthesis with consumption of other organisms. Clear distinctions between producers and consumers, or between predators and prey, therefore do not always apply.

Fig. 11. The naked ciliate Strobilidium sp.. Image taken by Alexandra Kraberg, courtesy of Plankton Net, Alfred Wegener Institute for Polar and Marine Research.
Fig. 12. Dinoflagellate feeding on a ciliate via the peduncle. Reproduced with permission from Wiley-Blackwell [15]


Ciliates (Fig. 11) are a particularly important group of microzooplankton. As their name suggests, they possess numerous hair-like features that fulfil several functional roles including movement, feeding and sensing the external environment. Heterotrophic ciliates focus on prey much smaller than themselves, ingesting bacteria and/or nanoflagellates. They play key roles in the microbial loop (see below).


Heterotrophic dinoflagellates frequently attack prey of equal or larger size than themselves (Fig. 12). Some feed by engulfing their prey whole, while others suck out the contents of their prey via a feeding appendage called the peduncle.

Crustacea

Planktonic crustaceans are in many ways analogous to the insects on land. They typically dominate zooplankton communities, representing a crucial link in oceanic food webs. The term crustacean is derived from the Latin word “crustaceus”, meaning ‘having a shell or crust’. This refers to the jointed armor that envelops their bodies, made from a tough material called chitin (Gk. chiton, a tunic). This rigid external skeleton restricts the growth process, and must first be shed before an individual can increase in size. Growth and development of all crustaceans is therefore achieved through a series of molts. It is common for the body form of an adult to be considerably different from that of the young.

Fig. 13. The copepod Calanus finmarchicus. Image taken by Daniel Mayor.

Copepods. In the words of Sir Alister Hardy, the world-renowned zooplankton biologist, “the crustaceans of the marine plankton par excellence are the Copepoda” [16]. Copepods are among the most abundant metazoans in the ocean and often dominate mesozooplankton abundance and biomass. They are found in all the seas on Earth, and have developed numerous strategies to survive in even the harshest conditions. Copepods may be herbivorous, omnivorous, carnivorous or even detritivorous. They typically have an array of specialized feeding appendages that enable them to effectively sieve out or grasp their food from the surrounding water. It is these oar-like feet that give copepods their name (Gk. Kope, Podo; oar-foot). The sheer numbers of these organisms necessitate that they play major roles in the ecology of our seas. Copepods are often the first prey of fish larvae, so a healthy population of these tiny creatures is essential for healthy fish stocks.

Fig. 14. The northern krill Meganyctiphanes norvegica. Image taken by Øystein Paulsen, courtesy of MAR-ECO.

Euphausids are relatively large (10-50 mm) shrimp-like animals that are more commonly known as krill (Fig. 14). They are thought to be omnivores, filtering out phytoplankton and similar-sized microzooplankton from seawater. Krill can form very dense swarms, especially in polar and subpolar waters. This is thought to confuse predators that are searching for an individual. Euphausids (Gk. Phausis; Shining light) are so-called because they have light-producing organs. Exactly why they produce light is not fully understood. It may be involved in mate-location, social interaction or camouflage. Krill are super-abundant, particularly in the polar seas. They represent a staple food source for a diverse array of animals, ranging from fish and penguins to seals and whales.

Fig. 15. The planktonic amphipod Hyperia macrocephala. Image taken by Uwe Kils.
Fig. 16. A pelagic ctenophore. Image taken by Marsh Youngbluth, courtesy of MAR-ECO.

Amphipods (Fig. 15) are another important type of planktonic crustacean. They have large (sometimes enormous), well-developed eyes at the front of their head. These, together with pincer-type feeding appendages, are used to actively seek out and capture their prey. The striking appearance of planktonic amphipods is reputed to have inspired the form of the creature in Ridley Scott’s 1979 film ‘Alien’. Some amphipods have a tendency to swarm like krill, whereas others live in association with gelatinous organisms such as jellyfish and salps. Upper ocean amphipods are eaten by an array of larger animals, including fish, birds and marine mammals. Deep water amphipods are important scavengers, and are always amongst the first animals to arrive when a new food source becomes available.

Many crustacean zooplankton undertake diel vertical migration, ascending towards surface waters at night and descending during daylight. The timing and amplitude vary among species, seasons and environments. Avoidance of visually hunting predators is considered a major driver, together with feeding conditions, metabolism and life-history requirements.

Jellies

The oceans harbour an enormous array of gelatinous organisms, or ‘jellies’ , that range in size from millimetres to meters. They have soft, translucent and often fragile bodies. The latter makes them notoriously difficult to sample in a quantitative manner. This has led to the significance of gelatinous organisms in marine ecosystems being underestimated historically. Cnidarians and ctenophores are primarily predators, whereas pelagic tunicates such as salps and appendicularians are suspension feeders. Some jellyfish contain endosymbiotic algae that provide sugars and other carbohydrates via photosynthesis.

Jellyfish are in fact not fish at all. They belong to a collection of organisms known as cnidarians. They are distinguished by the presence of nematocysts or cnidocytes, which are stinging cells. These serve as efficient weapons, firing tiny dart-like structures that deliver neurotoxins into their prey. Jellyfish are known to form large blooms. This largely reflects their seasonal reproductive efforts. It has been suggested that the frequency and magnitude of jellyfish blooms has increased as a result of overfishing. However, there are currently too few data to substantiate or refute this claim. Jellyfish are consumed by other jellyfish, as well as fish and turtles.

Ctenophores (Fig. 16) are distinct from jellyfish because they do not possess cnidocytes. They are more typically known as the comb jellies (Gk. kteis, ktenos; comb), owing to the eight rows of cilia which have a comb-like appearance. The rhythmic beating of these tiny hairs provides propulsion for these animals. The cilia refract light like a prism, giving rise to wave upon wave of rainbow colours that sweep over their body. In the 1980s an invasive American species of ctenophore was accidentally introduced into the Black Sea via a ship’s ballast water. Its rapid expansion greatly intensified food-web disruption in an ecosystem already affected by eutrophication and overfishing, contributing to the decline of commercial fish stocks.

Fig. 17. The giant colonial tunicate Pyrosoma sp.. Image taken by D. Shale, courtesy of MAR-ECO.

Salps, doliolids, pyrosomes and larvaceans are pelagic tunicates. They range in size from a few millimeters to meter-long colonies (Fig. 17). Typically they have a barrel-shaped body that has two openings – one for taking water in, and the other for allowing water to be pumped out. Salps, doliolids and pyrosomes pump water through an internal mucus filter, largely by muscular contractions. Larvaceans use ciliary currents to draw water through an external gelatinous 'house', which is periodically discarded and rebuilt.

Chaetognaths

The chaetognaths (L. chaeta, a bristle; Gk. gnathos, jaw) are a particularly interesting group of zooplankton. They are commonly known as the ‘arrow-worms’ because of their slender arrow-like nature. Every ocean of the world contains a chaetognath representative, and they are typically amongst the commonest animals encountered in the plankton. Arrow worms are voracious predators, with curved bristles that act as powerful jaws. They hunt by remaining motionless in the water until an unsuspecting copepod moves within striking distance; then they dart forward and grasp their prey. It is easy to overlook arrow worms in live plankton samples because they are almost completely transparent, often only becoming visible when they move.

Feeding relationships within the plankton community

Fig. 18. Marine plankton in the food web. Schematic illustration of food-web relationships within the plankton community and their role in the marine ecosystem. The organisms shown in the boxes are only a few representative examples from a much broader spectrum of plankton species; the dominant species differ among marine habitats.

The preceding descriptions illustrate the diversity of feeding relationships within the plankton community. Phytoplankton provide food for many heterotrophic plankton, but the food web is not a simple sequence from phytoplankton to progressively larger consumers. Heterotrophic nanoflagellates and ciliates consume bacteria and small phytoplankton, while heterotrophic dinoflagellates may attack prey of similar or even larger size. Microzooplankton are themselves an important food source for copepods, krill and newly hatched fish larvae. Copepods may be herbivorous, omnivorous, carnivorous or detritivorous, whereas amphipods, chaetognaths, jellyfish and ctenophores prey mainly on other zooplankton. Many planktonic organisms therefore feed at more than one trophic level, and organisms that consume smaller plankton are often themselves prey for larger animals. Some protists combine photosynthesis with prey ingestion, while several bacteria, archaea and protists combine phototrophy with the uptake of dissolved organic compounds.

Part of the organic matter produced by phytoplankton enters the food web through the microbial loop. Dissolved organic matter is released through phytoplankton excretion and cell leakage or lysis, and through incomplete ingestion or “sloppy feeding” by zooplankton. Bacteria use this material as a source of energy and nutrients. They are grazed by heterotrophic nanoflagellates, which in turn are consumed by larger heterotrophic or mixotrophic protists such as ciliates and dinoflagellates. In this way, part of the dissolved organic matter is returned to the wider plankton food web. See Microbial loop for further details. Major food web relationships are schematically illustrated in Fig. 18.

Sampling the plankton

Larger plankton have traditionally been sampled with nets whose mesh size and design depend on the target organisms. Smaller plankton are generally sampled by collecting seawater, whereas fragile gelatinous organisms require specialized collection or imaging methods. Silk, such as the ‘bolting cloths’ originally used by millers to sieve flour, was used for net manufacture prior to the advent of nylon meshes. Plankton nets are typically conical in shape. They are towed or hauled through the water, funneling organisms towards the ‘cod end’ where they remain trapped.

Fig. 19. The map above shows the full network of routes that have been towed over the period 1930-2015, each with two letter route ID. Image reproduced with permission from SAHFOS. The Marine Biological Association has operated the CPR Survey since 2018.

Phytoplankton

Studies of the vertical distribution of phytoplankton species typically collect seawater samples with Niskin bottles. The constituent phytoplankton cells are subsequently preserved with Lugol’s iodine for later examination with a microscope. The chlorophyll content of seawater is frequently used as an indicator of phytoplankton abundance and biomass. Broad-scale (km) surveys of chlorophyll can be achieved by towing a fluorimeter through the water. Satellite measurements of ocean color provide estimates of surface chlorophyll and other optically active constituents over broad spatial scales. These observations enable global monitoring of phytoplankton distributions and seasonal patterns, although interpretation is more difficult in optically complex coastal waters.

Zooplankton

Modern biological oceanographers have a suite of nets and sampling arrays to help them collect and count zooplankton from discrete depths of the ocean. Broad-scale surveys can be undertaken by towing an optical plankton counter (OPC) behind a research vessel. Zooplankton are funneled into a narrow corridor and through a beam of light. An optical plankton counter records particles passing through the light beam and estimates their abundance and size from the resulting optical signals. Some zooplankton, especially dense aggregations and organisms with a strong acoustic contrast relative to seawater, can be surveyed acoustically. This type of survey uses techniques identical to those used by fishermen to locate fish. See Sampling tools for the marine environment for an overview of other sampling equipment.

The development of the Continuous Plankton Recorder (CPR), by Sir Alistair Hardy and colleagues in the 1920s and 1930s, represents one of the major milestones in plankton biogeography. Each CPR is a self-contained sampling device that collects animals onto an array of ‘silks’. They are designed to be deployed by merchant ships crossing the oceans (Fig. 19). This enables a far greater number of observations than would be achievable using dedicated research vessels alone. The CPR survey, which now spans more than 90 years, has revolutionized our understanding of the distribution and seasonality of plankton.

Modern developments

Automated imaging, imaging flow cytometry, in situ optical and acoustic instruments, autonomous platforms and satellite observations increasingly complement water samples and plankton nets. Traditional methods of biomonitoring are currently supplemented with techniques based on analyzing fragments of DNA (metagenomics and metabarcoding), RNA (metatranscriptomics), protein (metaproteomics), and metabolites (meta- and exometabolomics) in water and sediment samples. With these techniques a much wider spectrum of species can be detected than with traditional methods. However, the data cannot be easily interpreted in terms of species abundances and therefore need to be combined with more traditional sampling techniques. Further details can be found in the article Marine biomonitoring with environmental DNA .


Related articles

Marine microorganisms
Plankton bloom
Plankton remote sensing
Plankton remote sensing North Sea
Differentiation of major algal groups by optical absorption signatures
Remote sensing of zooplankton
Light fields and optics in coastal waters
The Continuous Plankton Recorder (CPR)
In situ monitoring of eutrophication
Sampling tools for the marine environment
Marine biomonitoring with environmental DNA
Harmful algal bloom
Chemical ecology
Functional metabolites in phytoplankton


References

  1. ↑ Barnes, R. and Hughes, R. 1999. An introduction to marine ecology. Wiley & Sons, Oxford
  2. ↑ 2.0 2.1 Sieburth, J. Mn., Smetacek, V. and Lenz, J. 1978. Pelagic ecosystem structure: Heterotrophic compartments of the plankton and their relationship to plankton size fractions. Limnology and Oceanography 23: 1256-1263
  3. ↑ Reynolds, R. A. and Stramski, D. 2021. Variability in oceanic particle size distributions and estimation of size class contributions using a non-parametric approach. Journal of Geophysical Research: Oceans 126, e2021JC017946
  4. ↑ 4.0 4.1 Kämpf, J., Newman, M., Doubell, M., Möller, L., Baring, R., Shute, A. and Redondo Rodriguez, A. 2021. A study of the seasonal and interannual variability of phytoplankton and zooplankton assemblages in a significant marine ecosystem. Oceanologia 65: 434—451
  5. ↑ Biller, S.J., Berube, P.M., Lindell, D. and Chisholm, S.W. 2015. Prochlorococcus: the structure and function of collective diversity. Nature reviews Microbiology 13: 13–27
  6. ↑ Flombaum, P., Gallegos, J.L., Gordillo, R.A., Rincón, J., Zabala, L.L., Jiao, N., Karl, D.M., Li, W.K.W., Lomas, M.W., Veneziano, D., Vera, C.S., Vrugt, J.A. and Martiny, A.C. 2013. Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus. PNAS 110: 9824-9829
  7. ↑ Partensky, F., Hess, W. R. and Vaulot, D. 1999. Prochlorococcus, a marine photosynthetic prokaryote of global significance. Microbiol. Mol. Biol. Rev. 63: 106–127
  8. ↑ Stonik, V. and Stonik, I. 2015. Low-Molecular-Weight Metabolites from Diatoms: Structures, Biological Roles and Biosynthesis. Mar. Drugs 13: 3672-3709
  9. ↑ Montresor, M., Di Prisco, C., Sarno, D., Margiotta, F. and Zingone, A. 2013. Diversity and germination patterns of diatom resting stages at a coastal Mediterranean site. Marine Ecology Progress Series 484: 79–95
  10. ↑ McQuoid, M.R. and Hobson, L.A. 1996. Diatom resting stages. Journal of Phycology 32: 889–902
  11. ↑ Stoecker, D.K., Hansen, P.J., Caron, D.A. and Mitra, A. 2017. Mixotrophy in the Marine Plankton. Annu. Rev. Mar. Sci. 2017. 9:311–35
  12. ↑ Smith, W.O. and Trimborn, S. 2024. Phaeocystis: A Global Enigma. Annual Review Marine Science 16: 417-441
  13. ↑ Stoecker, D. and Pierson, J. 2019. Predation on protozoa: its importance to zooplankton revisited. J. Plankton Res. 41: 367–373
  14. ↑ Saiz, E. and Calbet, A. 2011. Copepod feeding in the ocean: scaling patterns, composition of their diet and the bias of estimates due to microzooplankton grazing during incubations. Hydrobiologia 666: 181–196
  15. ↑ Hansen, P.J. and Calado A. J. 1999 Phagotrophic mechanisms and prey selection in free-living dinoflagellates. J. Eukaryot. Microbiol. 46: 382-389
  16. ↑ Sir Alistair Hardy, 1959. The open sea – its natural history: Part I The world of plankton, 2nd Edition. Readers Union, Collins, London


The main author of this article is Mayor, Daniel
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Citation: Mayor, Daniel (2026): Marine Plankton. Available from http://www.coastalwiki.org/wiki/Marine_Plankton [accessed on 30-07-2026]



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