12.12: Plankton

The term plankton includes all marine organisms and viruses that do not swim or are very weak swimmers and that do not live on or attached to the seafloor (Fig. 12-15). Plankton generally do not settle to the seafloor, and they have very limited or no control of their horizontal movements, so they drift with the ocean currents. Phytoplankton are the phototrophic autotrophs that produce more than 99% of the food used by marine animals. Zooplankton are planktonic herbivores, carnivores, or omnivores.

Figure 12-15. A typical plankton sample (photographed at 3× magnification) contains many species of both phytoplankton and zooplankton. This image shows mostly zooplankton.

Plankton are often categorized by size. The largest, which are almost exclusively zooplankton, are the macroplankton (>2 mm). Most plankton are microplankton (20 µm–2 mm), nanoplankton (5–20 µm), or ultraplankton (2–5 µm), and these size ranges are dominated by phytoplankton. The smallest plankton are picoplankton (0.2–2 µm), thought to be predominantly bacteria and archaea, and femtoplankton (<0.2 µm), thought to be primarily viruses. Because even the finest mesh nets used by biologists are too coarse to collect the smaller species, they have not been well studied. Less is known about nanoplankton than about microplankton and macroplankton, and very little is known about the even smaller plankton or their ecological importance. For this reason, in the following sections we discuss what is known about the microbial part of the food web that consists of bacteria, archaea, and viruses in the nanoplankton size range or smaller and then describe those groups of phytoplankton and zooplankton that are better studied. Most species among these better studied plankton groups are larger than nanoplankton and fall into the macroplankton or microplankton size range. However, it is important to remember that the distinction we make between the very small, less well understood plankton and the larger better-known plankton is a necessity driven by the relative lack of knowledge of the very small plankton. There are autotrophic and heterotrophic plankton in all size ranges and they act together in an ocean ecosystem that is dominated in most areas by microbial communities that are nanoplankton or smaller. As our understanding of these microbes improves (aided by modern techniques such as flow cytometry and genomic sequencing), the long-standing but outdated view of a large-phytoplankton food chain operating independently of a microbial food chain is being replaced by an integrated model of the microbial loop linked with the better known food web of larger species.

The better-known large marine plants and macroalgae, including kelp, seaweeds, and sea grasses, constitute only a tiny fraction of the photosynthetic species in the oceans and so are not described in this text. Macroalgae normally grow attached to the seafloor and are found on the beach only when they have been broken loose by storm waves or animals.

Bacteria and Archaea

The majority of plankton biomass is now known to be outnumbered by plankton species so tiny that they escaped detection until the 1990s. These poorly studied, microbial plankton are dominated by species of bacteria and archaea. Viruses are also included in this group although viruses are not considered by many to be living organisms. Extremely sophisticated techniques are needed to isolate and identify bacteria and especially archaea and viruses, but they exist in vast numbers in all ocean water. As stated above, concentrations of bacteria in ocean water average around 1 million per milliliter. Archaea and virus concentrations of millions per milliliter are also believed to be not unusual.

One group of photosynthetic bacteria (also known as cyanobacteria), Prochlorococcus, absorbs blue light efficiently at low light intensities, so it can grow throughout the depth of the photic zone. Prochlorococcus may be the most abundant component of the phytoplankton, especially in the tropical and subtropical oceans, and it is estimated to contribute 30% to 80% of the total photosynthesis in areas of the oceans where nutrients are scarce and in HNLC areas. Until recently it was thought that archaea existed primarily in extreme environments, but it is now known that archaeal species are found throughout the ocean environment and that some are phototrophic primary producers 

We are only just beginning to study the microbial populations of the oceans and the role that they play in life on our planet. However, what we do know is that these microbes are responsible for regenerating dissolved nutrients and metals previously removed from solution by living organisms. Thus, without microbes, the ocean productivity and the entire ocean food web would quickly stop entirely.

Although there are millions of microbes in each milliliter of seawater, which might suggest that ocean water is a thick biological soup, in reality microbes are so small that their environment is 99.99999% water or dissolved ions. As a result, from the microbe’s perspective life consists of being separated from its neighbors by what to it are long distances (hundreds of body lengths), and of waiting around for an occasional encounter with an organic particle or an occasional plume of nutrient from the waste liquids of larger organisms. In response to this, microbial bacteria and archaea have developed the ability to slow their metabolism, sometimes even essentially stop their metabolism and form a resting phase until they encounter food or the chemical energy source they need to grow and divide. Microbes have also developed abilities to rapidly adapt to changing opportunities and environments. They do so by transferring genes among themselves. Although this particular mechanism has not been fully demonstrated yet, the concept can be illustrated by the example that a species adapted to live in deep water might be able to pick up the gene for photosynthesis if it is transported into the upper layers where there is light. Interestingly much of this gene transfer may take place through virus-like gene-containing fragments of the host species chromosome. We do not know what all of the roles are that viruses play in ocean ecosystems. Mounting evidence suggests that they cause diseases in marine organisms spanning the entire range from the smallest bacteria to the largest whales but likely they also have a major role in gene transfer and adaptation in bacteria and archaea and even in animals and plants and they are now thought to have been the ultimate ancestors of all other life.

The discovery of the abundance and central role of bacteria and archaea, together with the discovery that viruses are even more abundant throughout the oceans and perform essential roles in the microbial effects on biogeochemical cycles and likely also in evolutionary processes, has revolutionized ocean sciences. Extensive research efforts are currently directed toward gathering a better understanding of these organisms and viruses and details of their role in ocean ecosystems. A billion or more years ago, the microbial food web in the oceans became a self-sustaining community of organisms, likely even before photosynthesis developed. The non-microbial world, including all eukaryotes, are late comers on the planet and are completely dependent on the microbes.

Phytoplankton

There are only three known planktonic species of macroalgae, all of the genus Sargassum. Sargassum grows in dense rafts, often many square kilometers in area, floating at the surface of the Sargasso Sea (Chap. 15). 

Phytoplankton are generally much smaller than 1 mm in diameter—mostly too small to be seen clearly by the naked eye. The oceans contain an abundance of phytoplankton dispersed throughout the surface waters at concentrations that may exceed 1 billion individuals per liter. There are tens of thousands of species of phytoplankton. A sampled phytoplankton assemblage always consists of many species, but in any individual sample one species is often dominant and far outnumbers all others (as pine trees do in a pine forest).

Phytoplankton communities consist of different species in different climatic regions, and concentrations range from very low in some areas (equivalent to deserts) to very high in others (equivalent to rain forests). Unlike most land plant communities, phytoplankton communities can vary dramatically in composition and concentration within hours or days. Such variability occurs in part because phytoplankton are often concentrated in patches (tens to hundreds of meters across) that drift with ocean currents. Patches of phytoplankton can develop when rapid reproduction exceeds the rate of dispersal by mixing processes. Because phytoplankton can double their population within a day, or in even less time under favorable conditions, patches may develop quickly in calm seas. Phytoplankton are also concentrated by Langmuir circulation (Chap. 8). Smaller or less dense cells tend to concentrate at the surface convergence between Langmuir cells. Larger or higher-density cells, which tend to sink, may be concentrated below the surface where subsurface Langmuir cell currents converge (beneath surface divergences). Phytoplankton can also be concentrated within mesoscale eddies.

Individual phytoplankton species respond favorably to slightly different light intensity levels, temperatures, and nutrient concentrations. Under favorable conditions, one or more species can reproduce rapidly and become the dominant species. If conditions change and another phytoplankton species prospers in the changed conditions, the second species can become dominant within a few days because grazing zooplankton can rapidly remove the previously dominant species.

Diatoms

Diatoms (Fig. 12-16a) are among the most abundant types of phytoplankton in many of the more productive areas of the oceans. They are relatively large single cells (up to about 1 mm in diameter) with a hard, organically coated external siliceous casing called a “frustule,” which is made of two halves much like a pillbox. The frustule is porous, allowing dissolved substances to diffuse through it and to be taken into or excreted from the cell. Because silica is denser than seawater, most diatoms contain a tiny droplet of a lighter-than-water natural oil to reduce their density and thus their sinking rate. Many diatoms have protruding, threadlike appendages and may link to form chains (Fig. 12-16a). These adaptations also reduce the cell’s tendency to sink and may decrease predation by small zooplankton. The threadlike appendages also increase the surface area over which nutrients can be taken up and light collected for photosynthesis.

   

   

Figure 12-16. There are many varieties and species of phytoplankton (a) Various species of diatoms - approximately 20x magnification. (b) Bioluminescent dinoflagellates (Noctiluca scintillans, each individual is about 200-2000 micrometers across. (c) A bloom of coccolithophores seen from space. Since each individual coccolithophore is only about 40 micrometers across (about 100 times too small to see with the naked eye) there must be an almost unimaginable number of individuals that make up this bloom.

Their oil droplet and relatively large size make diatoms a favored food source for many species of juvenile fishes and zooplankton. Consequently, food chains based on diatoms are generally shorter than those based on smaller classes of phytoplankton. Smaller phytoplankton must usually be eaten by small zooplankton before they can provide food for juvenile fishes and larger herbivorous and omnivorous zooplankton.

Diatoms reproduce asexually by cell division. At each division, which may follow the previous one by less than a day, the frustule separates, with one-half taken by each of the two new cells, and each cell then manufactures a new half. Consequently, one of the new cells is smaller than its parent (Fig. 12-17). After several such divisions, the now much smaller cells may reproduce sexually, restoring the cell size to its maximum.

Figure 12-17. Most diatoms reproduce primarily by cell division. At each division, the two halves of the diatom frustule separate, and each half secretes a new half to its frustule. However, because one of the new halves is smaller than its parents, the mean size of the diatom is reduced with progressive divisions. Eventually, a small diatom reproduces sexually by producing an auxospore that at first has no frustule. After some period of growth, the auxospore secretes a new frustule.

Dinoflagellates

Dinoflagellates range widely in size, but many are nanoplankton, smaller than most species of diatoms (Fig. 12-16b). Most species of dinoflagellates have two hairlike projections, called “flagella,” that they use in whiplike motions to provide a limited propulsion ability. Some dinoflagellate species use this propulsion to migrate vertically. This allows them to stay at a depth where light levels are best for photosynthesis during the day, or move deeper to reach higher concentrations of nutrients that may be unavailable at the surface. Because of this "best of both worlds" strategy, certain species tend to concentrate at a specific depth within the photic zone.

Many dinoflagellate species have an armored external cell wall made of cellulose, but others are “naked.” Because cellulose is decomposed relatively easily by bacteria and other decomposers, dinoflagellates do not contribute significantly to deep-ocean bottom sediments. Dinoflagellates are not always autotrophs. Some species are mixotrophic and are able to use dissolved or particulate organic matter as food, and some are predators. Indeed, many species can live and grow both autotrophically and heterotrophically.

Dinoflagellates are more abundant than diatoms in the open oceans far from land because the silica needed to construct diatom frustules is in short supply. However, phytoplankton biomass is substantially lower in most such open-ocean areas (Fig. 12-18a). Silica can also be scarce in coastal waters at certain times of year. These temporary silica-deficient conditions can lead to explosive blooms of dinoflagellates if other nutrient, temperature, and light conditions permit (Chap. 13).

Figure 12-18. The composition of the phytoplankton community is different in coastal, offshore, and open ocean waters. (a) Phytoplankton are much more abundant (note the logarithmic scale) in the surface layer waters of the continental shelf than they are in the Sargasso Sea, where nutrients are depleted. (b) In many areas, the relative proportion of diatoms generally decreases with distance offshore, reflecting the lower availability of dissolved silicate in the offshore waters. These data were obtained from the Caribbean Sea.

Coccolithophores and Other Types of Phytoplankton

Coccolithophores (Fig. 12-16c) are generally nanoplankton, and they are smaller and less abundant than diatoms and most dinoflagellates. Coccolithophores are single-celled flagellates whose external cell surface is covered by a mosaic of tiny calcareous plates (Fig. 12-16c). In certain areas, these plates are a major component of seafloor sediments (Chap. 6). Coccolithophores make up a greater fraction of the phytoplankton biomass in relatively nonproductive temperate and tropical open ocean waters than they do in coastal waters.

In most of the oceans, phytoplankton consist primarily of diatoms, dinoflagellates, and coccolithophores (Fig. 12-16). However, there are several other types of phytoplankton, including silicoflagellates (which have an intricate silica shell), cryptomonads, chrysomonads, green algae, and cyanobacteria (bluegreen algae).

Zooplankton

Zooplankton are animals that do not swim strongly enough to overcome currents and so drift with the ocean water. As is the case for phytoplankton, and for similar reasons, the distribution of zooplankton is patchy. Dense patches of zooplankton attract fishes and other predators. In contrast, nekton are marine animals that swim strongly enough to move independently of ocean currents, are distributed nonuniformly because they are attracted to food sources, and they often exhibit schooling behavior, which is discussed in Chapter 14. The nonuniform distribution of both plankton and nekton, together with the temporal variability of their populations, makes it difficult for biologists to obtain precise population estimates, even when large numbers of samples are taken.

Zooplankton consist of a bewildering array of species from many different groups of organisms. Many zooplankton species tolerate only narrow ranges of environmental conditions, especially temperature. Consequently, zooplankton species composition changes from one water mass to another and with depth. At any one location, many species are represented, including species that are bacteriovores (bacteria eaters), herbivores, carnivores, and omnivores.

Zooplankton belong to one of two categories based on the life history of the species. Species that live their entire life cycles as plankton are known as holoplankton. The larvae (juvenile stages) of species that later become free swimmers or benthic species are meroplankton. Meroplankton include many species of fishes, sea stars, crabs, oysters, clams, barnacles, and other invertebrates. Holoplankton are the dominant zooplankton in surface ocean waters, whereas meroplankton are more numerous in continental shelf and coastal waters. In tropical coastal waters, larvae of benthic species make up as much as 80% of all zooplankton.

Many zooplankton species tend to concentrate at the same depth and collectively migrate between the photic zone and aphotic zone each day, but the depth to which this diel vertical migration takes place differs depending on the species. Zooplankton also tend to collect at density interfaces between water layers because these interfaces inhibit (but do not prevent) vertical migration and sinking and thus collect food particles. When zooplankton are present in large numbers within a thin layer below the surface, they scatter or reflect sound and are observed by echo sounders as a “deep scattering layer.” This layer changes depth during the day as the zooplankton make their daily migration between the photic and aphotic zones.

There are too many important species of zooplankton to describe in this text, but the characteristics of the major groups of holoplankton and meroplankton are described briefly in the sections that follow.

Holoplankton

The most abundant holoplankton are copepods (Fig. 12-19a), euphausiids (Fig. 12-19b), and amphipods (Fig. 12-19c), which constitute 60% to 70% of all zooplankton in most locations. Copepods and euphausiids are both crustaceans, a class of invertebrates that includes crabs and lobsters. In the open oceans, most copepod species are herbivorous, whereas many coastal forms are omnivores. Copepods eat about half their body weight in phytoplankton or other food each day. They are abundant throughout the oceans and can double their population within a few weeks. Euphausiids are generally larger and reproduce more slowly than copepods. Euphausiid population doubling times are typically several months. Many euphausiids are omnivorous, eating smaller zooplankton, as well as their major food, phytoplankton.

   

   

   

Figure 12-19. Typical holoplankton. (a) Copepods such as this Scaphocalanus acrocephalus are extremely abundant in some regions of high primary productivity. (b) Euphausiids. (c) Amphipods (possibly Ampeliscidae) are a major food source for fishes (d) Foraminifera Orbulina universa. Only the spherical inner shell will remain in sediments. The central spherical chamber is about 0.5 mm diameter and the many bright white spots are symbiotic dinoflagellates. (e) Radiolarian (class probably Acantharea), northeast Pacific Ocean, approx 160x magnification, cell body approx 50 µm across. 

Euphausiids called “krill” are especially abundant in waters around Antarctica, and they constitute the principal food source of the abundant marine animals there. Baleen whales, including the blue, humpback, sei, and finback, feed directly on krill. These baleen whales gulp large volumes of water, then squeeze it out through net-like baleen plates suspended from the roofs of their mouths. Krill collect on the baleen and are removed by the tongue and ingested.

Two groups of single-celled amoeba-like microplankton have hard parts that become important components of sediment in some areas: foraminifera (Fig. 12-19d), with shells composed of calcium carbonate, and radiolaria (Fig. 12-19e), with shells composed of silica. Both are holoplankton and feed on diatoms, small protozoa, and bacteria, often capturing them on their many long, sticky, spikelike projections. Radiolaria and foraminifera are most abundant in warm waters. Individual species, especially of foraminifera, are very sensitive to small changes in water temperature and salinity. Because of the sensitivity to temperature, the species compositions of radiolaria and foraminifera in sediments are important indicators of past climates. Isotopic compositions of the shells of these organisms also provide a record of the temperature at the time they lived (Chap. 6).

The pteropods (Fig. 6-8b), another group of holoplankton, are also important in marine sediments. Pteropods are mollusks and are related to slugs and snails. In pteropods, the “foot” on which slugs or snails crawl is modified into a delicate transparent wing that undulates and propels the organism like a fin. This modified foot enables pteropods to migrate vertically hundreds of meters each day. Some pteropod species are carnivorous and do not have a shell. Others are herbivorous and have a calcareous shell that contributes to sediment, especially in tropical regions (Chap. 6), where they often occur in dense swarms.

Gelatinous Holoplankton

Many holoplankton differ from other holoplankton species because they have gelatinous bodies and are apparently not a major part of the food webs that lead to fishes and other marine animals exploited by humans. Although these species consume large amounts of other zooplankton, their gelatinous bodies provide little or no food for species at higher trophic levels.

The most familiar group of gelatinous holoplankton, the jellies (commonly but incorrectly called jellyfish or sea jellies but they are not fish and are not restricted to seawater), are cnidarians (phylum Cnidaria), a phylum which also include corals and anemones. All cnidarians have stinging cells, called “cnidocysts,” within which they have harpoon-like structures called “nematocysts” that can fire into their prey to inject toxins. In some species, the toxins are extremely strong and can paralyze or kill large fishes or even people.

Some jelly species (Fig. 12-20a-c) are very large in comparison with most other holoplankton, perhaps because their food value is low and they are the preferred food source for relatively few predators. Some, such as the moon jelly Aurelia (Fig. 12-20a) and Cyanea, are holoplankton. Others are meroplankton that spend part of their lives in the plankton, then settle to the seafloor, where they attach with their stinging tentacles extended upward. In the benthos they resemble their close relatives: anemones and corals.

   

   

   

   

Figure 12-20. Typical gelatinous holoplankton. (a) Jelly Aurelia sp. This one was in a unique marine lake called Jellyfish Lake in Palau. (b) This upside-down jelly (Cassiopeia andromeda, Papua New Guinea, about 10–15 cm across) generally lies on its “back” on the shallow seafloor to provide light to the symbiotic algae that live in its tentacles. (c) The Portuguese man-of-war (Physalia physalis) is a colonial jelly whose long tentacles bear stinging cells that can severely injure a swimmer. (d) Ctenophores, like this Mertensia ovum, propel themselves by pulsating the columns of bright hairlike cilia visible through the body. (e) Each individual of this colonial salp species, photographed after it had floated into shallow water and settled on the sand in Indonesia, may be 10 cm or more across. The colonies, which normally drift through open-ocean surface waters, may exceed several meters in length. 

Among the most unusual jellies are species, such as the Portuguese man-of-war (Fig. 12-20c), that appear to be a single organism but in fact are a colony. In colonial forms, many individuals of the same species form a cooperative group that appears to be a single organism. Each colony member has a specialized task, such as protecting the colony, gathering food, digesting food, or reproducing. In the Portuguese man-of-war, one colony member is filled with gas to provide flotation and a “sail” that can partially control the colony’s drift.

Widely occurring gelatinous plankton that are not jellies include ctenophores and salps. Ctenophores are transparent, bioluminescent organisms, some of which have long, trailing tentacles (Fig. 12-20d). Ctenophores propel themselves through the water by beating eight columns of hairlike cilia that are usually visible through the ctenophore’s body (Fig. 12-20d) and that give these species their common name, “comb jellies.” Small, rounded species of ctenophores are often called “sea walnuts” or “sea gooseberries.”

Salps (Fig. 12-20e) are the holoplankton species of tunicates, most of which are benthos. Tunicates are among the most advanced invertebrates. They are chordates (phylum Chordata), a phylum which also includes the vertebrates (fishes and mammals). This close relationship is difficult to envision from the simple form of the adult salps or other tunicates, but tunicate larvae closely resemble vertebrate larvae because they possess a primitive backbone called a notochord. Adult tunicates have a simple baglike form with two openings. Water is pumped into one opening (incurrent opening) and out the other (excurrent opening). Food particles are removed from the water by a mucous layer spread over the interior of the tunicate’s body.

Many salps are hollow and barrel-shaped, with incurrent and excurrent openings at opposite ends (Fig. 12-20e). They propel themselves slowly by pumping water through their bodies. Several salp species are bioluminescent. I witnessed a “magic moment” one moonless night on a research vessel sailing through an exceptionally dense patch of salps in the tropical Atlantic Ocean. For miles, the ship’s wake was a brilliant, sparkling light show, as a continuous stream of salps was disturbed by the wake and emitted pulses of light.

Meroplankton

Meroplankton are the eggs, larvae, and juveniles of species that spend their adult lives as benthos or nekton. Larvae or juvenile forms of the majority of benthic species, including clams, oysters, crabs, snails, lobsters, sea stars, sea urchins, corals, and sea cucumbers, spend their first few weeks of life as meroplankton. Many fish species also release eggs to the water column. These eggs, the larvae that hatch from them, and juvenile fishes that emerge from the larvae are meroplankton until the fishes become big enough to swim actively against currents as nekton. The release of eggs and larvae as meroplankton is an effective means of distributing species over wide areas (Chap. 14).

Many species of meroplankton larvae do not remotely resemble their adult forms (Fig. 12-21). In the past, numerous meroplankton have been named and classified as separate species, even though their adult forms were already well known. Modern scientific methods, particularly genetic studies, now enable us to match meroplankton with their adult forms.

   

   

Figure 12-21. Typical meroplankton. (a) A crustacean larvae. (b) A blue king crab larvae, zoeal stage 2 (Paralithodes platypus) (c) The first larval stage of a starfish. This stage is called a bipinnaria.