Plankton (marine mammals)

Plankton is the collective name given to the assemblage of free-swimming or suspended microscopic organisms considered too small to move independently of ocean currents. Large animals that are able to disperse under their own power are called nekton. The distinction between plankton and nekton, however, is sometimes blurred. For example, larger animals that are capable of limited self-propulsion, such as jellies and salps, are often included in the plankton. Large euphausiids, such as Antarctic krill, whose ability to actively determine and maintain their position is poorly understood, have been referred to as either macroplankton or micronekton. Phytoplank-ters are plants, and zooplankters are animals.

I. Phytoplankton

Phytoplankton consists of microscopic unicellular plants and forms the basis of marine ecosystems; nearly all life in the sea derives from the solar energy fixed in photosynthesis by these plants. Two factors control phytoplankton growth; light irradiance and nutrients. Light is only available in the top layers of the oceans, variably down to about 200 m or less, whereas nutrients are more abundant in deeper layers. Evolution of small size has enabled phytoplankton to absorb scarce nutrients through maximizing the ratio of surface area to volume. Small size, down to 2 (ini, also confers high buoyancy and a low sinking rate, keeping the cells near the surface. Grazers on the phytoplankton must also be ex-tremelv small in order to be able to feed on them. As well as being the “grass of the sea.” phytoplankton conies into cetologv in one of the common names of the blue whale (Balaenoptera mus-culus), “sulphurbottom.” which refers to a vellowish-brown layer of diatoms (single-celled epiparasitic algae) accumulating on the whale while it feeds in cold polar waters.

II. Zooplankton

Zooplankton consists of animals from several taxonomic groups from Protozoa to Vertebrata and is a main source of food for many marine mammals. Carnivorous, omnivorous, and herbivorous zooplankters have been found in the stomach of baleen whales; three groups of crustaceans are the most important: copepods, euphausiids, and amphipods. These and other planktonic animals have developed a wide variety of specialized mechanisms and techniques for feeding on phytoplankton and suspended particulate matter, including appendicular nets and guiding whorls in copepods, finely structured appendages in euphausiids, and ciliary movements in pteropods such as Liinacina and Cavolinia. “Basket feeding” in the Antarctic krill, Euphausia superba, is a kind of mass raptorial feeding. Nektonic animals have also developed filters by modifying gill rakers into functional sieves, e.g., as in the basking shark (Cetorhinus maximus) and whale shark (Rhincodon typus). Buccal teeth with well-developed accessory cusps also function as sieves in crabeater seals (Lobodon carcinophaga) and leopard seals (Hy-drurga leptonyx), which feed on krill. The most highly derived filtering system among the vertebrates is that of baleen in whales, which enables them to utilize varying sizes of minute particles suspended in the viscose water medium.

III. Plankton and Whales

Whales can feed efficiently on zooplankters because they occur in dense swarms. Their patchy distribution and aggregation may be partially passive and due to oceanographic features such as Langmuir circulation but may also involve active processes on their part. When Antarctic krill swarm densely at the surface, the water is discolored with brick-red patches due to their carotenoid pigmentation. A swarm is usually a species-specific phenomenon, as in birds or fishes. In a particular feeding ground in the subantarctic area, sei whales (Balaenoptera borealis) and southern right whales (Eubalaena australis) feed on copepods (Calanus tonsus) that swarm at a density of 21,000-23,000 individuals/m3, or 29-35 g/m3. The background density in the same region is fewer than 1/m3 (Kawamura, 1974). Based on acoustic data, Antarctic krill has been found to swarm at a density of some 60,000 individuals/m3 or more or >33 kg/m3 (Miller and Hampton, 1989). The baleen whales exploit these swarms, harvesting immense amounts of biomass in a process that could be compared to seining pelagic fish schools. The concentration of biomass by swarming zooplankton is incredible; it has been estimated that the energy represented by the phytoplankton in 5 million gallons of seawater is equal to that in only 1 pound of black beans (Fraser, 1969), yet baleen whales may efficiently collect some 4% of body weight daily by exploiting swarms of zooplankton that feed on the phytoplankton (Fig. 1).

The stomach contents of a whale usually consist of zooplankton of a single species, one of only a limited number of major species (Table I), depending on location and species of whale. This is due to the swarming habit, which does not occur in all planktonic crustaceans (Kawamura, 1980a). Sometime, however, two or more species may be found in a single stomach due to accidental engulfment or to feeding on swarms of different species. The dominant copepods in the North Pacific are Eucalanus bungii and Metridia paeifica and in the Southern Ocean Calanus propinquus, Calanoides acutus, and M. geriachei, but these are not fed on by whales because they do not swarm. In temperate and tropical waters, the Bryde’s whale (Balaenoptera edeni) feeds on the swarming euphausiids Pseudeuphausia latifrons, Thysa-noessa gregaria, Euphausia recurva, and E. diomedeae. No swarming copepods have been found in the Indian Ocean, despite high species diversity in that region. Balaenopterid and bal-aenid whales each show similar food habits in different oceans and seas but exhibit different food preferences (Bowen and Siniff, 1999); the former feed by “swallowing” (engulfing) large amounts of water, which allows them to also feed on squid and fish of suitable size as well as euphausiids, whereas balaenids feed largely by “skimming” for copepods (Mitchell, 1974). However, the bowhead whale (Balaena mysticetus) is also known to feed near the bottom on planktonic amphipods in the Arctic. Gray whales (Eschrichtiidae: Eschrichtius robustus) feed mainly on benthic amphipods in the Bering and Chukchi Seas but also consume planktonic crustaceans and fish eggs and larvae in the southern parts of their range (Darling et al, 1998).

Figure 1 Krill (Euphausia superba Dana) gushing out from a fin whale stomach in the Antarctic Ocean. Note uncontaminated composition with other prey items.

TABLE I

Zooplankton Species Representing Major Components of the Stomach Contents of Whalebone Whales

“Adapted from Kawamura (1974, 19S0a.b, 1982) and Gaston (1982). ^&New Zealand waters. ^<?Patagonian waters.

TABLE 11

A Rough Summary of the Number of Species Found in the Stomachs of Baleen Whales”

“The anomalously high number of copepod species in South African waters may reflect supplemental feeding en route to and from the northern breeding ground.

&Five pteropod species could be added.

rDoubtful because no scientific name is known, but “squid.”

IV. Plankton and Seals

Seals also feed on plankton, but to a lesser extent than baleen whales, with the degree depending on species, age, breeding condition, and season. Some Antarctic seals eat euphausiids. Two species (Euphausia superba and E. crystaUorophias) were found in 1 of 48 Weddell seals (Leptonychotes weddellii), 94 of 100 crabeater seals, 37 of 159 leopard seals, and 9 of 21 Ross seals (Ommatophoca rossii) (Laws, 1984). The high proportions for crabeater and leopard seals reflect the specialization of their dentition for straining. In addition, Ross and Weddell seals feed on benthic and near-bottom planktonic amphipods, isopods, and mysids. In leopard seals (Hydrurga leptonyx), the proportion of krill in the diet decreases with age; krill were found in the stomachs of 87% of juveniles (vs 23% overall). Larga seals (Phoca largha) and ice-breeding harbor seals (P. vitulina) in the Okhotsk Sea feed largely on fish, but pups of harbor seals, ribbon seals (Histriophoca fasciata), and ringed seals (Pusa hispida) feed on swarming euphausiids, such as Thysanoessa iner-mis and T. raschii, which appear in association with the pack ice of their habitat (Kato, 1982). This planktonic food is important after weaning until the seals become able to catch fish or squid.

V. An Efficient (but Vulnerable?) Ecology

The marine ecosystem is a world of size-dependent structures, based at the bottom on suspended organic particles of phytoplankton and zooplankton. Annual global marine primary production of about 2 X 1010 tons of carbon has been estimated to produce some 2.4 X 10s tons of fish after passing through up to five trophic levels (Ryther, 1969). The baleen whales shorten this chain, as do other top krill-eating predators in the Antarctic such as seals and penguins. Solar energy is transferred to large animal production with greater efficiency than in any other ecosystem. A 30-m blue whale can consume vast numbers of 4—5-cm krill. Similarly, a 0.3-cm copepod is available to a 13- to 16-m sei whale. The ratio of predator length to prey length is as great as 8300:1. These are the most extreme cases of size disparity between predator and prey in marine ecosystems (Berta and Sumich, 1999). Relatively few species of prey are involved (Table II), e.g., only 17-20 of the 85 or so known species of euphausiids and only 7-10 of the approximately 1800 known species of copepods (Mauchline, 1998). This may indicate ecological vulnerability of the baleen whales, as they have few options for substitution of food species. The distribution and abundance of the largest baleen whale stocks are/were determined by the distribution of their favored densely swarming planktonic prey (Nemoto and Kawamura, 1977). The most marked case is that of the highly stenophagous Southern Ocean blue whale, which feeds exclusively on E. superba (Clapham et al, 1999).