Zooplankton and krill

BOLTOVSKOY DEMETRIO; BOLTOVSKOY ANDRES

Encyclopedia of the Antarctic

Routledge, Taylor and Francis

Lugar: New York; Año: 2007; p. 1105 - 1108

Zooplankton and Krill Zooplankton are marine or freshwater animals that live in suspension in the water column, from the surface to abyssal oceanic depths (>10,000 m). The swimming capabilties of these organisms are not strong enough to preclude their horizontal distribution from being governed by currents, but most are capable of maintaining a preferred depth range, and many can migrate vertically tens to hundreds of meters daily. Zooplankton includes a diverse range of organisms, from many single-celled organisms, often with elaborate calcareous or siliceous skeletons (Foraminifera, Radiolaria), gelatinous predatory (Hydromedusae, Siphonophorae, Scyphozoa, Ctenophora, Polychaeta, Chaetognatha) and particle-filtering forms (Pteropoda, Appendicularia, Salpidae), many crustaceans (Cladocera, Ostracoda, Copepoda), fish larvae, etc. The term “plankton”, which encompasses plants (phytoplankton) and animals (zooplankton), is derived from the Greek “planktos”, meaning wandering or drifting. Its first use is ascribed to the German biologist Viktor Hensen in 1887. Zooplankton can be classified according to size, life cycle traits, morphology, feeding mode, etc. Classifications based on size, most useful for methodological purposes (most plankton sampling gears are size-selective), recognize 7 size categories, ranging from ultrananoplankton (below 2 µm; bacteria, viruses), to megaloplankton (>20 cm, many jellyfish). Holoplankters are those organisms that spend teir entire life cycle in the water column, as opposed to meroplankters, that are not planktonic at some stage in their lives. For example, most marine benthic (bottom dwelling) animals like starfish, clams, mussels and snails, worms, crabs have a planktonic stage represented by their gametes (sperm and ova) and larvae. Also the early stages of fishes and squids are part of the plankton until they grow large enough to become strong swimmers, at which time they are considered to belong to the nekton. In total, ca. 7,000 species of marine zooplanktonic animals have been described, distributed among approximately 30 groups. The most speciose holoplankters are the copepods, a group of small (around 1 mm) crustaceans for which over 2,000 free-living marine planktonic species are  known. A few groups have 300-600 species (other crustaceans like Mysidacea and Amphipoda, the Hydromedusae – a group of small jellyfish, and the single-celled Radiolaria and Tintinnina), with most others ranging around 50-100 described species worldwide. Although most zooplanktonic taxa have representatives south of the Polar Front, the numbers of Antarctic species are very variable between groups: some are almost absent from the area (like the single-celled Acantharia, or the Heteropoda - planktonic snails), while others are very well represented. The most conspicuous zooplanktonic organisms are here the crustaceans (Copepoda, with over 100 species, Euphausiacea, Amphipoda, Ostracoda), followed by Chaetognatha, Salpidae, Polychaeta, Phaeodaria and Appendicularia. However, on average the waters around Antarctica are relatively species-poor, hosting some 1,500 described holoplanktonic species (as compared with 5,000-6,000 for the tropics and subtropics). Probably the most important Antarctic zooplankter is Euphausia superba or krill – a shrimp-like crustacean of the order Euphausiacea (the term “krill” is also applied to any of the ca. 80 species of these planktonic crustaceans). Krill have an elongated head-trunk region (cephalothorax) where the food gathering, manipulating and grinding limbs are inserted, and a muscular, segmented tail (abdomen) provided with five pairs of paddle-like swimming legs. Adult individuals are approximately six centimetres in length and weigh over a gram. Their body is reddish-transparent, with large black eyes and bioluminescent organs (photophores) on the lower side, but dead and preserved specimens are opaque. Krill feed mainly on phytoplankton, but planktonic animals are common in their diet as well. The abundance of zooplankters in sea-water is roughly inversely proportional to their size. Thus, bacteria, at less than 1 µm in diameter, are present in densities around 108-109 l‑1; protist concentrations (ciliates, Foraminifera, Radiolaria), whose size ranges from 10-100 µm, are usually around 10 to 103 l‑1; medusae (Hydromedusae), whose body measures around 5-15 mm, seldom reach densities over 0.05 l‑1; etc. But as opposed to diversity, abundance of zooplankton is not associated with latitude. Richest areas in numbers of individuals are located along the continental shelves. Antarctic waters have generally moderate to low zooplanktonic abundances, except during the short austral summer when reproductive activity is concentrated. One notable exception is that of krill, which forms huge swarms up to several hudreds of meters across, with densities of over 20 animals per liter and up to 15-20 kg per m3. Stocks of krill around Antarctica, are estimated at 300 to 1,000 million metric tonnes, with a potentially sustainable harvest of 150 million metric tonnes a year  (more than that of all other oceanic species of fish and shellfish throughout the world combined). One of the most outstanding features of the Southern Ocean is its extreme seasonal ciclicity, whereby the scarcity of light in the ice-covered waters determines a strongly food-limited winter, which alternates with an ice-free season when phytoplankton growth is active in the water column (see also Seasonality). The life cycle of most Antarctic zooplankters is finely tuned to these changes. Most herbivorous copepods (e.g., Calanus propinquus, Calanoides acutus, and many others) feed actively on the early spring (October) phytoplankton bloom and shortly thereafter reproduce in the surface waters. Newborns graze vigorously in these upper phytoplankton-rich layers throughout the summer, and as they grow they sink in the water column. By May, when the pack ice starts waxing and food becomes scarce, they have reached an advanced larval stage and are located in deeper waters (250 and up to over 1000 m, depending on the species). Here they spend the winter in a dormant state, rising back to the surface in spring, where they feed, mature and reprooduce. Other organisms, while also finely tuned to Antarctic seasonality, have a different reproductive strategy. Euphausia superba (krill) spawns in the upper layers (each female may lay up to 10,000 eggs at a time) and the fertilized eggs slowly sink; at 2000-3000 m the newborn hatch and start swimming upwards. They reach the surface in an early larval stage and start grazing. During their fist year krill complete their 9-stage larval development, during the second year they graze and accumulate reserves, and by the third year of life they mature sexually and reproduce. During the summer E. superba is an active filterer of the microplankton suspended in the water column, but during the winter, when food particles in the water are very scarce, it scrapes the undersurface of the ice pack harvesting algae and other microorganisms. Zooplanktonic organisms are key elements in marine food webs. They are active consumers of phytoplankton, and are in turn fed upon by other zooplankton, cephalopods, fish, and marine birds and mammals. In some areas, like the Southern Ocean, their importance is enhanced by the fact that the single-celled sarcodines and ciliates feed upon the abundant nanoplanktonic and ultrananoplanktonic particles (bacteria, autotrophic flagellates), and are in turn consumed by larger zooplankters, like copepods and euphausiids (see figure). The size of these bacteria and flagellates is too small for the larger zooplankters to efficiently feed on them; thus, the protists convert the organic matter into a size more easily available to higher levels of the food web. On the other hand, the grazing and predatory activity of some zooplankters can be so intense that they strongly affect the abundance and biomass of other plankton, including fish larvae, thus reducing fish populations. In the Southern Ocean, krill is a key component of food webs as it constitutes the major direct or indirect source of food of most of the larger Antarctic animals, such as fish and squid, seabirds, seals and  whales (see figure). The abundance of krill has called the attention of scientists and fishermen since its discovery, and in the early 1970s commercial fishing was started in Southern Ocean waters. Krill landings increased swiftly from 59 tonnes in 1973 to ca. 50,000 tonnes in 1982-1986 (with a drop to ca. 200,000 tonnes in 1983–85, probably partly due to the Argentina-United Kingdom war over the Falkland Islands). However, between 1987 and 1993 catches shrank to around 100,000 tonnes remaining at those levels until the present (in the 2002/2003 season 116,866 tonnes of krill were caught by a total of five countries: Japan, South Korea, Ukraine, the United States and Poland, with the first three accounting for over 80% of the catch). The main reason for this decline is the remoteness of the area. Profitabiity is aso reduced by the fact that krill has powerful enzimes which break down the krill’s tissues soon after death, spoiling the catch, and the high concentrations of fluoride in the shell (which is toxic and has to be removed before the meat is fit for consumption). In addition, market demand for krill has decreased in the last decade in association with the strong worldwide increase in the production of farmed shrimp. In 1991 the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR) has set a precautionary catch limit of about 10% of the biomass, but since 1993 catches have been far below these threshhold values. Krill biomass, however, has also decreased considerably over the past century. In the southwest Atlantic sector, which  contains >50% of Southern Ocean krill stocks, mean densities have declined significantly since the 1970s, probably as a response to waning winter ice-coverage. While krill has a circumpolar distribution, highest concentrations are found in the area around South Georgia and the Antarctic Peninsula, which is where most of the fishing activities take place. Krill are caught by large freezer trawlers and processed on board into products for human consumption, domestic animals (cattle, poultry, pigs), farmed fish and sport fishing bait. Krill are especially rich in vitamin A.