A few common plankton species. 1 and 2. copepod species,

3. planktonic amphipod (hyperidae), 4. arrow worm, 5. and 6.

cladocerans, 7. barnacle larvae, 8. ophiuroid larvae,

9. capelin larvae, 10. cod larvae

Drawing: Kristín Líf Valtýsdóttir


Lions mane jellyfish (Cyanea capillata).

Photo: Erlendur Bogason


Average zooplankton biomass in spring (g dry weight m-2) at 0-50 m depth on the Siglunes section

Source: The Marine Research Institute

The zooplankton is the second step in the traditional food chain of the ocean. That is, they eat the phytoplankton and are themselves in turn eaten by small fishes. This is of course a generalization as many zooplankton species eat other zooplankton, or even fishes.

Most of the zooplankton is made up of very small, almost microscopic species. The smallest are single celled protists that feed mostly on bacteria. On the other hand, some zooplankton species can grow quite large such as the jellyfish.

Zooplankton groups

The zooplankton consists of many other groups of animals. Well known groups, such as crustaceans, snails, and annelids (segmented worms) contain species that are planktonic all their life. Other phyla are less well known, but nevertheless an important segment of the zooplankton. These include salps, comb jellies (or ctenophores) and arrow worms (or chaetognatha). The two latter groups are important predators of other zooplankton, including fish larvae.

The most important and best studied zooplankton groups in Icelandic waters are copepods and euphausids (or krill), both crustaceans. Only 86 species of euphausids are known in the world, thereof only 4 are very common in Icelandic waters. However, the scarcity of species is only half the story, as some of these few species can be super-abundant. Euphausids are omnivorous (that is eat both phytoplankton and zooplankton) and are themselves very important food for many fish species and baleen whales. Euphausids look a bit like shrimps but are smaller (they are often confused with shrimp juveniles), usually 15-35 mm fully grown.

The copepods are smaller, the larger species usually the size and shape of a rice grain. There are many species around Iceland, or about 100. Usually the smaller ones (such as species of the genus Temora and Acartia) are associated with coastal areas, while the largest species (such as Calannus hyperboreus and C. glacialis) live in cold Arctic offshore waters. The most common species around Iceland is C. finmarchicus. This single species is commonly 60-80% of the spring zooplankton bloom. During the summer time C. finmarchicus is found near the surface, but hibernates in deeper offshore waters during winter. Copepods, especially C. finmarchicus, are the main food of herring and capelin, as well as of juvenile stages of numerous other fish species (including cod).

Zooplankton abundance

Surveys have been conducted annually since 1961 to estimate the zooplankton biomass around Iceland. Large variation occurs between years, but the general trend is that the highest abundance was recorded before 1965. After that a marked cooling trend began and lasted for several years. Although the biomass has oscillated, it was generally low until 1990 when it tended to increase again. However, it has still not reached the same levels as that observed in the early 1960´s.

References and further information

Hreiðar Þór Valtýsson, University of Akureyri