Zooplankton

The term zooplankton is derived from the Greek words zoo and plagktos, which mean “animal” and “wandering,” respectively. Zooplankton are aquatic animals that possess little or no swimming capabilities and are found in marine and freshwater ecosystems. They are generally unable to swim against the flow and thus drift with the water current.

Zooplankton usually implies the complete community of such animals found drifting within water bodies such as lakes, rivers, and oceans. The great diversity of organisms comprising zooplankton includes species sensitive to changes in the biological, chemical, and physical aspects of the environment, as well as others less sensitive to environmental factors. This variable sensitivity to environmental factors makes it possible to use zooplankton as indicators of environmental quality and climate change.

Most zooplankton are microscopic in size and include representatives of the phylum Rotifera— the “wheel-bearing” or ciliated animals, such as Keratella quadrata, as well as many crustaceans, including the strikingly iridescent epipelagic copepods belonging to the genera Sapphirina and Copilia. Also included are single-celled protozoans, some annelids, and certain much larger coelenterates, such as the jellyfish Aurelia aurita. Crustaceans in the Order Cladocera, such as waterfleas in the genus Daphnia, are often the most abundant zooplankton in freshwater, while genera in the Class Copepoda, such as Calamus, Diaptomus, and Cyclops, are abundant in marine waters. In coastal waters, insects can be a significant component of the plankton community.

Links Between Producers and Consumers

In marine and freshwater food webs, zooplankton are one of the most significant links between the producers and higher consumers. Most zooplankton are primary consumers—herbivores that graze on plants—although zooplankton may also be carnivorous, feeding on other zooplankton species. Some may be parasitic for part of their life or detrivores feeding on organic matter suspended in the water column. A water-filtering mechanism helps zooplankton extract and feed on the chlorophyll-containing aquatic organisms known as phytoplankton. Consisting mostly of unicellular or colonial algae, the photosynthesizing phytoplankton are the foundation of the aquatic food web. Phytoplankton abundance increases seasonally in response to greater sunlight availability and is followed shortly by an increase in zooplankton abundance. This cycle continues year after year, and the productivity of zooplankton is thus directly dependent on phytoplankton productivity. Zooplankton are an important dietary component of secondary consumers such as planktivorous fish larvae, herring, and salmon, which in turn are consumed by tertiary consumers and top predators in the food chain such as harbor seals, sharks, and killer whales.

Compilation of biogeographical, ecological, taxonomic, and genetic information on the world’s zooplankton is a huge task being conducted at universities and institutes around the world. Protocols have been established for sampling zooplankton found at the water surface and at different depths in the water. In addition to traditionally used nets and trawls, advanced technologies developed for submersible vehicles, remote sensing, optical sensor systems, and molecular genetics are critical to understanding the global patterns of zooplankton distribution. Zooplankton production varies with available solar energy, nutrients such as nitrogen and phosphorous, water temperature, and various other physical and biological characteristics of the aquatic environment. Since the majority of these characteristics vary with latitude and altitude, both are important determinants of plankton production and biomass. The tropical zone demonstrates low productivity in general as a result of the relatively rapid uptake of nutrients by phytoplankton,while sunlight is the main limiting factor in the otherwise nutrient-rich Arctic region.

Climate warming is fragmenting and depleting the sea ice cover, which functions as a heat insulator and unique habitat for a range of organisms, including zooplankton. Climate warming has also been associated with changes in water circulation patterns between the Arctic, Pacific, and Atlantic oceans, with implications for both flora and fauna. Species of boreal plankton from the Pacific and Arctic oceans are known to have begun traveling into the North Atlantic, resulting in changes in species assemblages. Studies have linked the surface warming of southern California’s Pacific waters by 2.7 degrees F (1.5 degrees C) at some places since the 1950s to an 80 percent decline in the biomass of the 20–200 mm sized macrozooplankton during the same period. Carbon dioxide, a greenhouse gas that contributes to climate warming, is absorbed at the ocean surface, and the carbon component is taken up by phytoplankton. Zooplankton contribute to the ocean’s greenhouse gas buffering ability by grazing on carbonfed algae and then descending and releasing the carbon in deeper ocean waters.