Disruption to the plankton cycle
Essential single-celled organisms
Plankton is a vital food source for life in the ocean. Phytoplankton, algae and cyanobacteria, take up nutrients dissolved in the water, grows, and undergoes cell division. Biomass is thus produced, on which zooplankton such as copepods feed. The zooplankton, in turn, is eaten by fish and their larvae. Plankton therefore plays a key role in the biogeochemical cycle of the ocean. Disruptions to plankton development caused by climate change will thus have a critical impact on the functioning of the entire pelagic system.
Copepods belong to the crustaceans. They are found in both saltwater and freshwater. Although most of
these animals are only a few 100 micrometres to a few millimetres in size, they are the most species-rich group of the crustaceans (around 14,000 species), and make up the largest share of marine zooplankton. Copepods therefore represent an important food source for fish and other pelagic animals.
Faltering plankton growth
Plankton predominantly comprises short-lived organisms. As a rule, these reproduce so rapidly that several generations may be produced within a single year. The development of planktonic organisms generally follows a regular annual cycle that begins with a spring bloom of the phytoplankton. At this time the increasing light availability promotes a rapid increase in the abundance of phytoplankton. Only a few weeks after the winter minimum the biomass reaches its annual peak value, following which it undergoes a continuous decrease. This is, for one, because of the zooplankton feeding on the phytoplankton, but also because of large amounts of the dissolved plant nutrients being consumed during the bloom and sinking to greater depths. So the phytoplankton finds ever decreasing amounts of nutrients in the water.
In nutrient-poor and cold marine regions the spring bloom represents the only influx of nutrition for the zooplankton during the year, while in other regions it represents the greatest such influx. So the spring bloom is also very important for the nourishment of fish that feed primarily on zooplankton. The benthic organisms, in turn, also benefit from the large amounts of organic material sinking to the sea floor, the dead remains of both phytoplanktonic and zooplanktonic organisms.
Because the plankton consists of short-lived organisms, it reacts rapidly to physical and chemical changes in the ocean and to fluctuations in nutrient availability. The size of populations can sometimes vary greatly within a few days or weeks. Depending on conditions the actual composition of the plankton assemblage can change, with certain species suddenly becoming predominant. Variations due to climate change have definitely already been observed. Some of these are consistent with expectations. Just like the earlier fruit tree blossom on land, the spring bloom of plankton begins earlier in many marine regions. In addition, the ranges of some planktonic species are shifting toward the poles in response to ocean warming.
One example is the northward expansion of a characteristically temperate-zone copepod, Calanus helgolandicus, a small crustacean that is displacing Calanus finnmarchicus, a species native to Scandinavian latitudes. Since both species are important sources of food for fish and have similar food requirements, this will probably not have grave impacts on the functioning of the ecosystem. But not all changes in plankton communities are so benign. In some cases warming of the water causes zooplankton offspring to hatch too early and starve. This has been demonstrated in water-tank experiments. >