Marine ecosystem
WOR 1 Living with the oceans. A report on the state of the world’s oceans | 2010

Plankton cycle

Disruption to the plankton cycle

> Recent experiments and studies show that climate change – and global warming in particular – is pushing established biological systems off balance. This can have a devastating effect on some organisms. What is most disturbing is that the natural rhythm of the ocean’s most important food source, the phytoplankton, is changing

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.

The copepods 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.

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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.

Mounting threat of harmful algal blooms

Harmful algal blooms (HABs) are massive growths of toxic or otherwise harmful phytoplankton. HABs are becoming ever more frequent worldwide. It is not yet known, however, why this is. Eutrophication, the increased concentration of nutrients in the water, is considered to be the main cause, but climate change also appears to play a role. Harmful algal blooms normally occur in the summer months when the water column is thermally stratified. A warm, light surface layer overlies a colder, heavier deep layer. The warmer the surface water, the more pronounced the temperature gradient is at the thermocline between the layers.

5.6 > The mauve stinger, Pelagia noctiluca whose painful sting is normally not deadly to humans, has become increasingly abundant in the Mediterranean Sea in recent years.

5.7 > The dinoflagellate Karenia brevis occurs primarily in the Gulf of Mexico. Its nerve poison, Brevetoxin A, can lead to inflammation and asthma attacks in humans.

5.8 > Small crustaceans like this Calanus-species are widely distributed in the oceans and one of the most important food sources for fish.

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5.6 > The mauve stinger,  Pelagia noctiluca whose painful sting is normally not deadly to humans, has become increasingly abundant in the Mediterranean Sea in recent years. © David B. Fleetham/
5.7 >  The dinoflagellate Karenia brevis occurs primarily in the Gulf of Mexico. Its nerve poison, Brevetoxin A, can lead to inflammation and asthma attacks in humans. © 
5.8 > Small crustaceans like this Calanus-species are widely distributed in the oceans and one of the most important food sources for fish.  © Arco/NPL Kim Taylor


A strong temperature gradient prevents water masses from mixing at the thermocline, because the density difference between the cold and heavy water, and the warm and light water, acts as a barrier. Nutrients from greater depths are therefore prevented from circulating to the surface. So when the nutrients near the surface have been consumed by phytoplankton growth, there is no source of replenishment. The vertical barrier between a zone with enough light and insufficient nutrients, and a zone with insufficient light and abundant nutrients, which is characteristic of the summer, is thus reinforced. Relatively large mobile phytoplankton species have an advantage here. With vertical migration they can move back and forth from the deeper nutrient-rich water to the shallower layer penetrated by light where photosynthesis is carried out. Such species include numerous dinoflagellates and, especially in the Baltic Sea, cyanobacteria, which can regulate their specific gravity to rise and descend like a diver. One problem with this is that both groups include numerous toxic species. If mussels consume these organisms, then the mussels become dangerous or even lethal to humans. The planktonic organisms may also release some of the toxins directly into the water. In some cases these are even detectable in aerosols, small droplets wafting in the air that are produced by breaking waves in the surf. An especially notorious culprit is the dinoflagellate Karenia brevis, whose periodic blooms off the coast of Florida cause mortality of fish, poisoning of mussels, inflammation in swimmers and, in extreme cases, asthma attacks in visitors to the beach. Experts attribute the increased incidence of these blooms to general climate warming. As mentioned, there are also numerous toxic groups of cyanobacteria. Investigations so far have focused on cyanobacteria that live in freshwater – especially in waters that are used as sources for drinking water or where bathers are in danger from cyanobacteria. But toxic strains of various cyanobacteria such as Nodularia spumigena have also been verified in the Baltic Sea.

Trouble with jellyfish

Beside more frequent HABs scientists are also observing explosive growth of jellyfish populations. The impacts of such proliferation are quite well known: injured swimmers, clogged fish nets, feeding competition for fishes, and predation of fish eggs and larvae. The possible causes of these burgeoning populations are somewhat controversial. One significant problem, presumably, is overfishing. Fish that feed on zooplankton are in feeding competition with the jellyfish. If the fish are absent, then the jellyfish have an abundance of available food. It is also known that jellyfish are more robust than many species of fish, especially in the sense that they can tolerate much lower oxygen concentrations. Oxygen deficiencies in the oceans, in turn, occur increasingly as a result of eutrophication. More biomass is created because of eutrophication. As a result, more organic material sinks into the deep water where it is decomposed by oxygen-consuming microorganisms. The result is a general decrease in oxygen. Climate change, which causes warming of the ocean surface, can exacerbate this situation. The warming slows down exchange processes because the oxygen-rich surface water mixes less with the colder deep water. Only small amounts of the oxygen consumed by microorganisms at greater depths are replaced. Increasing jellyfish scourges could therefore be a result of combined stressors. Climate change will therefore lead to a restructuring of the pelagic biocoenoses, which will disadvantage the classical food chain “phytoplankton – zooplankton – fish”. Jellyfish, on the other hand, will benefit from that. Presumably overfishing and eutrophication of coastal waters will have additional synergistic effects that will worsen the situation. Textende