From microorganisms to large marine mammals: A variety of interrelationships connects animals and plants in the ocean. The marine food web is not just defined by the immediate “eat and be eaten” – the exchange of organic nutrients and vital functions is equally crucial. We humans may consider us as part of this system: The ocean is a source of food and oxygen, a carbon sink, location of important resources and much more. Ocean acidification and other aspects of global climate change are about to alter the food web – with consequences for all life on earth.
Phytoplankton, microscopic plants of sizes between 0.2 micrometres and 0.2 millimetres, forms the basis of the marine community. As primary producers, these organisms build up biomass, using light, carbon dioxide and other inorganic substances to photosynthesize. As part of this process, they also release oxygen. Zooplankton, tiny animals, take up part of this biomass as food. Both plankton groups also give off organic substances into the seawater, which are then processed by bacteria and enter the system again as nutrients. Fish also live on plankton and return organic matter into the circulation.
Several experiments with the KOSMOS mesocosms showed that the smallest phytoplankton benefits, if additional carbon dioxide is dissolved in the seawater. Pico- and Nanophytoplankton grow more strongly under these conditions – consuming nutrients that larger photosynthesizing plankton such as about diatoms would have used. Also the zooplankton is excluded from the boom at the base.
The phytoplankton also include the single-celled calcareous algae, coccolithophores such as Emiliania huxleyi. This tiny all-rounder produces considerable amounts of biomass and calcite – a relatively stable form of calcium carbonate – and releases a climate-cooling gas. Laboratory and field studies have shown that elevated carbon dioxide concentrations stimulate Emiliania huxleyi’s photosynthesis, but its calcification and growth is impaired.
Prominent calcifying zooplankton are the pteropod species Limacina helicina and Limacina retroversa. The “sea butterflies” form their shells from aragonite, a relatively soluble form of calcium carbonate. If seawater contains less carbonate ions due to ocean acidification in the future, the swimming snails have to spend too much energy to build their houses – it remain thin and porous or brittle. The animals do not seem to be able to benefit from the additional phytoplankton growing under these conditions. In extreme cases, the acidified water might even dissolve their shells. Larger fish and marine mammals, as well as seabirds will be missing an important food source.
Experiments with copepods showed that climate change can transform zooplankton into “fast food” of poor quality: The organisms that make up about 80 per cent of the zooplankton actually benefit if more phytoplankton grows because more carbon dioxide is dissolved in the water. But overall, the fatty acid composition in the body of the crustacean is affected negatively by acidification and rising water temperatures. Food webs, which are influenced by the food quality – not by the sheer mass of supplies – would be deteriorated.
Jellyfish and gelatinous creatures seem to suffer less from aspects of global climate change. The approximately one millimetre large Larvacee Oikopleura dioica thrives in warmer or CO2-rich waters. Its pronounced hunger can disturb the food web significantly. But on the other hand, the “marine snow” produced by Oikopleura transports carbon down to the sea floor – and a new carbon dioxide can be processed on the surface.
During the past years, scientists have found out how ocean acidification – in some cases combined to other factors such as rise in temperatures, eutrophication or loss of oxygen – affects isolated species. A couple of studies even shed light on marine communities. The current challenge is to draw a more comprehensive picture and assess reactions of the marine ecosystem to a multitude of stressors. In addition to laboratory studies, experiments with the KOSMOS mesocosms or the benthocosms facilitate these evaluations as well as observations at naturally-acidified sites such as the coral reefs of Papua New Guinea or model calculations.