The warming of the oceans means that the climatic zones in the ocean are shifting. As a result, many marine organisms follow the cooler water and shift their distribution areas further towards the poles. However, there are important differences, as scientists have now discovered. According to this, zooplankton could shift more than phytoplankton and smaller species could benefit more than large ones. This could lead to an imbalance in the food chains.
The plankton – microscopic creatures floating in the water – form the basis of the marine food chains. Single-celled algae provide food for tiny plankton animals, which in turn are prey for fish and marine mammals. In addition, plankton also play an important role in the global carbon cycle. The plankton algae bind carbon dioxide from the air through photosynthesis. When they then die, they sink to the sea floor and the carbon bound in them is stored for a long time and withdrawn from circulation.
Shift towards Pole
Because most plankton species are adapted to certain water temperatures, their distribution is influenced by the warming of the oceans. Areas of cooler water are shifting further to the poles in the course of climate change, in the tropics the ocean temperatures rise to previously unusual values. Studies have long suggested that with this shift in the marine climatic zones, the distribution of plankton is also shifting and that many species are expanding their areas of distribution to higher latitudes. What has been missing so far, however, has been more precise data on which plankton groups react how strongly to climate change.
Fabio Benedetti from the Swiss Federal Institute of Technology in Zurich (ETH) and his colleagues have now examined this in more detail. To do this, they collected data on the distribution of 860 species of phyto- and zooplankton and, on this basis, created distribution maps for today’s seas and – with the help of a climate model – for the oceans at the end of this century. The simulation initially confirmed that the distribution of the plankton species will actually shift towards the poles. “We are seeing a polar shift in species distribution at an average speed of 35 kilometers per decade,” the researchers report. At the same time, the species diversity of the plankton is increasing overall because warmer water favors greater diversity, as the team explains.
Zooplankton reacts differently than phytoplankton
But the shifts also lead to growing imbalances because not all plankton groups react to the warming of their habitat to the same extent. In the areas with water temperatures above 25 degrees, the diversity of phytoplankton continues to increase, but in the case of zooplankton, the species tend to thin out. “As a result, the diversity of the zooplankton could decrease slightly in the tropics, but it increases in the temperate and subpolar latitudes,” report Benedetti and his colleagues. “In these latitudes, almost 40 percent of the phyto- and zooplankton communities will be replaced by species that have migrated towards the pole,” reports the team.
However, this means that, especially in the even cooler sea areas, there is an increasing number of plankton species that did not originally occur in the same habitat. As a result, their ecological relationships are also not coordinated with one another. “The polar wandering leads to a profound restructuring in the composition of the plankton communities,” explain the researchers. “On the surface, the number of species will develop positively in some marine regions. However, the increase in diversity could seriously threaten established marine ecosystems at higher latitudes and their functioning, ”says Benedetti.
Plankton sizes are shifting
There is also another factor: closer examination of diatoms and copepods as two shell-bearing representatives of phyto- and zooplankton showed that climate change affects smaller and larger species differently. For smaller species, conditions tend to get better with ocean warming; for larger species, they worsen. As a result, smaller plankton species are becoming more common and numerous, especially in the temperate and high latitudes, while larger species are becoming rarer, as Benedetti and his colleagues report. Such a shift in plankton sizes would not only have an impact on food webs and, for example, fish yields in our latitudes, but also on the carbon cycle.
Larger shell-bearing planktonic algae or animals are heavier and sink more quickly to the sea floor after they die – and with them the carbon stored in their tissues. The excrement of such larger plankton animals is also usually larger and sinks faster. Because such larger plankton organisms have so far mainly lived in the cold seas of the polar latitudes, they contribute significantly to the rapid transfer of carbon into the deep sea. But if they are replaced by smaller species in the course of climate change, this transport slows down – and this has an impact on the carbon cycle. How strong this influence will be in the future, however, is still unclear, as the scientists explain.
Source: Swiss Federal Institute of Technology Zurich (ETH Zurich); Technical article: Nature Communications, doi: 10.1038 / s41467-021-25385-x