The oceans have been far too warm since mid-March. Scientists are puzzled: reality doesn’t match their climate models. This applies not only to the intensity of the phenomenon, but also to the spatial extent and the point in time. What’s happening underwater? Helmut Hillebrand, Director at the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg, explains what the unusual heat means for sea creatures.
Southgerman newspaper: The oceans are warmer than ever, and the situation in the North Atlantic is particularly dramatic. Off the coast of Great Britain, for example, the water is five degrees warmer than normal. Does this affect sea creatures?
Helmut Hillebrand: Definitely. Sea creatures are even more sensitive to heat than organisms on land. The temperature in water does not change as quickly as in the air, so sea creatures also have a lower tolerance for temperature fluctuations than land creatures.
What happens when the creatures get too warm?
In extreme cases, they die or lose vital functions. The classic example of this is corals, which bleach with a relatively small increase in temperature because they shed the protozoa with which they live in symbiosis. But the warming of the water can have negative effects on many animals and plants.
In what way?
Creatures that live sedentary lives and are therefore unable to move to cooler regions are hit the hardest. These are not just corals, but many invertebrate organisms, such as mussels or bryozoans. Mass extinctions of such organisms have already been observed during previous heat waves, for example in the Mediterranean Sea. Fish and other mobile creatures can usually flee the heat and swim to cooler ocean regions. A long rise in temperatures such as the one we are currently witnessing can result in animals appearing hundreds to thousands of kilometers further poles.
Helmut Hillebrand is Director at the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg and Professor of Planktology. He is particularly interested in the mechanisms that limit and change biodiversity in the oceans.
(Photo: Monika Feiling)
Are the heated areas then empty?
No, we observe that thermophilic animals and plants are also spreading poleward. Across all observed species, the distribution in the sea shifts on average six kilometers poleward every year. As a result of these shifts, communities restructure because established food webs suddenly no longer function in the same way. Certain predator-prey combinations, for example, can simply be omitted if one cannot move with them. This will also have a negative impact on biodiversity in the oceans.
Are there any specific examples?
Yes, seabirds, although they are not aquatic and mobile. But they get into big trouble when the fish they feed on move to cooler regions. Unlike fish, birds cannot stray indefinitely from land where they nest and raise their young. They have to cover significantly greater distances to fly after the fish. Because this costs a lot of energy, the birds’ ability to successfully feed their offspring decreases. This is also considered one of the reasons why the population of puffins has shrunk in many of their European settlement areas. The puffins feed mainly on small fish such as sand eels. Because of the rise in temperature, however, sandeels have migrated north to follow their main food source, zooplankton. However, the puffins cannot stray that far from their nesting rocks on the coast.
Is the currently observed warming of the oceans so strong that it can trigger such effects?
Yes, that is my fear. But the problem is also that the current heat wave has built up relatively quickly and that huge areas of sea are affected. So it wasn’t easy for the mobile sea creatures to dodge either. They had to travel long distances in a relatively short time to get to cooler regions.
Are there other consequences?
Definitely long-term. The current high temperatures are, so to speak, an extreme of the phenomenon that has been observed for a long time, that not only the atmosphere is warming up, but also the oceans. In addition to the shift in distribution, a second frequently observed effect is that sea creatures become smaller at higher water temperatures. Why this is so is not yet fully understood. But it affects a lot of organisms: fish, but also phytoplankton, for example. This often consists of unicellular organisms in the order of micro to millimeters. They are the basis of most marine food webs, so this can have far-reaching consequences.
Which?
Phytoplankton is primarily eaten by zooplankton, i.e. small invertebrates; on the coast but also of filter feeders such as mussels. The eating tools of these creatures are often adapted to certain sizes of phytoplankton and cannot cope with smaller cells. In extreme cases, it can no longer be eaten at all. Smaller phytoplankton also sink more slowly to the sea floor than larger ones.
Why is this a problem?
The phytoplankton plays a central role in the function of the oceans as a carbon sink. Similar to plants on land, tiny algae absorb CO₂ to grow. If the phytoplankton dies or is eaten and excreted again, aggregates are formed that sink to the sea floor and with them the bound CO₂. So this so-called biological carbon pump brings CO₂ from the atmosphere into the deep sea. The smaller the cells are, the slower they sink and the slower the carbon pump works.
Are heat waves worse at sea than on land?
That is hard to say. The fact is, however, that the creatures in the sea are affected at least as much as the creatures on land. It’s just not that obvious to humans because it’s all underwater.