How copepods defy ocean acidification

Success story of a resistance fighter

They are small, inconspicuous – and extremely important for the food web in the sea. That’s why biologist Dr Barbara Niehoff at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research is investigating how copepods are able to survive in acidifying waters. Up to now her experiments have indicated that the small crustaceans she is preoccupied with are genuine born survivors. However, what makes the tiny animals so robust? And could acidification possibly become a threat for them in an indirect manner?

Not even the smallest reaction

The biologist responds to the term “dulled senses” with a comparison: “Copepods are like willows. They bend to the harsh environmental conditions and are consequently much more resistant than strong oaks.” While other dwellers have long vanished from the oceans, copepods have repeatedly adapted themselves to new living conditions in the course of the past 100 million years – and chances are they will also withstand present-day climate change. After all, so far they seem quite unimpressed by Barbara Niehoff’s endurance tests. 

In laboratory tests the biologist has repeated subjected three copepod species* that dominate zooplankton in Arctic waters to carbon dioxide concentrations which are nearly ten times higher than those today. “We started our tests with the toughest living conditions. In this case with a carbon dioxide concentration that we expect in 300 years at the earliest. In this way we wanted to find out what adverse effects acidic water can cause in the worst case,” she recounts. The plan was to subsequently reduce the concentrations. “Until we reach a point at which we can determine: as of here something changes in the animal compared to its present-day behaviour.”

However, even at the extremely high concentrations effects on the creatures failed to materialize – no matter where Barbara Niehoff examined her copepods: in laboratories on board the AWI research icebreaker Polarstern, at the institute in Bremerhaven or in field experiments off the coast of Spitsbergen and Norway. Somewhere and some time or other, she thought, the copepods will react to the declining pH value. In this process she examined various aspects: How much do the animals eat? How much oxygen do they need? Do they lose weight? Do their gonads develop normally? And finally: How do the various enzymes in the individual cells react? But the small crustaceans did not react at all. They developed normally, ate and breathed as if nothing had happened.

“Their senses are just dulled,” jokes a colleague when AWI biologist Dr. Barbara Niehoff recounts how her small test subjects defy ocean acidification. Simple, yes. But she would not call copepods dull. In fact, the tiny creatures prove to be genuine born survivors – even if they appear rather inconspicuous at first glance.

Copepods are small crustaceans that resemble water fleas. With their paddle limbs they more or less catapult themselves through the water and extend their antennae, which are often longer than the entire front part of their body. A transparent chitin shell surrounds their cigar-shaped trunk and gives Barbara Niehoff an unrestricted view of the inside of the animals: the intestines, reproductive organs, heart and the large oil sac. “Even a snail has a more complex structure,” she explains. Nevertheless, these simple creatures are more impressive than they look.

* The three species

Dr Barbara Niehoff is investigating the following three copepod species:

  • Calanus finmarchicus
    A species that primarily lives in the temperate zones, but is transported to the Arctic with the North Atlantic Current.

  • Calanus glacialis
    A species that lives primarily in Arctic shelf areas.

  • Calanus hyperboreus
    A giant among copepods with a body length of up to three centimetres. This species primarily inhabits Arctic waters, but can also be found further south, down to the Gulf of Maine.

Nourishing favourable meal

However, no result is also a result. In this case it is even a very good one for all those for whom it really matters: namely the animals in the ocean that feed on copepods – and they include fish, sea birds and whales. If they suddenly had less to eat, we humans would also feel the impact. On the one hand, copepods owe their popularity on the menu of various ocean dwellers to their widespread range. In principle, they live everywhere: in extremely salty waters, at hot thermal springs and in the ice of the polar regions. On the other hand, they often occur in very high densities. Alone the three copepod species with which Barbara Niehoff is occupied form up to 80 percent of the total zooplankton biomass in the Arctic. Their gigantic oil sac – in comparison to their body size – additionally makes these three species an especially nourishing meal. The animals are thus far more than just a snack – they are an important element of the food webs of the Arctic.

Indirect consequences

According to the experiments, therefore, ocean acidification alone presumably cannot have much impact on copepods. Only recently did Barbara Niehoff’s colleagues discover something astonishing that reinforces this presumption. Copepods have a special talent: they can regulate the pH value in their bodily fluids.

AWI scientists observed that the pH value of some copepods was only around six during their winter dormancy. As a comparison: a pH value of six corresponds to the degree of acidity of human urine. Seawater, by contrast, is slightly alkaline with a pH value of eight. However, as soon as copepods woke up from winter dormancy and started to eat, their inner pH value went back up to eight.

Does this mean we can give the all-clear for copepods? Not entirely. Copepods primarily feed on unicellular algae and their community will probably change as a result of climate change. In future, therefore, the number of smaller algae could increase. Furthermore, if sea ice retreats earlier in spring, algae will also reproduce earlier in spring – that means at a time when the copepods are still dormant in the depths of the Arctic Ocean.

Barbara Niehoff’s next research questions are, therefore: What do copepods eat if their favourite food, diatoms, has nearly vanished already when the animals wake up from their winter dormancy? Can they also live on significantly smaller flagellates? Initial tests indicate to the biologist that Arctic copepods might reach their limits here. This is because if they have to ingest less or inferior food, the animals no longer grow optimally and their population could decline – possibly with severe consequences for the entire Arctic food web. Indirectly, therefore, acidification and warming might just have an impact on the otherwise so resistant copepods.