Some of the smallest marine species are actually the most important, because all other life depends on them. Phytoplankton are probably the most important, but zooplankton, which are just above them in the food chain, are also extremely important.

A small copepod called Calanus finmarchicus is the most important zooplankton species in cold North Atlantic waters. Researchers have now investigated how Calanus copepods react to rising temperatures and changing access to food.

These copepods thrive best in cold waters. There is strong evidence that the species has already started migrating northwards because the ocean has become warmer.

Given that the planet’s average temperature is rising, this news is not very encouraging.

“We see that when the temperature rises, Calanus eggs hatch more quickly. However, the offspring are smaller and fewer of the eggs hatch,” said PhD student Sidonie Rousseau at NTNU’s Department of Biology.

She has been studying Calanus finmarchicus in connection with her doctoral research. Unfortunately, this is not the only news we should be concerned about:

• Mortality rates are rising.
• The larvae’s metabolic rate increases up to a certain temperature, before decreasing again. This means they require more energy.
• Increased availability of food cannot fully compensate for the higher energy costs of surviving at higher temperatures.

All these findings suggest that Calanus copepods are exposed to more stress when temperatures rise, putting them under greater physical strain.

In copepods such as Calanus finmarchicus, body size is directly linked to their ability to reproduce. The smaller they are, the fewer offspring survive and go on to reproduce themselves. Small size also means reduced ability to store fat, with the consequence that fewer of them manage to survive the winter.

As a result, this means that there will also be less food for other animals that feed on Calanus copepods. In practice, this can mean less food for fish, and consequently less cod and herring for fish-eaters like us.

Just like other animals, Calanus copepods can adapt to environmental changes. Therefore, some people will undoubtedly say that everything will be fine – and there is a good chance they will be right, for a while.

However, there will always be a tipping point after which things will not be fine anymore.

“We also investigated how the different sexes respond differently to oxidative stress, which we induced using a chemical,” explained Rousseau.

Oxidative stress occurs when cells accumulate harmful molecules that can damage proteins, DNA and other vital cell components.

“To measure how stressed the Calanus copepods were, we examined the activity of genes involved in the antioxidant defence system – the system that protects cells against this type of damage.”

The researchers found that males are more sensitive to oxidative stress than females. Up to a certain temperature, the females were able to increase the production of antioxidant genes, but this response eventually reached its maximum capacity as well. So, there is a limit.

“Males also use their energy differently than females. For example, they swim more actively and therefore use energy more rapidly. When exposed to oxidative stress, they may run out of energy sooner and die more quickly than the females.”

This is worrying because the species already has fewer males than females. If male mortality rates increase even further, it could threaten the stability of the entire stock.

Much of this research takes place in laboratories, but the researchers also went out into the field to investigate whether low-frequency noise from offshore wind turbines affects the copepods. In this respect, the news is more encouraging.

“The sound triggered little physiological response in the copepods, suggesting that the ecological consequences are limited,” Rousseau said.

However, the researchers have not yet been able to determine anything concrete about the potential long-term effects of this low-frequency noise.

“Collectively, the results show that physiological stress in Calanus finmarchicus is closely linked to life stage, sex and the types of external influences they are exposed to,” Rousseau said.

We should also note that these influences can have serious consequences, even if they are not directly lethal to Calanus copepods.

“If we affect their metabolism so that the copepods burn energy faster, their energy reserves also become depleted more quickly. This affects reproduction, how Calanus finmarchicus develop, and whether they manage to survive the winter,” explained Rousseau.

Most researchers generally prefer not to sound the alarm, even if it sometimes seems that way when their work is reported. But this is serious.

“Although our results do not suggest that the Calanus finmarchicus stock is in immediate danger of collapse, they show that the species is vulnerable to rising temperatures, and that this risk may be greater when multiple environmental stress factors occur simultaneously,” she said.

Reference:
Sidonie Rousseau, Wassim Ameur, Elise Thiebaut, Tamer Hafez, Dag Altin, Bjørn Henrik Hansen, and Kang Nian Yap. Sex differences in stress response in the marine copepod, Calanus finmarchicus. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 2025 329:5, R771-R783 https://doi.org/10.1152/ajpregu.00120.2025