Birds and fish in the Disko Bay no longer have access to the same amount of two Arctic fat-rich copepod species as they did 25 years ago, according to recent research.
“It’s like eating peas instead of donuts.”
This is how Professor Torkel Gissel Nielsen, DTU Aqua, describes a significant shift at the bottom of the food chain—a shift in the composition of copepod species. In collaboration with Senior Researcher Eva Friis Møller, Aarhus University, he demonstrates this shift in a new article recently published in the scientific journal Limnology and Oceanography.
Copepods make up the food basis for fish and many sea bird species in the area, which depend on being able to build an insulating layer of fat during the short summer in order to survive the winter. Over the past 25 years, the composition of copepod species in the Disko Bay has changed dramatically. 25 years ago, two large Arctic species particularly rich in fat constituted the majority of the copepod biomass, while the smaller Atlantic species Calanus finmarchicus, which contains less fat, constituted the rest—averaging 39 per cent in the years 1992-2001. According to the new study, the opposite is now the case. In the period 2005-2018, the smaller copepod species accounted for an average of 64 per cent of the biomass.
“To cover their need for fat, the fish must now eat more and swim longer. There has been a shift at the bottom of the food chain, which can have as-yet-unknown consequences,” says Torkel Gissel Nielsen.
Less ice and warmer water
Torkel Gissel Nielsen and his research group have been working for many years to understand the consequences of climate change in Western Greenland. The new study, carried out in collaboration with Eva Friis Møller, Aarhus University, also looked at the presence of sea ice in the Disko Bay since 1992. It showed that while most years until the mid-1990s the ice layer remained solid and deep for up to six months, the layer has become more unstable in recent years—some years resulting in very little or no ice at all in the area. And this affects the composition of copepod species.
“We can see that more Atlantic water, which is slightly warmer, enters these areas, resulting in less ice. The fat-rich copepods are adapted to areas with sea ice and can live long without food deep under the ice. But those who come with Atlantic waters depend on a constant food supply. With no ice, sunlight enters the water, sustaining the phytoplankton that in turn feeds the smaller species for a longer period. The larger, fat-rich species then lose their competitive advantage in the area,” says Torkel Gissel Nilsen.
Copepods under pressure from several directions
Less ice in the Disko Bay not only makes the area more attractive to the Atlantic copepod species, it also offers better opportunities for other types of activities, such as sailing cruises and oil drilling—two activities that may further increase the pressure on the Arctic copepod species C. hyperboreus. The results of another study recently published in Aquatic Toxicology show that exposure to even very small amounts of the substance ‘pyrene’ from crude oil can have fatal consequences for the copepods and their eggs.
The researchers collected copepods (C. hyperboreus) in September, after they had eaten enough to sustain them through the winter and had travelled towards the ocean bottom to hibernate. Instead, they got to hibernate in large refrigerated containers at DTU Aqua, where researchers could expose some of them to small amounts of pyrene and examine its effects. C. hyperboreus does not eat during the winter but lives off its fat reserves, which it also uses to produce eggs in the spring. Yet it turned out that they were still affected by pyrene, which is very fat-soluble.
“We used very low pyrene concentrations compared to what may be found a few kilometres from an oil platform. The adult copepods were not affected by the pyrene. But the eggs they laid after having been exposed to the pyrene during hibernation did not develop well, and many of them did not hatch at all. And when the females started eating again in preparation for the next winter, they died. They were simply not able to eat and gain weight, while both the eggs and adult females in the control group, which had not been exposed to pyrene, were fine,” says Torkel Gissel Nielsen.
In the past, researchers have shown that the other Arctic copepod species, C. glacialis, has a similar sensitivity to long-term exposure to low concentrations of pyrene.
Shifts in the food chain may have consequences
Copepods constitute the main food basis for fish, birds, and large marine mammals, such as the bowhead whale. The vast majority of Greenland’s economy is based on the fishing of relatively few species, so a shift to smaller copepods less rich in fat can affect the composition and production of fish.
“Most fish are dependent on copepods at their earliest stages of life—and in Greenland, the very fat-rich copepods are usually the most common species in the fishing grounds. When smaller copepod species become the norm, it may affect the animals that feed on them, because they are harder to see and the animals need to swim longer distances to eat enough to cover their need for fat,” says Torkel Gissel Nilsen.
Read the scientific articles:
Limnology and Oceanography:
Borealization of Arctic Zooplankton—smaller and less fat zooplankton species in Disko Bay, Western Greenland
Aquatic Toxicology:
Delayed effects of pyrene exposure during overwintering on the Arctic copepod Calanus hyperboreus