We are currently examining precisely this in two species of cod: the Atlantic cod and the Polar cod. One of them, the Atlantic cod, is a generalist. It lives both in the North Sea, around Iceland and in the Barents Sea and along the East and North Coast of North America. It tolerates water temperatures between zero degrees and 20 degrees Celsius and presumably does also not react so sensitively to lower pH values in the sea. By contrast, the Polar cod only occurs in the Arctic, at water temperatures from around zero to four degrees Celsius and will therefore presumably encounter difficulties in coping with higher temperatures and carbon dioxide concentrations in the water. As water temperatures rise, the Atlantic cod swims further and further to the North and therefore into the habitat of the Polar cod. We presume that both species hunt the same prey and are therefore in food competition. If the Polar cod is weakened by the rising water temperatures, for example, then it may certainly be the case that the Atlantic cod wins the upper hand and spreads to the detriment of its arctic cousin.
How do you determine whether and how ocean acidification impacts the cod?
We firstly look at the entire fish: does its metabolic rate alter at a lower pH value? Does it swim faster or slower in acidic or warmer water? We conduct a type of fitness test with the fish in our swim tunnel in Bremerhaven. The fish is in a chamber and must swim against water current. By slowly increasing the flow speed, we can measure its metabolic rate and optimum swimming speed. The principle is the same as for humans: if we run, we also need more energy and that will be reflected in an increased metabolic rate. In the same way as people on a treadmill, we can also measure when the maximum speed of the fish is reached in the swim tunnel. For example, we can examine whether the performance limit is associated with different temperatures and carbon dioxide concentrations in the water. These measurements are an example of how we examine a fish at the so-called whole animal level. But we can also analyse what for example is happening at the genetic level , i.e. in the DNA, of the fish. Ultimately, we wish to create an overall picture, from the processes in the cell through to the whole animal level. Only in this way can we understand how a fish reacts when the water becomes warmer or more acidic.
How are these laboratory results then transferred to the real ecosystem?
We cannot of course simulate all the environmental conditions in the laboratory as they occur in the ecosystem but we can test the impact of individual conditions consecutively. For example, we can first investigate how a fish reacts to warmer water and how it reacts to more acidic water. Finally, we can then test how both factors, i.e. warming and acidification, together influence the fish. We can even go further by not only observing one species but as many species as possible. However, there are limits to this because we do not hold the complete ecosystem in the aquarium and cannot work with every individual animal. Therefore, we select certain species which, for example, together form a food chain or a part of a food chain. This is because it could of course be that one fish species can adapt to the ocean acidification but that its main prey is particularly endangered. The more factors we take into consideration, the better indications we can give as to what is happening in the ecosystem.