More heat = Less oxygen

For many species, the threat of global warming is more than too much heat. You can get temperature-related effects that may not be what you expect. Subtle environmental change can interact with physiological needs to weaken the ability of a species to maintain a viable population even when the temperature rise is too mild to kill individual organisms.

Biologists usually don't know enough about a species to assess this risk. Their knowledge comes largely from laboratory research on bits and pieces of a species' physiology. This method doesn't give much insight into how an animal as a living whole responds to climate change in its natural habitat.

Now, 10 years of research that combined laboratory study with field work has given that kind of insight into the decline of eelpout populations in the southern part of the North Sea.

To state it in a nutshell, Hans O. Pörtner and Rainer Knust at Germany's Alfred Wegner Institute for Polar and Marine Research in Bremerhaven have shown that a relatively mild rise in water temperature can reduce the water's ability to hold dissolved oxygen, and, at the same time, it increases the fish's need for more oxygen to maintain its vigor.

The researchers discovered that, over the past five decades, eelpout populations declined when summer temperatures rose and vice versa. They also found that warmer summers reduced eelpout population the following year.

The results were published in the journal Science two weeks ago. In an accompanying commentary, Danish scientists Tobias Wang at Aarhus University and Johannes Overgaard at the National Environmental Research Institute in Silkeborg noted that "it remains difficult to establish increased temperature as the mechanistic cause for the [eelpout] population decline." Yet they acknowledged the correlation of the lock-step relation between temperature and population size as documented in the field with the effect of temperature on the fish's oxygen needs as documented in the laboratory.

They found this correlation between laboratory and field data to be "persuasive."

Drs. Pörtner and Knust take a larger view. They see their work illustrating that a supply-and-demand mismatch for oxygen "is the first mechanism to restrict whole-animal tolerance to thermal extremes." In simpler terms: More heat means less oxygen, and less oxygen means animals have a harder time of it.

That mechanism cuts in even before it becomes hot enough to kill the animals or make them migrate. The scientists suggest that the relationship between rising warmth and oxygen supply could become a "unifying principle" in the effort to gain "a cause-and-effect understanding of the influence of climate change."

The impact of climate change may also be expressed through indirect effects of the interaction of environmental change and physiology.

The warming-induced shift from large to small forms of microscopic animals called copepods in the southern North Sea caused the Atlantic cod to starve and contributed to its decline. Satellite data have established a lock-step correlation between tropical and subtropical surface water temperature and the abundance of microscopic algae – the so-called "grass of the sea" on which almost all marine animals ultimately depend.

But algae need nutrients that well up from below. Warmer surface water tends to cap that upward flow. The productivity of the algae then declines. This affects the whole tropical and subtropical marine food chain.

As Drs. Wang and Overgaard put it: "Old fashioned physiology can be essential for understanding how temperature determines the geographical distribution of animals."