Fish, Fish Food, and Lake Temperature

Living things change the earth. You need only look at an anthill, a beaver dam, or a shopping mall to notice that many animals tinker with their environment, consciously or not.

The tinkering can be hard to notice. Sometimes, in unexpected places, what seems to be the straightforward working of geophysical principles turns out to have a biological component. For example, a group of Canadian scientists recently examined how fish indirectly affect the temperature of the lakes they inhabit.

People who swim in northern lakes quickly learn that the water temperature is not uniform from top to bottom. By late spring, a temperate-zone or boreal lake usually is thermally stratified: its water has different temperatures, and different patterns of temperature change, in layers.

The warmest water--no surprise--is found at the top. That's where the sun's effect is most easily detected. Although solar energy is reflected away by the lake surface, and back-scattered by particles in the water, some is absorbed directly by the water.

The thickness of this warm layer is determined by what students of the subject call the mixing depth, the depth to which wind action stirs in the warmer surface water. Water in the lowest layer, below the mixing depth, stays uniformly cold.

At least, conventional wisdom held that wind was the main factor determining the depth of the warmer layer. Theory held that a lake's mixing depth could be predicted quite well if one knew the amount of wind and the orientation of the lake with regard to that wind. Puny winds stir the sun-warmed surface water into the lake to a meager depth; stronger winds do a better job of mixing more water down deeper. Winds blowing in the direction of the longest dimension of a lake have more effect than winds blowing across it.

That was appealingly simple--too simple. By the mid-1980s, researchers had come to understand that, at least in some lakes, water clarity played an important role in setting mixing depth. New measurements showed that greater water clarity meant more solar radiation absorbed by the water and less solar energy lost back to the air. So, given two otherwise identical lakes, the one with the clearer water will be warmer.

Sometimes a lake is murky because of suspended sediments or dim because its waters have leached through peat bogs or other sources of brown coloring; more often, the murkiness comes from plankton. Small phytoplankton have the most obvious effect. These miniscule floating plants reduce water clarity, and that leads to lower water temperatures. The sun's energy just can't penetrate past all those little green cells.

Lakes usually aren't pools of cold green soup mostly because waterborne grazers, from microscopic protozoa to mosquito larvae, devour phytoplankton. By reducing the phytoplankton population, these little animals work to clear the water and thus to make it warmer.

But the food chain doesn't stop there. Fish eagerly snap up the plankton-eating animals reducing the number of grazers. This enhances plankton populations; more fish mean chillier, murkier water.

To test that view of how the water temperature-water creature relationship works, the Canadian scientists first gathered information from two similar-sized lakes in Ontario. The lake with few fish had deeper mixing depths and warmer waters in summer (by as much as 13 degrees Centigrade several meters down) than the lake with many fish.

Then they tried suspending fish-proof enclosures in lakes; the enclosures turned into liquid islands of clearer water, abundantly stocked with plankton-eating animals. Finally they took advantage of an experiment by others in which a small lake had been deliberately acidified. It proved to have both clearer water and warmer temperatures to a greater depth after the phytoplankton population was reduced by the acidification.

The Canadian researchers were content to conclude their report by noting that lake organisms can influence their physical environment and thus contribute to ecosystem stability. I think they've also given a possible reason why a good fishing hole is unlikely to be a good swimming hole.