Few things in nature are as beautiful as lakes. Lake water is often crystal clear and one can see to great depths. Generally they have a placid presence, but this belies the extent of the biological and physical activity that goes on in every lake.
In particular, they illustrate how living, non-human organisms actively shape the physical environment. This has been elegantly demonstrated by the work of Asit Mazumder and colleagues of the University of Montreal, Canada.
Lakes have a well-defined thermal structure. In southern Canada, lakes separate into three thermal layers in summer. The upper layer, the epilimnion, is freely circulating and is warmest. The coolest layer, the hypolimnion, is relatively stagnant at the bottom. The metalimnion is an intermediate layer of rapidly declining temperature.
The water density increases with depth as temperature declines. The thermal structure of the lake exercises powerful effects on biological, chemical and physical processes in the lake. For example, the movement of nutrients from the hypolimnion to the epilimnion can be inhibited by the density barrier at the metalimnion.
Biological organisms in these lakes can be divided into four functional groups arranged into an ascending food chain, where each link in the chain feeds on the link below and is eaten by the link above. Photosynthetic algae (microscopic plants) make up the first level. These organisms engage in photosynthesis using the power of sunlight to unite carbon dioxide and water to build up their own substance and releasing oxygen as a by-product.
The other biological groups in the lake ultimately rely on the algae for energy. The overall growth of algae depends partly on the supply of nutrients such as phosphorus.
The second biological group is the zooplankton, invertebrates no longer than one quarter of a centimetre, many of whom feed on the smaller algae. The third biological group is zooplankton-eating fish, such as yellow perch. The fourth group is the fish-eating fish, such as pike, that feeds on the zooplankton-eating fish.
Predators control the abundance of their prey. Thus, strong growth in fish-eating fish numbers will cause a decline in the zooplankton-eating fish. This in turn will allow the zooplankton to blossom which, in turn, will drain the lake of small algae - although the latter reduction can be mitigated by a strong supply of nutrients.
Lake water is heated by sunlight. A small fraction of the sunlight that penetrates the water is used by the algae to power photosynthesis. The rest of the sunlight is scattered back into the atmosphere or is stored as heat in the lake water.
As light passes through the water, it becomes attenuated by suspended particles, dissolved material and the water itself. In coloured lakes (where dissolved organic compounds tint the water) most of the sunlight is absorbed by the tinted water. In non-coloured lakes, the algae absorb some light and scatter more. Lakes with dense populations of small algae are murky as a result of scattering and absorption of light, whereas lakes with few algae are very clear. Sunlight can obviously penetrate to much greater depth in a clear lake than in a murky one.
Asit Mazumber and colleagues have carried out ingenious experiments in Canadian lakes to demonstrate how the living organisms affect and adjust the thermal structure of the lake in a manner that tends to maintain a stable balance between the four groups of organisms. The researchers enclosed eight sections of a lake with plastic walls, each section 25 feet in diameter and 50 feet deep. In some sections they removed all the zooplankton-eating fish - the other sections were left unchanged.
The removal of the zooplankton-eating fish in some enclosures allowed the population of zooplankton to soar, which in turn ate up the algal population, causing a marked increase in water clarity. The enhanced clarity allowed sunlight to penetrate deeper into the water, raising the temperature at depths as low as the top of the hypolimnion.
After several months the temperature of the deeper parts of the clearer enclosures was 25 per cent higher than the temperatures at corresponding depths in the enclosures whose fish stocks had not been touched.
Traditionally it has been assumed that the heat structure of a lake is determined by physical factors. For example, it was assumed that transport of heat to the lower depths was carried by wind-activated currents and turbulence. But the studies carried out by Mazumder show that the biological organisms in a lake have a large influence on the heat structure of the waters.
Mazumder's work shows that the biological life of a lake can regulate the lake to its own advantage. Consider, for example, a sharp rise in the population of fish-eating fish in the lake. As I described before, this will cause a sharp decline in the algal population. Since the algae are critically important for maintaining the food supply for the rest of the biological organisms in the lake, their decline would have a very deleterious effect on the entire pyramid of life in the lake if it were left unaltered.
However, a sharp decline in the algal population improves water clarity and allows the lake to warm up at deeper levels allowing new algae to grow at greater depths than would ordinarily be the case. This will tend to restore the algal population of the lake.
The Gaia Hypothesis advocated by James Lovelock proposes that life on earth actively shapes the physical conditions of the planet. The work of Mazumder on the Canadian lakes illustrates a specific example of this Gaia behaviour.
William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.