7 - Lake Ecosystems

Abstract

An ecosystem is a composite system consisting of an ecological community and its environment. Here, the oligotrophic northern basin of Lake Biwa in Japan and the eutrophic Lake Mendota in the USA are investigated and compared. Energy flows between a lake and its surroundings are due to direct solar radiation, diffuse solar radiation, reflected solar radiation, downward and upward infrared radiation, the evaporation of water, and heat conduction–convection. The corresponding entropy flows are estimated by means of physical methods, and the net entropy inflow into the lake is obtained. From the change in heat storage and mean temperature of the lake, the change of the lake’s entropy content is also calculated, which is the sum of entropy inflow and entropy production within the lake. Hence, the entropy production in the lake is numerically obtained. The monthly entropy production turns out to be a linear function of the monthly absorbed solar radiation energy. A comparison of the entropy productions of Lake Biwa and Lake Mendota indicates that entropy production increases as the progress of the degree of eutrophication and hence with time. From a comparison of senescent lakes, on the other hand, it turns out that entropy production decreases with time in the course of hypereutrophication. Thus, the entropy production in lake ecosystems shows an increase with time in the initial stage of development and a decrease with time during the later process in senescent stage. This is the diphase characteristic of organisms and ecological communities shown in early chapters.