Stability and Multiple Steady States of Hypereutrophic Ecosystems

Abstract

Due to their high biomass and metabolic rates hypereutrophic water bodies easily disequilibrate. Growth of fish may be affected both by low and high photosynthesis. In consequence of its shallowness, the water body is highly sensitive to fluctuations in the physical environment, and the ecosystem is frequently in a transient state. Stability was measured as the statistical variation of a time series in relation to the arithmetic mean.

In sewage lagoons, the adjustment toward a new steady state of the O²-concentration corresponded to a rate of 1– 1.5mg/l/d. In a village pond, the transient phase was characterized by rates up to about 4 mg/l/d.

In continuous-flow laboratory models of sewage lagoons operated under constant external conditions, the fluctuations in the abundance of the particular species were high indicating internal instability. On the other hand, the average community structure was equilibrated, and the removal rate of dissolved organic substances fluctuated within very narrow limits.

As dense phytoplankton blooms and overgrazing were observed in the ponds as well as in the laboratory models, these obviously represent alternative equilibrium states which may occur at the same set of external conditions. Clear-water periods caused by mass growth of Daphnia in the absence of fish make the application of the phosphorus loading concept to hypereutrophic water bodies difficult.

While a hypereutrophic state of a natural water body in most cases is an unwanted side-affect of human activity, sewage treatment lagoons, fertilized fishponds and primary reservoirs represent examples of man-made systems which are intentionally hypereutrophic. Here excessive growth of phytoplankton produces oxygen for waste treatment or useful biomass, or incorporates dissolved phosphate from non-point sources preventing water reservoirs from eutrophication. If there is a probability that the quality of a limnic ecosystem can turn into a state at which previous water uses are hampered or impossible, the mere knowledge of the average conditions is of little use. It is the borderline cases that may be the most critical. In this contribution, the short-term variations are considered in some detail.