Concepts for Restoration: Nutrient Loading, Thresholds and Alternative Stable States

Abstract

Shallow groundwater seepage lakes are difficult to manage and to restore once they are eutrophied. Effective management and rehabilitation of such lakes need strict and systematic planning based on solid concepts. The model most commonly used for eutrophication is the nutrient loading concept which relies on the limiting nutrient theory. Although not particularly designed for lakes primarily dependent on groundwater, nutrient loading models can be adapted and used to investigate the nutrient input and nutrient balance from the aquifer. These models can also be used to make predictions and to calculate thresholds or limits which must be reached to improve water quality. The theory of alternative stable states provides another excellent concept. Clear water, macrophyte dominated stages can switch to turbid conditions characterized by high algal concentrations. Such forward switches are often associated with anthropogenic pressure or changes in the hydrological regime. Recreational activities often enhance eutrophication processes resulting in a system collapse and an almost spontaneous switch to an alternative state. Backward shift to the original, macrophyte dominated stage can be difficult to attain. Return times are often prolonged due to hysteresis as a result of resilience. Concepts are outlined in general and then specified for the ecosystem in question.