Clear and Turbid Water in Shallow Norwegian Lakes Related to Submerged Vegetation

Abstract

Timms and Moss (1984) suggested that fertile shallow lakes may have alternative stable states, a clearwater state with dense vegetation and a turbid water state dominated by phytoplankton and with little submerged and floating-leaved vegetation. This phenomenon has also been observed and discussed by several other authors (e.g., Irvine et al., 1990; Jeppesen et al., 1990; van Donk et al., 1990; Blindow et al., 1993; Scheffer et al., 1993). According to the model of Scheifer (1989, 1990), the main controlling factor for the two alternative states is the turbidity of water regulating the vertical light penetration. When the nutrient level increases, phytoplankton growth is often stimulated, which in turn increases the turbidity. This leads to increased light attenuation and a reduction of the maximum growth depth of submerged vegetation. Increased nutrient concentrations will therefore reduce the bottom area covered by submerged vegetation until it is virtually absent. In shallow lakes with most bottom areas at similar depths, the change from high plant cover to plantless bottoms may be abrupt. Uncovered sediments in shallow lakes are much more vulnerable to resuspen-sion and more easily give rise to turbid water during periods of wind and wave action than in lakes with plant-covered sediments. Benthivorous fish may be favored under these circumstances and add to the turbidity by foraging on the sediment. A more-or-less stable turbid state may be the result.KeywordsShallow LakeSubmerged VegetationTurbid Water StateIsaac Newton TelescopeArmagh ObservatoryThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.