Seasonal changes of mechanisms maintaining clear water in a shallow lake with abundant Chara vegetation

The study is based on monitoring data on the seasonal variation during four (1996–1999) vegetation periods, as well as long-term summer data on submerged vegetation, nutrients, light, phytoplankton and zooplankton in Lake Krankesjön, a shallow, calcium-rich, moderately eutrophic lake in southern Sweden.The lake has been in the clear water state with abundant submerged vegetation since the end of the 1980s. Somewhat lower summer biomass of submerged macrophytes during 1997 and 1999 indicates a temporary instability of the clear water state. During these 2 years, summer transparency was about 1.2–2.1 m, while concentrations of total phosphorus and chlorophyll (Chl) a were about 26–40 and 8–18 μg l^?1, respectively.Summer biomass of submerged macrophytes was higher during 1996 and 1998. In both years, a distinct increase in light availability and decrease in concentrations of nutrients and chlorophyll occurred simultaneously with the development of dense Chara vegetation. Summer values for transparency were about 2.0–2.5 m, while concentrations of total phosphorus and Chl a were about 20–32 and 4–11 μg l^?1, respectively.Summer biomass of crustacean zooplankton was below 250 μg l^?1 during all 4 years. A peak abundance of Cladocera (mainly Bosmina longirostris) during May or June caused only a short-term reduction in chlorophyll concentrations that was more pronounced in 1997 than in 1996.Measured light attenuation during 1999 was closely correlated with light attenuation calculated from the amount of suspended solids, chlorophyll concentrations, and water colour. Detritus contributed most to the total amount of suspended solids, while chlorophyll was the main contributor to light attenuation.A long-term decrease of the ratios between chlorophyll and total phosphorus suggests that phytoplankton in the clear water state is limited by factors other than total phosphorus concentrations. Increased sedimentation rate, carbon limitation, allelopathy and a lower bioavailable fraction of the total amount of phosphorus are possible explanations, while nitrogen limitation and grazing from zooplankton probably are of minor importance.Possible reasons for the “instability” of the clear water state during 1997 and 1999 are discussed. Unusually high water level as well as cold and windy weather during the spring of 1996–1999 may have caused a slow and late growth of the plants and thus a temporary instability. However, a tendency for an increase in total phosphorus concentrations and sediment accumulation along the wind-protected shores during the clear water state indicate the possibility of a long-term destabilization which contradicts the alternative stable states model.