Alteration of phytoplankton structure in Lake Lucerne due to trophic conditions

Abstract

During the last 45 years, Lake Lucerne (Switzerland) was characterized by four major periods of varying trophic state: 1) the oligotrophic period (1955 to 1969): total phosphorus concentrations below 20 mg m⁻³, nitrate-nitrogen 300 to 400 mg m⁻³; 2) a period of accelerated eutrophication (1970 to 1977): total phosphorus concentration 20 to 30 mg m⁻³, nitrate-nitrogen 400 to 500 mg m⁻³; 3) a moderate mesotrophic period (1978 to 1988): total phosphorus concentration 20 to 25 mg m⁻³, nitrate-nitrogen 500 to 600 mg m⁻³; and 4) a period of growth limiting phosphorus concentration (1989–2001): total phosphorus concentration 5 to 10 mg m⁻³, nitrate-nitrogen 600 to 700 mg m⁻³.

The biomass, evenness, species richness and biodiversity of the plankton community was analyzed over 40 years. Annual means of biomass were not closely correlated with nutrient loading. Biodiversity, evenness and monthly changes in biomass proved to be better indicators of environmental changes. Clear water periods due to zooplankton overgrazing were observed during the mesotrophic period. The growth of phytoplankton appeared to be limited by phosphorus. Blooms of nitrogen gas‐fixing (N₂) cyanobacteria occurred only during the 1971 to 1983 period.