Increasing algal biomass in Lake Vänern despite decreasing phosphorus concentrations: A lake-specific phenomenon?

Abstract

Algal blooms in lake waters are frequently associated with elevated nutrient levels, in particular phosphorus. Here we show, with long-term data from Sweden’s largest lake, Lake Vänern, that total phytoplankton biomass has significantly increased since the 1980s at the same time as total phosphorus and inorganic nitrogen concentrations have significantly decreased (non-parametric Mann-Kendall test: P < 0.01). Lake Vänern was not the only lake showing such a pattern: evaluating long-term data from 13 small oligotrophic reference lakes revealed the same pattern. Other synchronous changes observed in Sweden’s oligotrophic lakes included strongly decreasing sulfate concentrations and increasing water temperatures. We found that alkalinity, water temperatures, silica and sulfate concentrations were the strongest predictors for total phytoplankton biomass at all three sites in Lake Vänern when we used partial least squares regression models with 11 input variables. From our results, we suggest that decreasing acidity in combination with increasing water temperatures has been more important for phytoplankton growth than total phosphorous and nitrogen concentrations. Thus, we conclude that phytoplankton biomass in oligotrophic small and large lakes is strongly associated with climate and atmospheric depositional changes, and consequently needs to be managed beyond catchment scales.