Water level decrease and increased water stability promotes phytoplankton growth in a mesotrophic subtropical lake

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Abstract
Global warming and climate variability can promote changes in freshwater ecosystems that may result in increased frequency of algal blooms. These effects, however, depend strongly on local factors. Peri Lake is a cyanobacteria-dominated shallow lake that has experienced a strong reduction in water level, as well as increased occurrence of algal blooms. The goal of this study was to determine the influence of water level on phytoplankton biomass in Peri Lake. We hypothesise that increased algal blooms are associated with reduction in water level, possibly in response to changes in rainfall patterns. We measured meteorological, physical, and chemical variables over 5 years, as well as chlorophyll-a levels. We observed a strong reduction in total rainfall in the last year of the study, in which the flow of lake water nearly ceased. Phytoplankton biomass increased despite a lack of evidence of water heating or eutrophication. Our results suggest that changes in rainfall regime alter lake characteristics such as retention time and water stability (i.e. increased occurrence of diurnal stratification events), producing favourable conditions for growth of cyanobacterial biomass. These effects perhaps better explain increases in algal biomass in this system than do temperature or nutrient availability alone.

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