Planktonic Communities in Florida Softwater Lakes of Varying pH

In 20 softwater Florida lakes (pH range 4.7–6.8), phytoplankton diversity was significantly higher in the less acidic lakes; mean numbers of species in two groups (1, pH < 5.3; 2, pH > 5.6), each consisting of 10 lakes, was 10.8 and 16.5, respectively. Blue-green algae were dominant in less acidic lakes, while green algae were the most common group in acidic lakes. Phytoplankton abundance also was lower in acidic lakes, but covariation of phosphorus concentrations with pH confounded interpretation. Trends in zooplankton communities along the pH gradient were less pronounced. More acidic lakes had more species than the less acidic lakes, but three measures of species diversity showed no significant differences between the two groups. Zooplankton abundance was lower in the acidic lakes, but the relationship between abundance and pH showed much scatter. Lake productivity probably affects zooplankton abundance more than pH does in the sample group. No clear trends were observed in relative abundance of major zooplankton groups across the pH gradient. Six acid-tolerant species (including Diaptomus floridanus, Eubosmina tubicen, and Daphnia ambigua) were dominant in all 20 lakes.