Environmentally‐mediated consumer control of algal proliferation in Florida springs

Abstract

Summary Reduced grazing can lead to increases in autotroph biomass and changes in taxonomic composition similar to those associated with nutrient enrichment. In Florida's iconic spring ecosystems, algal proliferation has become widespread, yet the causes remain unclear. We tested three linked hypotheses: (i) loss of top‐down grazer control explains algae proliferation, a change often attributed to nitrate enrichment; (ii) grazer control is mediated by dissolved oxygen (DO) concentration; and (iii) an algal‐dominated state may persist if biomass exceeds a critical level beyond which grazers can no longer constrain accumulation. We tested these hypotheses using hierarchically nested benthic surveys of algal, vascular plant and gastropod biomasses along with physicochemical measurements in eleven springs spanning gradients in nitrogen enrichment, algal cover and DO. We observed a significant and temporally consistent negative association between algal and gastropod biomasses (R2 = 0.38), with gastropods displaying the strongest explanatory power in multivariate prediction models that explained 45% of algal variation in the best fitted model. A modest but significant positive bivariate association was observed between gastropod biomass and DO (R2 = 0.23); a multivariate model including temperature, velocity, canopy cover, submerged aquatic vegetation and conductivity explained 56% of gastropod variation. Residuals from a linear relationship between gastropod and algal biomasses were strongly bimodal above a threshold grazer biomass of 20 g dry weight m−2 (c. 235 snails m−2) suggesting alternative states of high and low algal abundance. These observations support the hypothesis that gastropods have the potential to control benthic algae in Florida's springs, identify DO as a partial explanation for variation in grazer abundance and imply potentially important hysteretic behaviour in top‐down algal control.