Interactive effects of phosphorus and zooplankton grazing on cyanobacterial blooms in a shallow temperate lake

Cyanobacterial blooms are becoming increasingly common worldwide. These blooms can be moderated by grazing and/or nutrient availability, but the interactive effects of these processes are not well understood. We examined the interactive effects of phosphate and copepods on growth of phytoplankton (algae and cyanobacteria) in a shallow temperate lake. Field sampling of nutrient and chlorophyll a concentrations was conducted weekly from May through October, 2013. Five two-factorial experiments, spanning pre-, mid-, and post-bloom periods, were conducted with unfiltered lake water incubated with amended copepods, phosphate, or both. Changes in chlorophyll a concentration were used to calculate net phytoplankton biomass growth rates, and cell counts were performed on selected experiments to calculate growth rates of six microplanktonic taxonomic groups. Field data revealed cyanobacterial bloom development in July and decline in September. Experimental results indicated that phytoplankton growth increased with added phosphate pre-bloom, and decreased with added copepod grazers post-bloom, but that a more complex interactive (phosphorus x copepods) effect was observed immediately prior to and during peak bloom times. More specifically, the addition of phosphate and copepods enhanced ciliate growth pre-bloom, while selective grazing by copepods reduced dinoflagellate growth mid-bloom, possibly enhancing cyanobacterial growth and bloom duration via trophic cascade effects.