Do Algal Blooms Reduce Grazing by Having High Oxygen or High pH conditions?

Daphnia magna mortalities were associated with elevated oxygen and pH during experiments with high initial nitrate concentrations, resulting in high algae concentrations (Chl a) in Closed Ecological Systems. With lower initial nitrate concentrations, Daphnia populations survived much longer and grazed the algae to low abundance (Chl a), with lower oxygen and pH levels.  These simple experimental systems do not allow exchange with the open atmosphere, and conditions probably exceed those observed in natural systems. Given the increased frequency of reported algal blooms, are oxygen and pH increased, and are there any observations that zooplankton are avoiding edible algal blooms, reducing grazing pressure?  Our Closed Ecological Systems are 250 ml to 1-liter systems containing 3 species of green algae (Ankistrodesmus, Scenedesmus, and Selenastrum) that are presumed non-toxic and suitable foods for the grazer, Daphnia magna. Unidentified microbes are also present. The medium is chemically defined. Given high nitrate, pH can exceed 10, and oxygen can exceed 28 mg/l (the limit of our instrumentation). In naturally occurring blooms, unusually high pH and oxygen could be irritating to zooplankton, even if nonlethal, and might interfere with normal zooplankton diurnal migration behavior. In lakes with automated pH, oxygen, and Chl a monitoring, are measurements during algal blooms available, and is there evidence that grazer behavior is modified during edible algal blooms?  

Presentation Preference: Oral

Primary Presenter: Frieda Taub, University of Washington (taub@uw.edu)

Authors:

Frieda Taub, University of Washington  (taub@uw.edu)

Kate McGrath-Flinn, University of Washington  (katemf16@uw.edu)

Nattlie Stillwell, University of Washington  (nstill77@uw.edu)

Rachel Kasbohm, University of Washington  (rkasbohm@uw.edu)