Sunfish as zooplankton control agents to improve yields of wastewater-cultivated algae

Wastewater algal cultivation (WAC) in outdoor raceway ponds is a promising means of removing polluting nutrients from water while producing biomass feedstock for products like fertilizer or biofuel. However, algal cultivation in open ponds is often plagued by underproduction due to contamination by zooplankton. The use of biological controls, i.e., natural predators to control zooplankton, was evaluated as a potential low-cost solution in this study. Eighteen pilot-scale raceway ponds were inoculated with cultured algae and naturally-occurring zooplankton; bluegill sunfish (Lepomis macrochirus) were tested as biological control agents to suppress zooplankton. We installed cages in the ponds to protect the fish from the paddlewheels and control the area the fish could access. The cage either surrounded the paddlewheel - in which case the fish had access to the remaining area - or the cage was opposite the paddlewheel, in which case the fish had access to the smaller area inside the cage. We also implemented fishless controls. After 19 days, ponds with cages surrounding the paddlewheel had 93% more zooplankton, but the fish had reduced zooplankton by 89% on average across cage treatments. Cage placement did not affect algal biomass, but ponds with fish had 45% higher algal dry weight and 84% higher algal biovolume. However, no differences in nutrient levels were recorded among treatments after 19 days. We concluded hardy zooplanktivorous fish represent a promising means of boosting algal production in WAC, although further research is needed to determine whether the change in algal biomass could eventually translate to improved nutrient removal.