Water depth influences algal distribution and productivity in shallow agricultural lakes

Abstract Agricultural activity can alter water quality and quantity resulting in lakes facing increasing interactive stressors including eutrophication, suspended sediment and water withdrawal. This study examined the relationships between water depth, water quality and algae in shallow agricultural lakes in the lower Mississippi River Basin (LMRB) to assess how lake depth influences algal structure and function, focusing on factors that typically regulate growth, that is, light, temperature and nutrients. Lake water and sediment cores were collected in shallow (mean = 0.8 m) and deep (mean = 1.7 m) locations in three lakes seasonally for 1 year. Additionally, interactive effects of light and temperature on algal growth were investigated through a laboratory experiment to determine the relative importance of each in regulating algal biomass and net oxygen production. In these shallow, turbid, eutrophic lakes, deeper water had lower nutrient concentrations and temperature across all seasons and clearer water in the summer. Shallow areas had more phytoplankton and periphyton. Water temperature had a stronger correlation than light with both phytoplankton and periphyton biomass. Algal biomass generally decreased with increasing water temperature, but biomass was better predicted by increasing nitrogen availability, and productivity declined with greater phosphorus availability. Water depth was therefore likely influencing different locations of the same lake towards different algal stable states. Mitigating excess algal production is an important goal towards limiting hypoxic conditions in the LMRB, and increasing water storage could help moderate temperature, nutrients and turbidity effects that contribute to algal blooms in lakes receiving agricultural runoff.