Interactive Influence of Turbidity and Light on Larval Bluegill (Lepomis macrochirus) Foraging

In a series of in situ enclosure experiments with larval bluegill (Lepomis macrochirus), we demonstrate that turbidity from suspended sediments reduces bluegill consumption of crustacean zooplankton, primarily cyclopoid copepods and copepod nauplii. However, this reduction occurred only when light intensity in parts of enclosures fell below a threshold, estimated at < 450 lx. Following recent studies demonstrating copepod diel vertical migration in response to predators, it appears that copepods in our experiments used low-light strata as a refuge. Without this apparent refuge present, larval bluegill consumption increased with increasing turbidity, but prey were smaller on average. Thus, prey biomass consumed by larval bluegill did not differ with turbidity in high-light conditions. We postulate that the shift to smaller prey across taxa at higher turbidity, when light intensity exceeded 450 lx, derives from increased prey–background contrast. In low-light conditions, larval bluegill consumed larger, but fewer, zooplankton with increasing turbidity, resulting in lower prey biomass consumed. Thus, we demonstrate the field conditions causing negative turbidity effects on larval fish foraging success, and thus growth and recruitment.