Indirect versus Direct Effects of Freshwater Browning on Larval Fish Foraging

Abstract

Fish foraging and energy flow are both predicted to decline with freshwater browning due to reductions in light availability. Studies investigating these predictions have focused on juveniles and adults; however, the larval stage represents a critical period in fish development. We investigated the indirect versus direct effects of browning on zooplankton-larval fish interactions by altering water color with SuperHume (absorbance at 440 nm = 1.6 - 10.8 m-1). Phytoplankton and zooplankton densities were monitored across experimental tanks in the laboratory for one month leading up to fish spawning. Larval largemouth bass were then introduced to assess indirect effects on fish feeding rates and growth. Direct effects on foraging of largemouth bass and bluegill were determined with separate short-term feeding experiments. Browning did not directly alter the ability of larval fish to capture prey. However, significant indirect effects on larval fish foraging, growth, and survival were observed as phytoplankton and zooplankton decreased with increased browning. Our data suggest lake browning will reduce energy transfer to larval fish due to a reduction in prey availability but not visual foraging.