Indirect versus direct effects of freshwater browning on larval fish foraging

Abstract

Fish foraging and energy flow between trophic levels are predicted to decline with freshwater browning due to decreased light availability. Studies investigating these predictions have focused on juvenile and adult fishes; however, the larval stage represents a “critical period” in development. We investigated the indirect (i.e., prey availability) versus direct (i.e., visual foraging) effects of browning on zooplankton–larval fish interactions by altering water colour with SuperHume (absorbance at 440 nm = 1.6–10.8 m−1). Phytoplankton and zooplankton densities were monitored across treatments for 1 month to simulate the time leading up to fish spawning in the spring. Larval largemouth bass (Micropterus salmoides) were then introduced to assess indirect effects on fish feeding, growth, and survival. Direct effects on largemouth bass and bluegill (Lepomis macrochirus) foraging on zooplankton were determined with separate short-term experiments. Browning did not directly alter the ability of larvae to capture prey. However, indirect effects on larval fish foraging, growth, and survival were observed as phytoplankton and zooplankton abundance decreased with increased browning. Our data suggest browning may reduce energy transfer to larval fish due to a reduction in prey but not visual foraging.