In‐stream structure alters density‐dependent fish effects in stream ecosystems

The independent effects of in‐stream structure (ISS) and fish foraging on stream properties have been well documented, but few studies have explored the interactive effects of ISS and fishes on streams. Herein, we tested the independent and interactive effects of ISS and a generalist fish (Blacktail shiner, Cyprinella venusta) on suspended organic matter (SOM), benthic algae, invertebrate density and fish growth using experimental mesocosms. We found that Blacktail shiner foraging affected all of the ecosystem properties; however, in some cases, the fish effects differed between mesocosms with and without ISS. Specifically, mesocosms with ISS provided greater surface area for invertebrate colonisation and enhanced food resources for Blacktail shiner. As a result, benthic foraging by Blacktail shiner was reduced in these mesocosms. The reduced benthic foraging in turn enhanced benthic algae and benthic invertebrates via a bottom‐up, nutrient excretion pathway. The ISS‐dependent effects of fish on these stream properties, however, were only evident at low and intermediate fish densities (1 and 2 fish·m^?2 respectively). This was likely because at the highest fish density (4 fish·m^?2) intense fish foraging overrode any mediating effects of ISS. Furthermore, fish growth decreased with fish density because of intraspecific competition, but this negative effect on growth was reduced in mesocosms with ISS because of the increased forage base. However, the positive effect on fish growth was weak and only marginally significant. Our data suggest that fish‐mediated effects on streams are context dependent, changing with microhabitat availability (e.g. ISS) and density of the fish population.