Spatial and temporal variation in the effects of fish and crayfish on benthic communities during stream drying

Omnivorous central stonerollers (Campostoma anomalum) and crayfish (Orconectes meeki meeki) inhabit streams in the Boston Mountain ecoregion of Arkansas, USA, and can have substantial effects on stream ecosystem structure and function through foraging and ecosystem engineering activities. Seasonal summer drying is extensive in these systems and might alter interactions between these consumers and stream benthic communities. Electrical exclusion of consumers was used to examine changes in the effects of fish and crayfish on algal abundance, sediment deposition, sediment organic matter content (as ash-free dry mass [AFDM]) and invertebrate density on tiles during stream drying. Replicate paired exclusion/exposed treatments were placed in 8 pools for two 30-d periods in June and August 2006 during an extended period of stream drying. Consumer exclusion increased algal chlorophyll a 1.5X, sediment organic matter content 2.3X, sediment dry mass 3.4X, and chironomid density 21 X relative to levels on exposed tiles. Exclusion effects were greater in August than in June, but effects were not evident in all pools. In June, the strength of consumer effects on chlorophyll a was positively related to % canopy cover, whereas effects on dry mass were positively related to crayfish density, fish predator presence, and % canopy cover but negatively related to depth. In August, the strength of consumer effects on chlorophyll a was positively related to fish predator presence and effects on AFDM were positively related to stoneroller density. These results demonstrate the importance of environmental context when determining how fish and crayfish influence stream functioning and suggest that mechanisms that generate heterogeneity in streams deserve greater attention, especially because disturbances, such as stream drying, are predicted to intensify with climate change.