Effect of Nutrient Enrichment and Large Benthic Consumers On Stream Ecosystem Structure

Stream ecosystem structure can be strongly influenced by both bottom-up and top-down effects and these effects may be greater in nutrient poor systems.  We examined the joint effect of nutrients and large consumers on benthic community structure in a manipulative field experiment conducted in a low nutrient, second order Ozark stream.  We installed quadrats containing consumer (electrically excluded large consumers or open to consumers) and nutrient treatments (nutrient enriched or control tiles) in a fully factorial design.  Tiles were collected from the stream after 28 days and response variables measured from each tile were chlorophyll a, Chironomidae biomass, ash free dry mass, and dry mass.  Large consumers excluded were mainly Meek’s crayfish and central stonerollers, both omnivores, but affecting different components of benthic structure.  Nutrient enrichment and consumer exclusion had significant interactive effects on benthic structure.  The interaction was caused by strong consumer effects in control nutrient treatments, and no consumer effects in nutrient enrichment treatments.  In control treatments, consumers reduced ash free dry mass, dry mass, and chironomid biomass, with no effect of consumers in nutrient treatments.  We found that nutrient enrichment can reduce effects of benthic consumers and this suggests that nutrient enrichment could alter food web dynamics and energy flow in stream systems other than typical bottom-up effects.  This has important implications for systems likely to become nutrient enriched.  Food webs and benthic structure may be dramatically altered by nutrient enrichment of nutrient poor Ozark streams.