Multiple stressors and stream ecosystem functioning: climate warming and agricultural stressors interact to affect processing of organic matter.
Measures of organic matter processing have been proposed as indices of stream ecosystem function to complement structural measures (water quality, taxonomic composition) in stream health monitoring. However, to be meaningful for ecosystem management, functional responses must be informed by knowledge of multiple stressors in the face of global change. We manipulated nutrient concentrations (two levels), fine sediment (two levels) and water temperature (eight levels, 0-6°C above ambient) in 128 streamside mesocosms to determine the individual and combined effects of these stressors on decomposition rates of fresh and pre-dried leaf packs and cotton strips. Raised temperature affected all seven studied decomposition measures. Leaf biomass loss rates were linearly positively affected, while strength loss rates of leaves and cotton strips showed nonlinear, weakly unimodal responses. Respiration of fresh leaves was positively affected by warming, but respiration of dried leaves showed the opposite pattern. Nutrient enrichment enhanced dried leaf biomass loss and respiration, and cotton tensile strength loss. Sediment addition increased loss of fresh leaf strength and biomass, but decreased dried leaf respiration. Interactions among stressors occurred in three cases. The positive effect of raised temperature on biomass loss of dried leaves was weaker in mesocosms with added sediment, the negative effect of sediment on dried leaf respiration annulled the positive effect of nutrients, and cotton strength loss increased strongly under enriched nutrients but less so when sediment was added. Synthesis and applications. Sediment greatly increased decay rates of fresh (living) leaves, but not of dried leaves or cotton strips. Leaf condition can strongly affect decomposition processes, with important consequences for streams that naturally receive freshly fallen leaves and have high sediment loads. Nonlinear responses of decomposition to rising temperature highlight how a warming climate may alter organic matter processing in complex ways. Our finding that dried leaf biomass loss responded positively to nutrient enrichment but not sediment addition, while fresh leaf biomass loss responded positively to sediment addition but not nutrient enrichment, suggests the two measures may offer complementary information in the functional assessment of multiple stressors in streams.