Climate modulates the magnitude of the effects of flow regulation on leaf-litter decomposition

The need of water for human use has led the impact on running waters of flow regulation to be of a global-scale. Although the effects of this impact have been widely investigated, efforts have been focused on large dams, so information about small reservoirs and their effects on ecosystem functioning is lacking. A recent collaborative project (IMPARIOS) addressed the effects of flow regulation by small impoundments on leaf-litter decomposition, a key function in low order streams which contributes greatly to the global carbon cycle. Flow regulation was found to affect ecosystem functioning reducing decomposition rate by altering shredders, but the magnitude of change varied among the different sub-climatic regions. The current project examined whether climatic variables modulate the effect of flow regulation on decomposition. For this, 19 bioclimatic variables were studied in relation to the leaf-litter decomposition rate and associated variables (sporulation rate and richness of aquatic hyphomycetes, and richness, density and biomass of total macroinvertebrates and shredders) in 17 streams impacted by regulation structures distributed in four sub-climatic regions within Spain. Overall, decomposition was slower below structures and climate influenced the magnitude of reduction. Effect sizes were negatively related to the seasonal changes in temperature and precipitation and to the general water deficit of the locations. In the future, the forecasted increase of seasonality in precipitation and temperature and the expected increase of number of dams to meet the needs of growing population may exacerbate the effects of flow regulation, altering nutrient recycling and the carbon cycle globally.