Organic litter dynamics in headwater streams draining contrasting land uses

Climate change could alter fluxes of organic matter and macronutrients through freshwater ecosystems potentially affecting stream organisms. However, riparian controls on litter dynamics offer an opportunity to adapt headwaters to climate change by protecting or restoring riparian vegetation. We assessed how riparian land cover and climatic variability affected the supply, retention and downstream transport of particulate organic matter (POM) in headwaters—the most extensive small water bodies in temperate landscapes. Leaf litter inputs, benthic stocks and suspended organic matter were measured nominally monthly in second–third-order streams draining broadleaf woodland, conifer, acid moorland and circumneutral moorland over four years with varying discharge. Streams draining broadleaf woodland received more leaf litter from the riparian zone than conifer and moorland and transported higher concentrations of CPOM and FPOM at base flows. Broadleaf sites had higher CPOM stocks, even after hydrological events that reduced CPOM in conifer and moorland sites. In contrast, FPOM dynamics reflected hydrological conditions irrespective of land cover. These results show how some organic matter fractions in streams are sensitive to hydrological conditions, illustrating how wetter climates will influence FPOM exports. Nevertheless, riparian broadleaves have the potential to offset climatic effects on organic matter processing in headwaters through the replenishment and retention of CPOM.