Running waters as ecosystems

Abstract Energy flow (primary and secondary production) and nutrient cycling (decomposition and nutrient regeneration) are ecosystem processes. Rivers both fix carbon and mineralise it, importing more from the catchment (allochthonous) than they fix (autochthonous) and exporting more than they respire. The balance between primary production and ecosystem respiration varies. Light usually limits primary production, with a lesser role for nutrients. Respiration depends on temperature and a supply of easily decomposed organic matter. Adding nutrients accelerates decomposition. Much terrestrial carbon is imported into rivers, transformed and transported downstream, but is animal production mainly fuelled by allochthonous or autochthonous material? Retention within the channels is required for breakdown. Microbes, and (initially) particularly fungi, mineralise detritus. Detritivorous animals reduce its particle size. Despite the important role of detritus in forested headwaters, animals also require algal resources. Carbon is also exchanged between the riparian system and the stream via feeding links across the habitat interface.