Carbon in catchments: connecting terrestrial carbon losses with aquatic metabolism

For a majority of aquatic ecosystems, respiration (R) exceeds autochthonous gross primary production (GPP), These systems have negative net ecosystem production ([NEP] = [GPP] - R) and ratios of [GPP]/R of <1. This net hetero trophy can be sustained only if aquatic respiration is subsidized by organi c inputs from the catchment. Such subsidies imply that organic materials th at escaped decomposition in the terrestrial environment must become suscept ible to decomposition in the linked aquatic environment. Using a moderate-sized catchment in North America, the Hudson River (catchm ent area 33 500 km(2)), evidence is presented for the magnitude of net hete rotrophy. All approaches (CO2 gas flux; O-2 gas flux; budget and gradient o f dissolved organic C; and the summed components of primary production and respiration within the ecosystem) indicate that system respiration exceeds gross primary production by <similar to>200 g C m(-2) year(-1). Highly C-14 -depleted C of ancient terrestrial origin (1000-5000 years old) may be an i mportant source of labile organic matter to this riverine system and suppor t this excess respiration. The mechanisms by which organic matter is preser ved for centuries to millennia in terrestrial soils and decomposed in a mat ter of weeks in a river connect modem riverine metabolism to historical ter restrial conditions.