Carbon in catchments: connecting terrestrial carbon losses with aquatic metabolism

Abstract

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 heterotrophy can be sustained only if aquatic respiration is subsidized by organic inputs from the catchment. Such subsidies imply that organic materials that escaped decomposition in the terrestrial environment must become susceptible to decomposition in the linked aquatic environment.

Using a moderate-sized catchment in North America, the Hudson River (catchment area 33500 km²), evidence is presented for the magnitude of net heterotrophy. All approaches (CO₂ gas flux; O₂ gas flux; budget and gradient of dissolved organic C; and the summed components of primary production and respiration within the ecosystem) indicate that system respiration exceeds gross primary production by ~200 g C m^-2 year^-1. Highly ¹⁴C-depleted C of ancient terrestrial origin (1000–5000 years old) may be an important source of labile organic matter to this riverine system and support this excess respiration. The mechanisms by which organic matter is preserved for centuries to millennia in terrestrial soils and decomposed in a matter of weeks in a river connect modern riverine metabolism to historical terrestrial conditions.