METABOLISM AND ORGANIC-CARBON FLUXES IN THE TIDAL FRESH-WATER HUDSON RIVER

We summarize rates of metabolism and major sources and sinks of organi c carbon in the 148-km long, tidally influenced, freshwater Hudson Riv er. The river is strongly heterotrophic, with respiration exceeding gr oss primary production (GPP). The P:R ratio averages 0.57 (defined as the ratio of GPP to total ecosystem respiration) if only the aquatic p ortion of the ecosystem is considered and 0.70 if the emergent marshes are also included. Gross primary production (GPP) by phytoplankton av erages approximately 300 g C cm(-2) yr(-1) and is an order of magnitud e greater than that by submersed macrophytes. However, the river is de ep, well mixed, and turbid, and phytoplankton spend a majority of thei r time in the dark. As a result, respiration by living phytoplankton i s extremely high and net primary production (NPP) by phytoplankton is estimated to be only some 6% of GPP. NPP by phytoplankton and submerse d macrophytes are roughly equal (approximately 20 g C m(-2) yr(-1) eac h) when averaged over the river. Emergent marshes are quite productive , but probably less than 16 g C m(-2) yr(-1) enters the aquatic portio n of the ecosystem from these marshes. Heterotrophic respiration and s econdary production in the river are driven primarily by allochthonous inputs of organic matter from terrestrial sources. Rates of metabolis m vary along the river, with depth being a critical controlling factor The P:R ratio for the aquatic portion of the ecosystem varies from 1 in the mid-river to 0.2 in the deeper waters. NPP is actually negative in the downstream waters where average depths are greater since phyto plankton respiration exceeds GPP there; the positive rates of NPP occu rring upriver support a downstream advection of phytoplankton to the d eeper waters were this C is largely respired away by the algae themsel ves. This autotrophic respiration contributes significantly to oxygen depletion in the deeper waters of the Hudson. The tidally influenced f reshwater Hudson largely Bts the patterns predicted by the river conti nuum model for larger rivers, However, we suggest that the continuum m odel needs to more clearly distinguish between GPP and NPP and should include the importance of autotrophic respiration by phytoplankton tha t are advected along a river. The organic carbon budget for the tidall y influenced freshwater Hudson is balanced to within a few percent. Re spiration (54%) and downstream advection into the saline estuary (41%) are the major losses of organic carbon from the ecosystem. Allochthon ous inputs from nonpoint sources on land (61%) and GPP by phytoplankto n (28%) are the major sources to the system. Agricultural erosion is t he major source of allochthonous inputs, Since agricultural land use i ncreased dramatically in the last century, and has fallen in this cent ury, the carbon cycle of the tidally influenced freshwater Hudson Rive r has probably changed markedly over time. Before human disturbance, t he Hudson was probably a less heterotrophic system and may even have b een autotrophic, with gross primary production exceeding ecosystem res piration.