Hudson River Fishes and their Environment

Hydrologic Control of External Carbon Loads and Primary Production in the Tidal Freshwater Hudson

Nina F. Caraco and Jonathan J. Cole


Abstract.—Organic material can enter aquatic systems by de novo gross primary production (GPP) from inorganic carbon within the system (autochthonous sources) or by import from the surrounding watershed and upstream aquatic systems (allochthonous sources). We analyzed the spatio–temporal variation in these two organic carbon sources for the 150-km long tidal freshwater Hudson during 2 years (1999 and 2000). Gross primary production was calculated from dawn– dusk sampling at 2-km intervals over the length of the Estuary for dissolved oxygen, pH, and DIC. Allochthonous inputs (All) were also calculated for the same periods based on organic carbon measurements and hydrologic flows. The relative importance of these two organic carbon sources (GPP/All) varied by over 30-fold across space and time. This variation was related to hydrologic inputs; on average 1999 (the second driest summer in the past 15 years) had 10- fold greater GPP/All than did 2000 (the wettest summer in 15 years). Associated with the high GPP/All in 2000 there was higher carbon dioxide supersaturation and net heterotrophy as compared to 1999. We suggest that hydrologically driven variation in GPP/All may also impact the food web and the dominant organic carbon source for fish and invertebrate consumers in the river. Natural abundance 14C measurements could potentially trace these two food sources as they vary in space and time.