Publication

We made direct measurements of the partial pressure of CO2 (P-CO2) in the tidal-freshwater portion of the Hudson River Estuary over a 3.5-yr period. At all times the Hudson was supersaturated in CO2 with respect to the atmosphere. P-CO2 in surface water averaged 1125 +/- 403 (SD) mu atm while the atmosphere averaged 416 +/- 68 mu atm. Weekly samples at a single, mid-river station showed a pronounced and reproducible seasonal cycle with highest values (similar to 2000 mu atm) in mid-to-late summer, and lowest values (similar to 500 mu atm) generally in late winter. Samples taken along the length of the 190-km section of river showed a general decline in CO2 from north to south. This decline was most pronounced in summer and very slight in spring. Diel and vertical variation were small relative to the standing stock of CO2. Over six diel cycles, all taken during the algal growing season, the mean range was 300 +/- 114 mu atm. CO2 tended to increase slightly with depth, but the gradient was small, about 0.5 mu mol m(-1), or an increase of 190 mu atm from top to within 1 m of the bottom. For a large subset of the samples (n = 452) we also calculated CO2 from measurements of pH and total DIC. Calculated and measured values of CO2 were in reasonably good agreement and a regression of calculated versus measured values had a slope of 0.85 +/- 0.04 and an r(2) of 0.60. Combining our measurements with recent experimental studies of gas exchange in the Hudson, we estimate that the Hudson releases CO2 at a rate of 70-162 g C m(-2) yr(-1) from the river to the atmosphere.