Bacterioplankton dynamics and organic carbon partitioning in the lower Hudson River estuary

Surface water samples collected at 10 stations in April 1996 along the enti re salinity gradient of the Hudson River estuary were fractionated into par ticulate (POC), dissolved (DOC; <0.45 mu m), high molecular weight (HMWOC; 10 kDa-0.45 mu m), and low molecular weight (LMWOC; <10 kDa) organic carbon . Bacterial concentrations, production and specific growth rates were also determined at each location. While HMWOC (6 to 26 mu M) exhibited nonconser vative removal relative to ideal dilution of river and seawater along the e stuary, DOC (176 to 324 mu M) showed a nonconservative excess along this sa linity gradient. These contrasting distributions suggest that the majority of DOC was exported to the ocean and consisted of low lability material, wh ile a reactive fraction of HMWOC was removed during estuarine mixing. Bacte rial abundances (5 to 16 x 10(8) cells 1(-1)), production (3.4 to 28.7 pg C l-1 d(-1)), and specific growth rates (0.09 to 0.66 d(-1)) varied significa ntly along the salinity gradient. These variables were positively correlate d with algal standing stocks (chlorophyll a) and even more coherent with HM WOC distributions. In contrast, bacterial metabolism varied independently o f POC, DOG, and LMWOC concentrations. Therefore, while HMWOC accounted for <10% of the DOG, this pool appeared to be very dynamic, possibly due to bac terial degradation. However, mass balance estimates indicate that bacterial uptake could remove at most 30% of the HMWOC, suggesting that abiotic proc esses such as flocculation are probably the major removal mechanism of HMW organic matter within the estuary. Lastly, contrary to previous results fro m the tidal freshwater section of the Hudson, strong coherence between prim ary and secondary production and the nonconservative excesses of DOC found in the lower estuary suggest that carbon and bacterial dynamics can vary qu alitatively along different reaches of this river.