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

Abstract

Surface water samples collected at 10 stations in April 1996 along the entire salinity gradient of the Hudson River estuary were fractionated into particulate (POC), dissolved (DOC; <0.45 μm), high molecular weight (HMWOC; 10 kDa-0.45μm), 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 μM) exhibited nonconservative removal relative to ideal dilution of river and seawater along the estuary, DOC (176 to 324 μM) showed a nonconservative excess along this salinity gradient. These contrasting distributions suggest that the majority of DOC was exported to the ocean and consisted of low lability material, while a reactive fraction of HMWOC was removed during estuarine mixing. Bacterial abundances (5 to 16 x 108 cells l-1), production (3.4 to 28.7 μg C l-1 d-1), and specific growth rates (0.09 to 0.66 d-1) varied significantly along the salinity gradient. These variables were positively correlated with algal standing stocks (chlorophyll a) and even more coherent with HMWOC distributions. In contrast, bacterial metabolism varied independently of POC, DOC, and LMWOC concentrations. Therefore, while HMWOC accounted for <10% of the DOC, this pool appeared to be very dynamic, possibly due to bacterial degradation. However, mass balance estimates indicate that bacterial uptake could remove at most 30% of the HMWOC, suggesting that abiotic processes such as flocculation are probably the major removal mechanism of HMW organic matter within the estuary. Lastly, contrary to previous results from the tidal freshwater section of the Hudson, strong coherence between primary and secondary production and the nonconservative excesses of DOC found in the lower estuary suggest that carbon and bacterial dynamics can vary qualitatively along different reaches of this river.