Dissolved organic matter and its utilization by bacteria during spring in the Southern Ocean

Concentrations of dissolved organic carbon (DOC) and nitrogen (DON) were measured during early austral Spring 1992 at a number of stations along the 6 degrees W meridian between 47 degrees and 60 degrees S. This included the Polar Front in the north, the zone of melting sea-ice in the south, and waters of the Antarctic Circumpolar Current in between. Concentrations of DOC were low in deep water (34-38 mu M) With generally similar or slightly higher values in the surface mixed layer (38-55 mu M). DOC:DON ratios are wider in surface water than in deep water, i.e. surface accumulations contain relatively C-rich dissolved organic matter. The highly variable distribution of the surface DOC was not related to hydrographic or biotic features (fronts, plankton development) indicating the lability and transient occurrence of this material. Growth rates of bacteria were determined in subsamples from 51 0.8-mu m-filtered batches of seawater incubated in the dark at in-situ temperature. Thymidine and leucine uptake and bacterial biomass change as well as changes in dissolved organic carbon in the batches, and oxygen consumption in parallel incubations correlated linearly over 2 weeks of incubation which allowed extrapolation to in-situ conditions. Bacterial growth in these experiments depended strongly on the amount of initial DOG. Growth in water from greater depth (1000 m) containing 38 mu M DOG was minimal, as were DOG-decrease and oxygen consumption. Higher rates were observed in surface water slightly enriched with DOG, and highest rates in surface water amended with DOG-rich melted sea ice. Bacterial growth efficiencies (biomass C-increase vs DOC consumed) were about 30%. The experiments showed that at least 40-60% of the DOC in excess of deep water concentrations was available to bacteria. (C) 1997 Published by Elsevier Science Ltd.