Differential supply of autochthonous organic carbon and nitrogen to the microbial loop in the Delaware Estuary

Abstract

Using stable isotope tracer techniques in 4-h bottle incubations, the importance of organic matter transfer from phytoplankton to heterotrophic bacteria (bacteria) has been re-evaluated in the Delaware Estuary, considering carbon (C) and nitrogen (N) cycles separately. The hypothesis is that the transfer of C and N from phytoplankton to bacteria varies both temporally and spatially along estuarine gradients in response to variation in factors such as terrestrial organic C supply, inorganic N speciation and concentrations, and extracellular release of dissolved organic matter by phytoplankton. The percentage of autochthonous dissolved organic C being assimilated by bacteria varied between 3% and 10% of primary production and was not related to the rate of primary production. The transfer of N was considerably more variable when compared to C transfer, averaging ca. 20% of phytoplankton N assimilation; individual experiments yielded rates as high as 50%. Unlike C, autochthonous dissolved organic N transfer appears to vary with the magnitude of primary production, and its assimilation by bacteria accounted for 0-56% of the total measured bacterial N uptake. The results highlight the importance of separate consideration of C and N elemental cycles in evaluating sources of organic matter to the estuarine microbial loop.