Bacterial transformation and transport of organic matter in the Southern California Bight

Abstract

The role of bacteria in transformation and transport of organic matter was studied in the Santa Monica Basin (SMB) during 1985–1990 as part of the California Basin Study (CaBS). Bacterial carbon in the euphotic zone was generally 0.5–2.0 times the phytoplankton carbon and a substantial fraction (14–62%) of the total particulate organic carbon (POC). In the euphotic zone, bacterial carbon demand was generally equivalent to 30–60% of the primary production. Below the upper mixed layer, bacterial carbon demand was calculated to be equivalent to a major fraction (≥80%) of the depth-dissipation of sinking carbon. Since bacteria contribute substantially to the POC pool and its dynamics, the role of bacteria as organic particles should be taken into account in studies of the biogeochemical and optical behavior of particulate carbon in the sea. Our measurements of concentrations and bacterial utilization of dissolved free (DFAA) and dissolved combined amino acids (DCAA) have lead to the realization of a virtual uncoupling between bacteria and DCAA at depths below ∼200. The DCAA pool was quite large (554–1057nM glycine equivalents) and did not vary much with depth. Despite the presence of a large DCAA pool bacterial carbon demand was very low. Even if DCAA were the onky pool of utilizable dissolved organic carbon (DOC) bacterial turnover time of DCAA pool would be on the order of 10 4d (compared with <10d in the surface waters). Calculations for the SMB show bacterial surface area to be the dominant biotic surface area, suggesting bacteria may play an important role in radionuclide biodynamics. In view of the quantitative significance of material flow through the microbial loop in SMB ecosystem, it is possible that protozoan predation on bacteria is a significant pathway for radionuclide remineralization and solubilization.