MICROBIAL-POPULATIONS IN SURFACE-FILMS - AMINO-ACID DYNAMICS IN NEARSHORE AND OFFSHORE WATERS OFF SOUTHERN CALIFORNIA

Microbial biomass and utilization (incorporation plus respiration) of dissolved free amino acids (DFAA) were studied in sea surface films or microlayers in nearshore and offshore waters off the coast of southern California during 1988 (September) and 1989 (October). Microbial biomass (bacterial numbers via cell counts and total microbial biomass via adenosine triphosphate concentrations), DFAA, and dissolved combined amino acids (DCAA) were always enriched in films compared with subsurface waters, with enrichment factors (ratio of microlayer value compared to 10-cm control value) often exceeding those measured previously in microlayers from diverse Atlantic and Pacific coastal and island environments. Higher enrichment factors were generally obtained for offshore stations than for nearshore sites. Glutamic acid utilization was greater in microlayers than in control waters at 9 of 13 stations. At those sites where higher utilization occurred in the 10-cm waters, the microbial populations were highly active and had been subjected to substantial wind mixing prior to sampling. Glutamic acid concentrations were increased in subsurface waters at these sites relative to other stations, apparently by the input of dissolved film constituents. Calculated turnover times were longer for films than for 10-cm waters, in large part because of the substantially larger glutamic acid pools in the films. Individual amino acids were not enriched in films compared with 10-cm waters Lo the same degree in both the DFAA and DCAA. The relative levels of the individual amino acids in the free and combined pools may have been influenced by their differential production or preferential utilization.