SOURCES AND REACTIVITIES OF AMINO-ACIDS IN A COASTAL MARINE-ENVIRONMENT

Amino acid analyses were performed on a suite of potential organic matter sources to coastal marine environments. They yielded no source-indicative compositional parameters, but microbes produced markedly higher carbon-normalized yields than did vascular plant tissues.

Sediment trap samples collected monthly for a year at 30 and 60 m in Dabob Bay, Washington, were also analyzed, as were subsamples from a sediment core taken at the same site. Amino acid yields from the trap samples exhibited spring and fall maxima that coincided with blooms, and a winter minimum that resembled yields in underlying sediments. Amino acids in all samples were primarily of marine origin and low yields in winter traps resulted from low planktonic production, selective amino acid loss, and resuspension of amino acid-depleted sediments. Nonprotein amino acid levels and the fraction of total N represented by amino acids are indices of diagenetic alteration. Amino acids accounted for 13-37% of the total organic C and 30-81% of the total N in the 30-m samples, and, on average, 10 and 37% of sedimentary C and N.

Midwater amino acid fluxes also displayed a winter minimum and spring and fall maxima, they were more pronounced than fluxes of bulk particles or C, due to a lesser influence of resuspension. On average, approximately 80% of the total midwater particulate amino acid flux was lost prior to incorporation in surface sediments. Amino acids were degraded selectively and represented 35 and 71% of the C and N remineralized at the benthic interface. The labile fraction closely resembled fresh plankton in its amino acid composition and carbon-normalized yield.

Although trapping of poisoned zooplankton may have caused some artificially high amino acid yields, diagenesis was apparently the primary control on calculated reactivities. Among the amino acids, reactivity patterns indicated relative preservation of diatom cell-wall protein. Within the sediments, amino acids were again degraded preferentially relative to organic C and N, but no downcore compositional changes were observed. Midwater and sediment burial fluxes of planktonic amino acids represented, respectively, approximately 13 and 2.5% of annual mean amino acid production by primary producers.