δ¹³C evidence of the origins and fates of organic carbon in estuarine and near-shore food webs

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Abstract
Origins and fates of organic C in estuarine and nearshore marine food webs of Hood Canal, a fjord extension of Puget Sound, Washington (USA), were evaluated seasonally using stable C isotopes (.delta.13C). .delta.13C was measured in: autotrophic sources in terrestrial, estuarine, and nearshore marine habitats; particulate and dissolved organic C (POC, DOC) in riverine, estuarine, and nearshore marine waters; primary and secondary consumers in selected estuarine and marine littoral and neritic food webs. Autotrophic sources in terrestrial, riverine, salt marsh-meadow, and neritic habitats were generally more 13C-depleted (averaging -30, -26, and -22%, respectively) than sources in estuarine and marine littoral habitats (-8 and -23%). The isotopic composition of neritic consumers indicated that their food sources originated directly from phytoplankton production. In contrast, the .delta.13C of primary and secondary consumers (-11%- -23%) in estuarine and marine littoral habitats suggested that the most important sources of organic C to these food webs were autochthonous, originating primarily from eelgrass, epiphytes, and macroalgae. Estuarine detritus deposits, sestonic POC, and sediments had widely ranging .delta.13C values (-9.7- -28.9%), indicating diverse origins. However, POC associated with surface foam in estuarine littoral waters and DOC in marine littoral waters both had isotopic values most similar to estuarine autotrophs and consumers, suggesting the potential importance of dissolved and foam organics in food-web pathways. Depletion as high as .DELTA. 8% occurred in autotrophs, DOC, and some consumer organisms in estuarine and nearshore marine habitats during winter. This seasonal depletion in .delta.13C originates from variable imports of dissolved inorganic C (DIC) transported by freshwater and may reflect the influence of extensive freshwater storage in Hood Canal during fall and winter. These results indicate the need for further elucidation of the complex biological and chemical mechanisms of formation, cycling, and degradation of particulate and dissolved organics from autochthonous sources to estuarine and nearshore marine food webs.