Kelps are highly productive seaweeds found along most temperate latitude coastlines, but the fate and importance of kelp production to nearshore ecosystems are largely unknown. The trophic role of kelp-derived carbon in a wide range of marine organisms was assessed by a natural experiment. Growth rates of benthic suspension feeders were greatly increased in the presence of organic detritus (particulate and dissolved) originating from large benthic seaweeds (kelps). Stable carbon isotope analysis confirmed that kelp-derived carbon is found throughout the nearshore food web.

Origins and fates of organic carbon in estuarine and nearshore marine food webs of Hood Canal, a fjord extension of Puget Sound, Washington (USA), were evaluated seasonally using stable carbon isotopes (6I3C). 613C was measured in: (1) autotrophic sources in terrestrial, estuarine, and nearshore marine habitats; (2) particulate and dissolved organic carbon (POC, DOC) in riverine, estuarine, and nearshore marine waters; (3) primary and secondary consumers in selected estuarine and marine littoral and neritic food webs. Autotrophic sources in terrestrial, riverine, salt marshmeadow, and neritic habitats were generally more I3C-depleted (averaging -30, -26, and -22%0, respectively) than sources in estuarine and marine littoral habitats ( -8 and -23"A). The isotopic composition of neritic consumers indicated that their food sources originated directly from phytoplankton production. In contrast, the 613C of primary and secondary consumers (-11 %O to -2 3 x 0 ) in estuarine and marine littoral habitats suggested that the most important sources of organic carbon to these food webs were autochthonous, originating primarily from eelgrass, epiphytes, and macroalgae. Estuarine detritus deposits, sestonic POC, and sediments had widely ranging 613C values (-9.7 to -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 A 8 O/m occurred in autotrophs, DOC, and some consumer organisms in estuarine and nearshore marine habitats during winter. This seasonal depletion in 813C originates from variable imports of dissolved inorganic carbon (DIC) transported by freshwater and may reflect the influence of extensive freshwater storage in Hood Canal during fall and winter. These results indlcate 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.

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Summary1. Biogeochemical tracers such as stable isotopes are often used to determine sources and pathways of organic matter through food webs, which may provide powerful indicators of environmental stress or insight into resource management issues. However, mixing models using such tracers are algebraically constrained and rarely attempted for more than two sources. This makes them of limited value for ecosystems that have numerous contributing sources. 2. We developed two complementary mixing models, SOURCE and STEP, which use linear programming techniques with multiple tracers, to estimate the dominant primary producer sources of consumers, and their diets and trophic levels, regardless of the number of sources and trophic steps. 3. SOURCE is used to estimate consumers' direct and indirect uptake of autotrophic sources and their trophic levels. STEP calculates an estimate of a consumer's diet, which may include autotrophs and/or heterotrophs. 4. The two models were tested using simulated data sets of producer and consumer tracer values and then used with published δ 13 C, δ 15 N and δ 34 S data from a study tracing organic matter flows in a saltmarsh estuary. 5. SOURCE and STEP accurately estimated flows and trophic structures in the simulations, with average errors of 0·07 -0·09 for SOURCE and 0·03 -0·05 for STEP, depending on the number of tracers used. 6. We illustrate two resultant food webs for the saltmarsh estuary showing possible interpretations of the SOURCE and STEP estimates. 7. Synthesis and applications . SOURCE and STEP can be used with stable isotope data to estimate accurately consumers' trophic levels, primary producer dependence and diets, even when the number of potential autotrophic sources or foods is larger than the number of tracers. SOURCE and STEP could be used to assess the roles played by individual species within food webs, to compare food webs across locations or over time, and to examine potential pollutant bioaccumulation in higher order consumers, among other potential applications. The  - code for both models is available at http:// staff.washington.edu/lubetkin.