HIGH TURNOVER OF INORGANIC CARBON IN KELP HABITATS AS A CAUSE OF DELTA-C-13 VARIABILITY IN MARINE FOOD WEBS

In a study to assess qualitatively the importance of organic matter derived from kelp production in the Aleutian Islands of subarctic Alaska, replicated samples of autotrophic sources and primary and secondary consumer organisms were sampled for deltaC-13 among sources, sites, (treatment) islands, and years. Unanticipated variation in the deltaC-13 of kelps occurred among overtly similar sites at different islands. Variation in the deltaC-13 of the surface canopy-forming kelp Alaria fistulosa was particularly extreme, ranging from - 15.5 to - 28.0 9 parts-per-thousand compared to the understory kelps, Laminaria spp. A.fistulosa deltaC-13 varied by as much as 6 to 7 parts-per-thousand among similar sites at a given island within years, and by as much as 3 to 4 parts-per-thousand between years at the same sampling site. In several cases, deltaC-13 variation was weakly tracked by some consumer organisms, suggesting that even detritus pathways through the food web can be localized and tightly coupled. Dynamic cycles in the concentration and deltaC-13 of dissolved inorganic carbon (DIC) and aqueous CO2 concentration ([CO2]aq) were measured at three sites on one island. The deltaC-13 or organic carbon fixed by A. fistulosa, calculated from diurnal DIC concentration and deltaC-13 measurements, varied by 15 parts-per-thousand and varied inversely with [CO2]aq concentrations. Local DIC variability, probably resulting from high productivity and decreased turbulence in dense kelp habitats, provides a possible mechanism of variation in kelp deltaC-13. The short-term variability in the deltaC-13 of organic carbon fixed by kelps indicates that sampling methodology and design must assess this potential variation in marine macrophyte deltaC-13 before making assumptions about the transfer of deltaC-13-invariate organic matter to higher trophic levels. On the positive side, a predictable relationship between [CO2]aq concentration and kelp deltaC-13 offers a potentially robust means to assess productivity effects on CO2 limitation in kelps and other complex aquatic macrophyte habitats.