High turnover of inorganic carbon in kelp habitats as a cause of ?13C variability in marine food webs

Abstract

In a study to assess qualitatively the importance of organic matter derived from kelp production in the Aleutian Islands of subarctic Alaka, replicated samples of autotrophic sources and primary and secondary consumer organisms were sampled for δ13C among sources, sites, (treatment) islands, and years. Unanticipated variation in the δ13C of kelps occurred among overtly similar sites at different islands. Variation in the δ13C of the surface canopy-forming kelp Alaria fistulosa was particularly extreme, ranging from-15.5 to-28.0‰ compared to the understory kelps, Laminaria spp. A. fistulosa δ13C varied by as much as 6 to 7‰ among similar sites at a given island within years, and by as much as 3 to 4‰ between years at the same sampling site. In serveral cases, δ13C 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 δ13C of dissolved inorganic carbon (DIC) and aqueous CO2 concentration ([CO2]aq) were measured at three sites on one island. The δ13C or organic carbon fixed by A. fistulosa, calculated from diurnal DIC concentration and δ13C measurements, varied by 15‰ 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 δ13C. The short-term variability in the δ13C of organic carbon fixed by kelps indicates that sampling methodology and design must assess this potential variation in marine macrophyte δ13C before making assumptions about the transfer of δ13C-invariate organic matter to higher trophic levels. On the positive side, a predictable relationship between [CO2]aq concentration and kelp δ13C offers a potentially robust means to assess productivity effects on CO2 limination in kelps and other complex aquatic macrophyte habitats.