Linking light attenuation and suspended sediment loading to benthic productivity within an arctic kelp-bed community

Annual growth and productivity of kelp in the Stefansson Sound Boulder Patch, located along the Arctic coast of Alaska, is regulated almost entirely by PAR received during the summer open-water period. Increased water turbidity during summer, often in response to storm activity, has been linked to low levels of ambient PAR and measurable decreases in kelp elongation. However, the relationship between PAR and water transparency has not been quantified, which compromises efforts to assess the effects of changing climate and weather conditions on kelp production. During the 2001-2002 summer periods, the inherent optical properties (IOPs) of Stefansson Sound waters were measured in conjunction with total suspended sediments (TSS) concentrations, which differed significantly between the 2 years, for input into a radiative transfer equation (RTE). In both years, the highest TSS levels (24.2 and 18.5 mg · L⁻¹ in 2001 and 2002, respectively) occurred in nearshore areas and were coincident with increased beam attenuations (13.8 and 8.3 m⁻¹). Lower TSS concentrations and attenuations were measured offshore. Data input to the RTE provided a TSS-concentration-specific attenuation coefficient that was used in a productivity model to estimate annual kelp productivities throughout the Boulder Patch based on modeled irradiance and averaged site-specific TSS concentrations. Production estimates varied across the Boulder Patch but were lower in 2001 (0.12-0.34 g C · g dwt⁻¹ · year⁻¹, where dwt stands for dry weight) compared to 2002 (0.24-0.80 g C · g dwt⁻¹ · year⁻¹). Production in both years was greater in offshore locations with lower TSS loads. Results suggest that PAR availability during the summer is heavily influenced by TSS concentrations, and that changes in storm intensity and frequency, associated with current warming trends, may have significant effects on the primary production of these unique benthic algal communities.