Calibrating Seasonal Cycles of Marine Primary Production in California Coastal Ecosystems as a Tool for Paleo-reconstructions of Food Web Dynamics

Primary producers are the foundation of the marine ecosystem, however the future of ocean productivity and the composition of algal communities in the marine food chain are uncertain in the face of a changing climate. Assessing how primary productivity (PP) has responded to past climatic changes could help to understand current planetary warming scenarios will affect the marine environment. Carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of the organics from bivalve shells represent a novel source of highly detailed information into past coastal PP and ecosystem trophic structure. Bivalve shells are ideal archives to develop long-term records of PP, as they feed at the base of the food chain, and shell material is abundant and readily preserved in archeologic and geologic archives. We sampled two bivalve species native to two different environments on the California Coast- Ostrea conchaphilia and Mytilus californianus- seasonally along with monthly seawater particulate organic matter to assess how seasonal changes in marine PP are recorded in bulk δ¹⁵N and δ¹³C values. The objectives of this study are thus: 1) Assess the temporal resolution of isotopic signatures in two tissue types; adductor and shell matrix protein 2) Analyze how isotopic signatures record seasonal changes 3) Identify any species-specific effects in the isotopic signature. We found seasonal variation in δ¹⁵N in the adductor tissue of both M. californianus (p<.001) and O. conchaphilia (p<.001), but not in δ¹³C (M. californianus p = .14; O. conchaphilia p = .15). On average, adductor tissue from littoral sites had a lighter δ¹⁵N value (11.02‰) than estuarine sites (12.64‰). Comparison of O. conchaphilia and Mytilus edulis, which inhabit the same sites, indicated no difference in δ¹⁵N or δ¹³C in the adductor tissue of two bivalve species occupying the same habitat. There was no significant seasonal variation in δ¹⁵N or δ¹³C of demineralized shell, consistent with the hypothesis that shell proteins average longer-term PP trends than adductor tissue. This study lays the groundwork for future applications for paleo-reconstructions of past PP, as the determination of temporal resolution, species specific effects, and environmental effects is essential before attempting to reconstruct past food webs from shell records.