In nearshore, soft-sediment habitats of the Salish Sea, eelgrass (Zostera marina L.) meadows have been identified as potential mitigators of ocean acidification (OA) because their photosynthetic activity can decrease pCO2, increase pH and provide refuge for organisms sensitive to OA. The diurnal light cycle controls photosynthetic production of eelgrass and therefore, along with tidal cycles, exerts strong controls on variations in pCO2 in nearshore environment. In this study, we investigate the carbon uptake rates for eelgrass under varying light, ambient pCO2 conditions and eelgrass densities (leaf area index). The magnitude of changes predicted based on experimentally derived photosynthetic rates, measured light and water depth in Padilla Bay, WA compare well with observed variability in the field. The ambient pCO2 conditions we tested, however, did not appear to be a major control in carbon uptake rates for eelgrass. Combining lab, model, and field results will strengthen our understanding of the variability of OA in the nearshore environment and help shellfish managers understand the drivers of that variability and inform further studies of its effects, such as potential OA refuge for shellfish and other sensitive organisms.