High and dry: Variation in net photosynthesis of the intertidal seaweed Fucus gardneri

The paradigm that net seaweed photosynthesis in the stressful intertidal zone decreases predictably with increasing tidal elevation has not been tested widely. We measured the variation in net photosynthesis rates of a common intertidal brown seaweed (Fucus gardneri) collected from 14 sites and two tidal heights on San Juan Island, Washington (USA) in water and air, in summer and winter. We also measured temperature, relative humidity, and thallus hydration in situ. Mean net photosynthesis in air was two orders of magnitude lower than in water, a greater mean difference than found among sites and between tidal elevations and seasons. Thalli never fully desiccated even in summer, and net photosynthesis was always positive. Rates in air and water varied little and nonsignificantly between tidal elevations and were not consistently lower at upper elevations, where in situ conditions were not always drier and hotter. Although photosynthetic rates per SE did not vary predictably across the tidal gradient, mid-elevation thalli gained 25 times more carbon because they were exposed to air for shorter periods. Thalli experienced stressful conditions that reduce net photosynthesis during exposure, but the length of time exposed to air, rather than desiccation, determined daily photosynthetic carbon acquisition.