Water samples were collected by Niskin bottle or by hand from just beneath the surface during low tide surveys at or near 10 GCE-LTER sampling sites in September 2003. The incorporation of radiolabelled bicarbonate in response to varying levels of illumination was measured using a photosynthetron apparatus. Photosynthesis-irradiance (P-I) curves constructed from these measurements will be used in conjunction with algal biomass and PAR versus depth measurements to estimate instantaneous gross primary production in the water at each GCE sampling site. This study was part of the GCE-LTER hydrographic monitoring program, and will be repeated quarterly.