Marine intrusions in a coastal lagoon enhance the negative effect of solar UV radiation on phytoplankton photosynthetic rates

Between March 1997 and May 1999, we studied the effect of solar UV-B (290 to 320 nm) and UV-A (320 to 400 nm) radiation on phytoplankton photosynthetic rates in a shallow coastal lagoon with periodic exchange with the Atlantic Ocean. In particular, we assessed whether the exchange with the ocean affected the severity of photosynthetic inhibition. During events of marine intrusion, the UV attenuation coefficient (K-d) in the brackish zone decreased up to 70% of the values observed under the influence of the freshwater discharge. The highest inhibition of the PAR-saturated photosynthetic rates by UV radiation (up to 55%) was observed in summer at the brackish zone in association with marine intrusion events. In the freshwater zone, the highest inhibition was up to 3-fold lower than in the brackish area due to higher K-d values in the UV range. On average for the entire system, near-surface primary production was reduced by ca. 25%, and the contribution by UV-A and UV-B was close to 2:1. Our results provide a baseline for future comparisons at a latitude where predicted trends in reduction of stratospheric ozone are significant. However, hydrological changes affecting river discharge and communication frequency with the ocean appear to be more important for the UV underwater climate and primary production than the expected increase in incident UV-B fluxes.