Effect of solar ultraviolet radiation on bacterio- and phytoplankton activity in a large coral reef lagoon (southwest New Caledonia)

Measurements of underwater solar ultraviolet radiation (UV-R, 280 to 400 nm), particulate (PPP) and dissolved (DPP) primary and bacterial (BP) production allowed us to infer the respective contribution of UV-B (280 to 315 nm) and UV-A (315 to 400 nm) in algal–bacterial interactions in the southwest lagoon of New Caledonia. Differences in and between activities were assessed by in situ exposure of plankton to full sun radiation and shielded for UV-B or UV-R. We found a coherent response of phytoplankton and bacteria in total UV-R inhibition. The deepest limit of UV-R influence was 8 m, and inhibition increased with oligotrophy to reach 55% for phytoplankton and 75% for bacteria, at 1 m depth. UV-B contributed up to 50% to the inhibition, especially near the surface and at the oligotrophic station. The UV-A effect on PPP was stronger than that of UV-B; however, bacteria were more sensitive to UV-B than were phytoplankton. When considering UV-A and UV-B separately, the inhibition response was linear without threshold. However, a threshold of 0.002 for PPP and 0.004 for BP was obtained when considering the ratio UV-B:UV-A. The rate of PPP normalized to chlorophyll concentration (PPchl) is proposed to reconcile the large range of inhibition encountered in the literature. A significant linear relationship predicted a PPP inhibition of 60% in surface water with no significant effect on excretion for cells characterized by a low PPchl, whereas PPP inhibition should be limited for cells characterized by a PPchl higher than 18 mg C (mg chl)–1 h–1, even if excretion (DPP) could be slightly stimulated.