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.