AME prepress abstract  -  doi: 10.3354/ame01204

ABSTRACT: Measurements of underwater solar radiation, particulate (PPP) and dissolved (DPP) primary and bacterial (BP) production allowed us to infer the respective contribution of UV-B (280-315 nm) and UV-A (315-400 nm) in algal-bacteria interactions in the SW 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 (280-400 nm) radiation. 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. UV-B contributed up to 50% to the inhibition, especially near 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 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 (PP^CHL) 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 PP^CHL, whereas PPP inhibition should be limited for cells characterized by a PP^CHL higher than 18 mg C mg CHL^-1 h^-1, even if excretion (DPP) could be slightly stimulated.