Effect of solar UVR on the production of particulate and dissolved organic carbon from phytoplankton assemblages in the Indian Ocean

Abstract

This study shows the effects of solar UVR on the photosynthetic parameters and the production of both particulate and dissolved organic carbon (POC and DOC, respectively) of phytoplankton assemblages from oligotrophic areas of the southeastern Indian Ocean. Samples from the 20% PAR depth were incubated on deck under different intensities of 2 different light regimes: full solar radiation (PAR+UVR) and excluding UVR (PAR). Phytoplankton production was measured by the 14 C method, obtaining size-fractionated POC (0.2, 2 and 20 µm), total POC and DOC. POC values were fit to a photosynthesisirradiance (P−E) model that includes photoinhibi- tion. Phytoplankton populations under PAR+UVR had a 10% lower maximum photosynthetic rate (Ps B ) and an 86% higher sensitivity to inhibition (β) than populations incubated under only PAR exposures. For some stations, cells larger than 2 µm showed higher Ps B and photosynthetic effi- ciency (measured as the P−E slope, α) than smaller cells, but no significant differences were found for β. In contrast, DOC production was significantly higher under PAR+UVR than under PAR exposures and did not follow the P−E model used for fitting POC. The analysis of the P−E values for DOC production showed that values remained constant under PAR intensities but increased with increasing irradiance in the presence of UVR. In some cases, inhibition of DOC production at high irradiances was also observed. These results demonstrate the relevance of UVR irradiance as an environmental driver for phytoplanktonic DOC production in oligotrophic waters and reinforce the important role of UVR on the physiology and ecology of tropical plankton.