Effects of reduced UV radiation on a microbenthic community during a microcosm experiment

A microcosm experiment was conducted over a 77 d period (from 6 July to 21 September 1999) to investigate the response of a microbenthic community to the long-term reduction of ambient ultraviolet-B (UVBR, 280 to 320 nm) and ultraviolet-A (UVAR, 320 to 400 nm) radiation. Intact sediment cores were harvested from a muddy intertidal flat in the lower St. Lawrence Estuary (Quebec, Canada) and placed in an outdoor flow-through system under 3 treatments: ambient solar radiation, reduced UVBR and reduced UVR (280 to 400 nm), Both autotrophic (primary production, microalgal biomass, accessory pigments, diatom composition and abundance and UVR absorbing compounds) and heterotrophic (meiofaunal abundance) components were investigated. It was found that neither nematode nor ontogenic stages of harpacticoid copepod abundances were affected by the treatments. The temporal variations of the nauplius, copepodite and copepod populations correspond to the normal ontogenic evolution of harpacticoids. The microphytobenthic community, dominated by small (less than or equal to15 mum) pennate diatoms, presented significantly higher carbon fixation under UVR protected treatments on Days 14 and 21, suggesting a simultaneously detrimental but weak effect of UVBR and UVAR on photosynthesis. Occurring in very low concentrations in the sediment, UVR absorbing compounds did not seem to be a major UVR-attenuation mechanism within the microbenthic community. The screening of UVBR and UVR also resulted in a stepwise decrease of microalgal biomass on Days 7, 21, 28, 35 and 42, and of diatom abundance on Day 35. The simultaneous accumulation of pheopigments likely reflected an enhanced grazing pressure under both UV-shielded treatments, presumably by microfaunal grazers. The fact that these noticeable changes were observed between the 3 treatments implies that both UVBR and UVAR influence the microphytobenthic community. There was a lack of treatment effect during the second half of the experiment when UVR fluxes showed their normal seasonal decrease. Thus, our results indicate that effects of ambient UVR on microbenthos occur only under high solar irradiance. During this period, both ambient UVBR and UVAR exerted a controlling effect on the microalgal standing stock in muddy intertidal flats in the lower St. Lawrence Estuary.