Effects of UV radiation and consumers on recruitment and succession of a marine macrobenthic community

The combined and interactive effects of climatic and ecological factors are rarely considered in marine communities. We designed a factorial field experiment to analyze (1) the interactive effects of ambient UV radiation and consumers; and (2) the effects of photosynthetically active radiation (PAR 400 to 700 nm), UVA (320 to 400 nm) and UVB (280 to 320 nm) radiation on a marine hard-bottom community in Nova Scotia, NW Atlantic. Species recruitment and succession on ceramic tiles were followed for 5 mo. We found strong negative UV effects on biomass and cover of the early colonizing macroalga Pilayella littoralis, whereas UVB was more harmful than UVA radiation. Consumers, mainly gammarid amphipods, increased P. littoralis biomass when UV was excluded, probably through fertilization. These initially strong and interacting UV and consumer effects on total biomass and cover diminished as species succession progressed. Species diversity was not affected by experimental treatments, but significant shifts in species composition occurred, especially at the recruitment stage. Red algae were most inhibited by UV, whereas sedentary invertebrates and some brown algae tended to increase under UV exposure. Consumers suppressed green and filamentous brown algae, but favored the other groups. Again, these effects diminished during the later stages of succession. We conclude that UV radiation can be a significant structuring force in early successional benthic communities, and that consumers can mediate its effects.