Effects of multiple stressors on marine shallow-water sediments: Response of microalgae and meiofauna to nutrient-toxicant exposure

Two types of common stressors acting simultaneously on shallow coastal ecosystems include increased anthropogenic nutrient loading and exposure to toxicants. Nutrients (inorganic nitrogen and phosphorus) and the polyaromatic hydrocarbon (PAH) pyrene were added singly and in combination, to study the combined effects of nutrients and toxicants on the base of the food web of a shallow-water illuminated sediment. The microbenthic community of natural sieved, sediment was used in a in a flow-through laboratory experiment lasting 28 days. Variables measured included benthic microalgal and meiofaunal biomass and composition, and meiofaunal grazing rates. The hypotheses were that (i) pyrene exposure affects meiofauna and their grazing rate, resulting in increased benthic microalgal biomass, and that (ii) pyrene effects depend on nutrient status, as found in previous mesocosm experiments. Our results showed that exposure to a low, environmentally realistic concentration of pyrene had a general negative effect on meiofauna and their grazing rates, although major taxonomical groups differed in response. A concomitant increase in benthic microalgal biomass suggested a cascading effect on the primary producers. Whether there was a significant interaction between pyrene and nutrient status depended on the variable measured. While toxicant effects on total meiofaunal grazing rate, chlorophyll a content of the sediment and microalgal composition depended on nutrient status, the effects on meiofaunal and algal biomass (based on cell counts) did not. We found only partial support for our specific hypothesis that pyrene effects were greater when nutrient concentrations were high. The mode of nutrient-toxicant interaction appeared to vary with variable; non-additive effects (antagonistic or synergistic) were more common than additive effects, but some effects could also be interpreted according to a comparative model. This apparent variation in interaction mode highlights the complexity of nutrient-toxicant interactions - and of multiple stressors in general - in the marine benthic environment, emphasizing the need to consider both structural and functional variables when assessing effects of stressor interactions. Moreover, the presence of potential indirect, food web-mediated effects underlines the need to test multiple-stressor effects using multitrophic communities.