Experimental warming and toxicant exposure can result in antagonistic effects in a shallow-water sediment system

It has been suggested that future warming will exacerbate the effect of local stressors such as toxicants. In this study the individual and combined effects of warming (+4 degrees C above ambient) and a toxicant (the antifouling substance copper pyrithione) on natural intact shallow-water sediment were studied in an outdoor flow-through facility. Functional (oxygen and inorganic nutrient fluxes in light and dark, bacterial production) and structural (biomass and composition of microphytobenthos and meiofauna) variables were measured. Warming was found to modify the toxicant response antagonistically, i.e. warming removed the negative effect of the toxicant exposure. This antagonism was found for functions depending on light (gross primary production, 24 h net oxygen fluxes, oxygen and silica fluxes). Most functional variables were, however, affected by warming alone, while structural variables were affected by the toxicant alone. At the end of the experiment, the system had 2 types of microalgal communities, a typical benthic algal mat and a floating periphytic mat. Both the benthic and floating microalgal mats were significantly affected by the toxicant alone, but in opposite directions. The biomass of the benthic algal mat was significantly higher under toxicant exposure, whereas the biomass of the floating periphytic mat was lower. Our results suggest that the effects of toxicants in aquatic environments may be reduced (rather than amplified) by warming. We also show that autotrophic communities can respond differently within the same ecosystem and that habitat may determine the mode of response to warming-toxicant exposures in aquatic environments.