Micropollutants interfere with selection by environmental gradients in natural phytoplankton communities
Multiple environmental stressors in the form of pollutants, habitat degradation, and climate change challenge the structure and function of natural communities. Effects of multiple stressors on population and community dynamics are mediated by selection on individual level trait-diversity. Phenotypes of individuals reflect both the past selection processes and the current functional properties of the community. Importantly, adaptive responses to future environmental change depend on phenotypic distributions of the present community. We examined how realistic exposure to micropollutants change individual level phenotypic-diversity and productivity in lake phytoplankton communities, and how these effects interact with natural environmental gradients, influencing community assembly. We exposed natural lake phytoplankton communities to a mixture of water-borne pharmaceuticals and personal care products (micropollutants). We used replicated phytoplankton communities that were confined to dialysis-bags and subjected to a temporally fluctuating natural lake environment. We sampled and mixed three compositionally different phytoplankton communities, and exposed these communities to three different levels of micropollutants (mimicking wastewater effluents, polluted rivers and polluted lakes, respectively). Replicated experiments were deployed at three different depths to study sorting of phenotypes over a natural environmental gradient. We measured the magnitude of effects of micropollutants, depth and their interaction, on phenotypic-diversity (measured by scanning flow-cytometry), biomass production, and deviation of communities from random assembly.
Experimental phytoplankton communities at each depth converged to similar patterns of phenotypic-diversity and productivity as local depth-specific resident communities, suggesting that community assembly over depth was predictable. This was not the case, however, when communities were exposed to micropollutants. Micropollutants and depth had significant negative effects on phenotypic-diversity (reduced trait variance) and positive effects on total biomass (increase in cell numbers), with the effects of pollutants being stronger than those of the steep depth gradient. The interaction between micropollutants and depth was antagonistic suggesting that micropollutants exposure counteracted the effects of naturally occurring selective filters. Importantly, increasing levels of micropollutants significantly reduced the ability of disturbed communities to converge to the same structure and function of control communities. Micropollutants significantly reduced phenotypic-diversity and increased susceptibility to ecological drift, impairing the capacity of natural communities to adapt to a changing environment. We show that realistic exposure to common micropollutants can reduce biodiversity and adaptive capacity of natural phytoplankton communities. These results are alarming, since complex mixtures of chemical pollutants are found in surface waters worldwide.