Many of Earth’s freshwater ecosystems have been lost or degraded over the past century following the industrial revolution and the dramatic increase in human population. One important issue is the discharge of wastewater into streams and other receiving bodies. The widespread establishment of wastewater treatment plants (WWTPs) has greatly improved surface water quality in developed countries. However, modern ‘secondary treated’ wastewater still contains a variety of micropollutants (MPs), which potentially exert negative effects on stream ecosystems, thus posing a risk to aquatic biodiversity and ecological integrity.
By comparing sites upstream (US) and downstream (DS) of WWTPs in three Swiss lowland streams, I assessed the influence of wastewater on stream ecosystem structure and function. Using a field survey approach, I investigated the influence of wastewater on the composition and trait characteristics (e.g., functional feeding groups) of benthic macroinvertebrate communities in spring and autumn 2013. In an experimental part, I focused on the detritivoredetritus interaction between Gammarus amphipods and leaf litter, which is an integral part of stream food webs. In a reciprocal transplant experiment (RTE), I tested for negative impacts (environmental stress) and potentially confounding positive responses (acclimation, local adaptation) influencing pollutant exposure effects. Wastewater can also affect fungal and bacterial conditioning of leaf litter and thus indirectly influence gammarid-mediated leaf-litter decomposition. This effect was investigated in an exploratory nutrient enrichment experiment (NEE) in one study stream.
All results were variable among streams, reflecting differences in abiotic and biotic factors and indicating site-specificity in response to pollution stress. However, I found consistent differences in the relative abundance of several taxa at DS sites compared to US sites, which may reflect differing pollution tolerance of these taxa. Gammarus amphipods were hyperabundant in all three streams, which supports their functional importance within these ecosystems. Survey data showed higher parasitism rates and better condition for gammarids at some DS sites, which may have reflected the influence of higher water temperatures and elevated nutrient concentrations below the WWTPs. In the RTE, I found some evidence for carryover effects of wastewater, where feeding rates of gammarids were impaired even after transplantation to US sites unaffected by wastewater. Furthermore, gammarid mortality in one study stream showed the potential for local adaptation and acclimation to the influence of wastewater. In the NEE, I showed some evidence for subsidy-stress responses to nutrient enrichment in microbial leaf conditioning.
My results highlight the importance of combining classical ecotoxicological approaches with ecology and evolutionary biology. Using such an approach will help us to better understand the magnitude of the threat that contaminants in wastewater (e.g., MPs) pose to freshwater ecosystems.