Size‐dependent sensitivity of stream amphipods indicates population‐level responses to chemical pollution
Authors: Taddei Anja , Räsänen Katja , Burdon Francis J. 
Published: January 2021

Global change assessments have typically ignored synthetic chemical pollution, despite the rapid increase of pharmaceuticals, pesticides, and industrial chemicals in the environment. The paucity of research on the ecological effects of these micropollutants undermines our efforts to address the freshwater biodiversity crisis. Understanding the responses of individual organisms to chemical pollution should help address this knowledge gap because individual‐level effects can cascade across populations, communities, and ecosystems with devastating consequences.

Inputs of treated municipal wastewater are a major source of micropollutants in receiving environments. Here, we assessed population and individual‐level influences of treated wastewater on freshwater gammarid amphipods (Gammarus spp.) in Swiss lowland streams in situ and tested effects of a micropollutant mixture on individual Gammarus fossarum using a common garden laboratory experiment. We hypothesised that population‐level effects of wastewater are mediated through asymmetric sensitivities of juvenile and adult gammarids to chemical pollutants. We expected that life‐stage specific sensitivities would reflect allometric theory relating body size to a wide range of organismal characteristics, including metabolism, growth, and mortality.

At the population level (i.e. field survey), we observed greatly reduced abundances of juvenile gammarids downstream of the wastewater discharge in three of the six sites surveyed, indicating the potential for demographic effects of pollution. At the individual level in a field transplant experiment, we found that the presence of wastewater led to a steeper positive relationship between gammarid body size and leaf consumption. In the laboratory experiment, we found that micropollutants had negative effects on consumption and growth rates, but a positive effect on survival. Differences in the relationship between gammarid body size and performance were subtle across treatments, although flatter slopes in growth appear to be a consistent response to chemical pollution. Faster growth rates appeared to be connected with reduced survival, placing individuals in control treatments and/or with smaller body size at greatest risk of mortality. Notably, juvenile gammarids had faster growth rates and lower survival than adults when exposed to micropollutants.

Our results demonstrate the potential for negative impacts of micropollutants in freshwater ecosystems. However, the results also show that organismal responses to chemical pollutants can be complex, whereby impacts at the individual level may act counter‐intuitively to population‐level dynamics. This highlights the need for more realistic experiments to better assess how organismal responses depend on life stage and body size, and how individual‐level effects propagate to higher levels of biological organisation. Our study shows how allometric theory can be used to examine the effects of stressors on underlying organismal biology, population demographics, and link with broader macroecological patterns.