Coping with environmental change : Lessons from isotope studies in a sentinel species

Abstract: Adaptive evolution and phenotypic plasticity are two key mechanisms by which natural populations avoid extinction in the face of environmental change. Evolutionary young systems, such as the Baltic Sea with low species diversity and steep environmental gradients, are good models for studying how wild populations cope with environmental changes, such as climate-induced alterations in food supply and exposure to environmental contaminants. My objective was to apply the ecological niche concept to understand how benthic deposit-feeding amphipods, the key species in the Baltic ecosystem, respond to these pressures. First, in the field study (paper I), we used compound-specific isotope analysis to compare trophic metrics between the two sympatric species Monoporeia affinis and Pontoporeia femorata, and link feeding to their reproductive success. In the experimental studies (papers II and III), the isotope niche size, a proxy for the trophic niche, was used to infer M. affinis capacity to cope with presumably adverse effects of suboptimal diet and toxic exposure. Finally, a modelling study employing field data was used to assess how exposure to environmental contaminants in the sediment affects reproductive success and isotope niche in M. affinis (paper IV). In these studies, we found a higher trophic position and a narrower trophic niche in M. affinis than P. femorata; moreover, the amphipod diet translated into the reproductive output (paper I). In addition to this interspecific variability, M. affinis responses to dietary stressors and toxicity were population-specific, suggesting local adaptations to cyanobacteria blooms. More specifically, the amphipods from the Bothnian Sea (BoS) assimilated diatoms more efficiently than those from the Baltic Proper (BP), which, on the other hand, readily assimilated cyanobacteria (paper II). Nevertheless, the BoS amphipods had a considerable capacity for coping with the cyanobacteria blooms that are projected to increase in this subbasin due to the climate change (paper II). Although the cyanobacterial toxicity was similar for M. affinis from the two subbasins, the different isotope niche and growth responses of the amphipods suggested local adaptation. Amphipods from two relatively nearby sites in the Baltic Proper showed similar signs of neurotoxicity when exposed to contaminant spiked sediment, however there were differences in isotope niche responses, with examples of both expansion and compression linked to contaminant exposure (paper III). The observations from these experimental studies were supported by the structural equation modelling based on the field data, which showed that contaminants may affect reproductive performance directly by inducing developmental aberrations and indirectly by altering the amphipod feeding and trophic niche (paper IV). Moreover, the model suggested that linkages between the amphipod reproductive performance and chemical pollution are stronger in BoS than in BP. In conclusion, the isotope niche concept provides a valuable tool for analyzing feeding and growth in these amphipods coping with changing environments, demonstrating geographic variability of the responses. The differences in responses of the Baltic Sea M. affinis suggest distinctive challenges for this sentinel species in different subbasins, which should be considered when using population abundance, reproduction and physiological responses in the effect studies and environmental assessments.