Understanding how key-species respond to anthropogenic stress such as chemical pollution is critical for predicting ecosystem changes. Little is however known about the intra-specific variability in the physiological and biochemical traits involved in contaminant exposure responses. Here, we explored this idea by exposing the Baltic amphipod Monoporeia affinis from two sites, one moderately polluted and one more pristine, to a sediment spiked with PAHs and PCBs. We evaluated the amphipods responses related to feeding, growth, a stress biomarker (acetylcholinesterase [AChE] inhibition) and stable isotope (delta C-13 and delta N-15) composition including isotope niche analyses. More adverse responses were expected in animals from the low-pollution site than those from the high-pollution site due to tolerance development in the latter. Amphipods from both populations showed a similar to 30% AChE inhibition when exposed to the contaminant spiked sediment. However, both controls and exposed amphipods from the high-pollution site had higher survival, nutrient uptake and condition status than the amphipods from the low-pollution site, which did not feed on the added diatoms as indicated by their isotope values. We found no signs of population-specific responses in physiological adjustments to contaminants with regard to classic ecotoxicological biomarkers such as AChE inhibition and growth status. Instead, isotope niche analyses proved useful in assessing contaminant stress responses at the population level.

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.

We present a common-garden experiment to examine the amphipod Monoporeia affinis, a key deposit-feeder in the Baltic Sea, a low diversity system offering a good model for studying local adaptations. In the northern part of this system, the seasonal development of phytoplankton is characterized by a single diatom bloom (high nutritional quality), whereas in the south, the diatom bloom is followed by a cyanobacteria bloom (low nutritional quality) during summer. Therefore, the nutrient input to the benthic system differs between the sea basins. Accordingly, the amphipod populations were expected to be dietary specialists in the north and generalists in the south. We tested this hypothesis using a combination of stable isotope tracers, trophic niche analyses, and various endpoints of growth and health status. We found that when mixed with diatomes, the toxin-producing cyanobacteria, were efficiently incorporated and used for growth by both populations. However, contrary to expectations, the feeding plasticity was more pronounced in the northern population, indicating genetically-based divergence and suggesting that these animals can develop ecological adaptations to the climate-induced northward cyanobacteria expansion in this system. These findings improve our understanding regarding possible adaptations of the deposit-feeders to increasing cyanobacteria under global warming world in both limnic and marine ecosystems. It is possible that the observed effects apply to other consumers facing altered food quality due to environmental changes.

According to ecological theory, two species cannot occupy the same niche. Using nitrogen isotope analyses (delta N-15) of amino acids, we tested the extent to which two sympatric deposit-feeding amphipods,Monoporeia affinisandPontoporeia femorata, partition their trophic resources. We found that trophic position (TP) and resynthesis index ( n-ary sumation V; a proxy for degradation status of ingested material prior to assimilation by the consumer) differ between species. The surface-feedingM. affinishad higher TP and intermediate n-ary sumation V, both pointing to a large contribution of metazoans in its diet.P. femorata, which feeds in the subsurface layers, had lower TP and a bimodal distribution of the n-ary sumation V values, supporting previous experimental evidence of a larger feeding niche. We also evaluated whether TP and n-ary sumation V values have consequences for amphipod fecundity and embryo viability and found that embryo viability inM. affiniswas negatively linked to TP. Our results indicate that the amino acid-delta N-15 data paired with information about reproductive status are useful for detecting differences in the trophic ecology of sympatric amphipods.