According to what is often referred to as the principle of competitive exclusion, no two species can have the same niche; the less effective competitor will be extinct from an area (Gause, 1932; Grinnell, 1904; Volterra, 1928). The basis for coexistence is, thus, the niche differentiation which is often achieved through resource partitioning (e.g., the theory of limiting similarity, Abrams 1983). In the species-poor Baltic Sea, the two co-existing and morphologically similar deposit-feeding amphipods, Monoporeia affinis, a glacial relict, and Pontoporeia femorata, of marine origin, dominate in abundance the benthic communities at soft sediment bottoms. Like all deposit-feeders in this area, the spring phytoplankton bloom constitutes the largest annual food input (Cederwall 1977; Elmgren, 1978; Lehtonen & Andersin, 1998). Experimental studies have demonstrated that M. affinis and P. femorata display both habitat and resource partitioning, with P. femorata burying deeper down in the sediment (Hill & Elmgren, 1987) and feeding more on aged organic matter when M. affinis is present (Byrén, Ejdung, & Elmgren, 2006). However, when released from the competition with M. affinis, P. femorata also feeds on the fresh material (Karlson, Nascimento, Näslund, & Elmgren, 2010), indicating an asymmetrical competition between

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.

Understanding how environmental factors affect the reproductive success of sentinel species is crucial for ecosystem-based management. The deposit-feeding amphipod Monoporeia affinis plays a key role in the Baltic Sea. Environmental contaminants in sediments can impact embryo development in Monoporeia, and assessing embryo aberrations helps monitor contamination levels and evaluate the environmental condition of the Baltic Sea. However, non-chemical stressors like food scarcity and bottom hypoxia can also influence the reproductive performance of these amphipods. Consequently, determining chemical pollution as a significant driver of reproductive health in wild populations across different basins facing multiple stressors is challenging.Here, we used PLS-SEM to analyze the links between reproductive health, sediment contaminants, environmental variability, genetic diversity, and nutrition in M. affinis. Our study encompassed data from 30 monitoring stations in the Western Gotland Basin and the Bothnian Sea. We found that amphipod reproductive health, assayed by embryo aberration frequencies, and resource utilisation, assayed by isotopic niche metrics, were directly impacted by chemical contaminants (metals and PAHs) and non-chemical factors (temperature). Additionally, the trophic niche played a significant mediating role in embryo aberration frequency. Furthermore, temperature moderated the relationship between chemical exposure and reproduction. However, we did not find any consistent variable representing genetic diversity using commonly applied metrics in population genetic analysis and mtDNA. Consequently, the contribution of genetics to reproductive health and trophic niche remains uncertain.Our PLS-SEM analysis reveals the significant impact of environmental contaminants on reproductive outcomes. Moreover, the connection between exposure and reproductive health is stronger in the Bothnian Sea compared to the Western Gotland Basin. These findings highlight the varying challenges faced by amphipod populations in these subbasins, emphasising the need to consider these differences in the overall environmental assessment. These results establish important relationships between pressure and indicators for sentinel species, supporting the effective and science-based use of biological effect indicators in the Baltic Sea.

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