We present a common-garden experiment to study the feeding plasticity of a key deposit-feeder in the Baltic Sea, a low diversity system presenting a good model for studying local adaptations. Based on the latitudinal gradient in phytoplankton blooms, with a single diatom bloom (high nutritional quality) in the north and a diatom bloom followed by cyanobacteria bloom (low nutritional quality) in the south, we expected populations of the amphipod Monoporeia affinis 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 the toxin-producing cyanobacteria, when mixed with diatoms, were efficiently incorporated and used for growth by both populations. However, contrary to the expectations, the feeding plasticity was more pronounced in the northern population, indicating genetically-based divergence and suggesting that this population is capable of ecological adaptation to the climate-induced northward cyanobacteria expansion in this system. Results provide important understandings for adaptations to increasing cyanobacteria in a future warming world in both limnic and marine ecosystems by invertebrates, but  are likely widely applicable to other consumers where questions on adaptations to altered food quality.