| Summary: | Fish diets, and the processes resulting from their interactions with the broader food web, are central to lake structuring. These interactions are both influenced by and influence the surrounding environment. Typically, fish diets derive primarily from the pelagic or littoral/benthic trophic pathways. Understanding the balance between these pathways is ecologically significant as they form the foundation for several management applications, notably fish removal. Arctic char (Salvelinus alpinus) is a key species in Greenland, given its dominant role in the predator-prey dynamics of Arctic lake food webs. This thesis investigated how habitat type and lake morphology shape the trophic positioning of Arctic char. By employing stomach content and stable isotope analyses, the dietary trends of char in two Greenlandic lakes of varying size were analyzed. Generally, fish from the smaller of the study lakes displayed a marked preference for littoral macroinvertebrates. The cause of this was the relatively larger littoral habitat of the slightly smaller, elongated, and shallower, though still 10 m deep, of the study lakes. Further, there was increased reliance on zooplankton by small char in the larger of the two study lakes during the study period from 2008 to 2019. These observations show the interconnectedness of lake morphology and fish feeding behavior. Large-scale fish biomass reductions, both intentional via biomanipulation and natural fish kills, significantly impact lake ecosystems. We analyzed the diets of roach (Rutilus rutilus) and European perch (Perca fluviatilis) in a eutrophic shallow lake over 18 months, before and after a major fish removal. The results revealed a shift towards increased zoobenthivory across all fish sizes post-removal. Additionally, a temporary surge in zooplankton biomass saw both species predominantly feeding on daphnids, likely causing their number to drop. This suggests that after lake restorations involving fish removal, intermittent zooplanktivory could lead to unexpected top-down ...
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