Global warming leads to an increased browning of lakes across the northern hemisphere. This browning can due to benthic light-limitation cause hypoxic bottom conditions which can have negative impact on the fish production in the northern lakes. In this study, I obtained data from a large-scale experiment to test if the removal of anoxic bottom conditions in the hypolimnion affects Eurasian perch (Perca fluviatilis) populations. This was done with a Before-After, Control-Impact design in two adjacent lakes by altering the oxygen conditions in the hypolimnion in one lake while the other remained as control. To analyse if experimental oxygenation of an anoxic hypolimnion altered perch performance, I analysed samples of individual growth (based on back-calculated growth from operculum bone readings), condition, diets and stomach fullness in perch before and after oxygenation from the control and impact lake. The results showed that growth, condition, and stomach fullness increased in the treatment lake but not in the control lake, suggesting that resource availability increased for perch with the removal of anoxic hypolimnion conditions. However, a decline in population abundances was also observed over time within both lakes, which may provide at least an alternative explanation of the observed responses. Still, I hypothesize that the oxygenation at least partially increased the resource abundance in brown lakes, thus increasing fish growth and size.

This thesis presents: 1) the first statistically rigorous support for the longstanding hypothesis that state of satiation modifies diel vertical migration patterns of deep-sea micronektonic crustaceans and fishes; and, 2) the first assessment of microplastic ingestion by deep-pelagic micronekton in the Gulf of Mexico and Straits of Florida. Deep-sea pelagic crustaceans and fishes significantly contribute to abundance and biomass of pelagic ecosystems, are frequently consumed by commercially valuable fishery species, and serve to transport both nutrients and pollutants between shallow and deep waters. The results presented herein will be valuable for assessing risk associated with potential biomagnification of plastic through consumption or indirect consumption of deep-sea biota. Moreover, these data demonstrate that the extent of feeding at depth by non-migratory taxa as well as non-migrating individuals of migratory taxa is substantial. Feeding at depth is usually excluded from biogeochemical models, and these data demonstrate that this is an important factor that must be included to obtain more precise estimates of active nutrient flux by micronekton.