As I live and breathe! : The effects of hypolimnic oxygenation on benthic macroinvertebrate and zooplankton assemblages

Sammanfattning: Exacerbated deep-water hypoxia, brought about by anthropogenic-derived dissolved organic carbon (DOC) input, has potentially detrimental impacts on organisms residing in freshwater ecosystems. Oxygenation of the water column is a potential management strategy to mitigate this impact, and used to alleviate DOC induced oxygen depletion. In this thesis, I investigated the effects of hypolimnic oxygenation on benthic macroinvertebrate and zooplankton biomass and size assemblages in an experimental dystrophic lake. The experiment, which took place between 2017-2020 in a dystrophic brown-water lake consisting of two interconnected basins, was performed according to a Before-After-Control-Impact design. One basin was continuously monitored under reference conditions while the oxygen concentrations of the other basin was experimentally increased halfway through the experimental period. Macroinvertebrates were sampled from the epi-, meta- and hypolimnion while zooplankton were sampled from throughout the water column (0-6 m) and, during 2020, also the hypolimnion (3-6 m) separately. Collected macroinvertebrate and zooplankton samples were analyzed for biomass, chironomid body lengths and zooplankton community compositions per basin and treatment. Mean benthic biomass, chironomid biomass and chironomid body length increased in response to oxygenation in the hypolimnion but no effect was found in the epi- or metalimnion habitats. Observed biomass responses were mainly driven by the increase in chironomid biomass. No differences were found in neither zooplankton community assemblages nor biomass following oxygenation. These results suggest that the lack of oxygen-rich habitats act as a major limiting factor for hypolimnic macroinvertebrates and especially for chironomids. Zooplankton, on the other hand, appear to be limited by resource availability as opposed to the abundance of suitable, oxygen-rich habitats only. The results suggest that oxygenation may have clear benefits for deep-water macroinvertebrates. However, as a management tool, it is ineffective as a means to increase zooplankton abundances.