Oxygen (O₂) availability is a critical factor for the health of any aquatic ecosystem. It not only determines which organisms will be able to survive, persist, or thrive, but also fundamentally affects biogeochemical cycling. This is especially evident in its connection to the short carbon cycle, an umbrella term describing how carbon moves through the ecosystem via processes such as photosynthesis, respiration, and decomposition. It is well established that globally both the open oceans and the coastal waters have experienced a decline in dissolved O₂, over at least the last six decades, with the coastal regions being the most affected. 

There is a strong link between eutrophication and temporary or permanent anoxia, since high primary productivity in surface waters drives oxygen loss in bottom waters as organic matter is remineralized by microbes. 

In this thesis we explore near-seafloor oxygen dynamics over time at two sites: a coastal oxic site and a euxinic (sulfide can in this context be seen as negative oxygen) deep-basin site in the Western Gotland Basin (WGB) of the eutrophied Baltic Sea. Using a benthic lander system equipped with a profiling conductivity-temperature-depth (CTD) sensor, we monitored conditions in the benthic boundary layer (BBL), directly above the seafloor, over time. 

At the coastal site, we observed O₂ variations of up to 30 μmol L^-1 occurring over timescales of just a few hours. From the data collected by a vertically profiling CTD and a down-looking acoustic doppler current profiler (ADCP) we calculated measures for stratification and turbulent diffusivity over time. We observed a counterintuitive correlation between increased O₂, increased temperature (T) and decreased salinity (S) concurrent with an increase in stratification. Normally, it would be expected that O₂ and T increase and S decrease when vertical mixing—rather than stratification, which inhibits vertical mixing— is strong. Based on vertical transects of T and S over time, together with variations in flow velocity and direction, we developed a conceptual model explaining this behaviour as divergence and convergence of the lower water mass due to changing flow fields (Paper I). These oxygen dynamics occur on temporal scales and within a distance to the sediment surface that isn’t captured by snap shot ship-based monitoring. At the end of the time-series, a high-velocity high-turbidity event was followed by what appeared to be an increase in sediment O₂ uptake rate. 

Building on Paper I, we investigated how frequent and significant event-driven episodes are for bottom water O₂ availability. Following a hiatus for a deep-basin measuring campaign (Paper III), the next measuring campaign at B1 ran from May 2022 to January 2023. We found a general trend of declining bottom water O₂ concentrations from mid-May, interrupted by vertical mixing events associated with regional downwelling. From start to finish these events lasted less than 3 days. In the absence of these events, the bottom waters would have turned and remained anoxic from the onset of thermal stratification (~May) until its breakdown (~ November) (Paper II). 

The field campaign for paper III was built on the hypothesis that the BBL is a biogeochemical hotspot where suspended particles stimulate microbial activity, driving elevated rates of carbon remineralization and nutrient regeneration relative to both the sediments below and the water column above. However, the deep basin BBL was found to be much less dynamic than the coastal equivalent, with low amount of suspended solids and low sulfate-reduction rates. However, we found that the depth-integrated rate of sulfide generation in the anoxic below-pycnocline part of the water column exceeded the diffusion-limited flux from the sediments, indicating that water-column processes are more important than sediment processes in sustaining anoxia in the WGB (Paper III).