Retracing hypoxia in Eckernförde Bight (Baltic Sea)

Abstract

Abstract. An increasing number of dead zoning (hypoxia) has been reported as a consequence of declining levels of dissolved oxygen in coastal oceans all over the globe. Despite substantial efforts a quantitative description of hypoxia up to a level enabling reliable predictions has not been achieved yet for most regions of societal interest. This does also apply to Eckernförde Bight (EB) situated in the Baltic Sea, Germany. The aim of this study is to dissect underlying mechanisms of hypoxia in EB, to identify key sources of uncertainties, and to explore the potential of existing monitoring programs to predict hypoxia by developing and documenting a workflow that may be applicable to other regions facing similar challenges. Our main tool is an ultra-high spatially resolved general ocean circulation model based on a code framework of proven versatility in that it has been applied to various regional and even global simulations in the past. Our model configuration features a spacial horizontal resolution of 100 m (unprecedented in the underlying framework which is used in both global and regional applications) and includes an elementary representation of the biogeochemical dynamics of dissolved oxygen. In addition, we integrate artificial “clocks” that measure the residence time of the water in EB along with timescales of (surface) ventilation. Our approach relies on an ensemble of hindcast model simulations, covering the period from 2000 to 2018, designed to cover a range of poorly known model parameters for vertical background mixing (diffusivity) and local oxygen consumption within EB. Feed-forward artificial neural networks are used to identify predictors of hypoxia deep in EB based on data at a monitoring site at the entrance of EB. Our results consistently show that the dynamics of low (hypoxic) oxygen concentrations in bottom waters deep inside EB is, to first order, determined by the following antagonistic processes: (1) the inflow of low-oxygenated water from the Kiel Bight (KB) – especially from July to October – and (2) the local ventilation of bottom waters by local (within EB) subduction and vertical mixing. Biogeochemical processes that consume oxygen locally are apparently of minor importance for the development of hypoxic events. Reverse reasoning suggests that subduction and mixing processes in EB contribute, under certain environmental conditions, to the ventilation of the KB by exporting recently ventilated waters enriched in oxygen. A detailed analysis of the 2017 fish-kill incident highlights the interplay between westerly winds importing hypoxia from KB and ventilating easterly winds which subduct oxygenated water.

Global-scale parameters for ecological models

“…Sea-bottom dissolved oxygen was inverse-correlated with sea-surface temperature at the global scale, and in the Baltic, Black Sea, North Sea, and Western Mediterranean, as inferable also by other studies; 63 , 64 , 78 , 83 – 85 …”

“…These observations agree with those of other studies; 14 , 74 – 77 Sea-bottom salinity was generally positively correlated with sea-surface salinity in most seas except for the Black Sea, due to peculiar deep and shallow thermohaline dynamics 78 . It was also positively correlated with sea-bottom temperature in the Adriatic, Aegean, Bay of Biscay, and Levantine, as can also be inferred by other studies 79 – 82 ; Sea-bottom dissolved oxygen was inverse-correlated with sea-surface temperature at the global scale, and in the Baltic, Black Sea, North Sea, and Western Mediterranean, as inferable also by other studies; 63 , 64 , 78 , 83 – 85 Sea-ice concentration had no significant correlation with the other parameters. …”

Radon (222Rn) as tracer for submarine groundwater discharge investigation—limitations of the approach at shallow wind-exposed coastal settings

“…Such offshore mixing has to be considered a highly influential process along the three stretches of coastline investigated in our study, in particular in the Eckernförde Bay. A recent study (Dietze & Löptien, 2021 ), which investigated offshore mixing in the Eckernförde Bay between 2000 and 2018 in high-resolution, showed highly variable water residence times (i.e. the time since the water entered the bay) and water ages (i.e.…”

“…The authors identified the wind direction as key driver for the intensity of variation. For instance, the abovementioned strong (north-easterly) winds that occurred in the week preceding our radon survey in the bay caused the water residence time in the inner bay to drop to under 10 days (Dietzen & Löptien, 2020 ). Wind-induced offshore mixing caused the discharged groundwater (enriched in radon) to be rapidly mixed with bay waters low in radon, resulting in less pronounced (or completely levelled-out) radon concentration anomalies close to the (physically identified) coastal SGD spots.…”

Environmental changes affect the microbial release of hydrogen sulfide and methane from sediments at Boknis Eck (SW Baltic Sea)

“…Enhanced OM degradation leads to elevated respiration rates, increased oxygen demand, consolidating developed hypoxia or even anoxia during certain times of the year (Lennartz et al, 2014). Sluggish deep-water renewal and restricted seafloor topography amplify local oxygen depletion (Dietze and Löptien, 2021). High benthic degradation rates under anoxic conditions support increased rates of SR and methanogenesis producing hydrogen sulfide and methane, respectively (Treude et al, 2005;Bertics et al, 2013;Maltby et al, 2018).…”