Dr. Peter Holtermann from Leibniz Institute for Baltic Sea Research Warnemünde, Germany, will talk about his current scientific work. (Photo credit: IOW/M. Rövensthal)
Online via Zoom on Monday 7 December 2026
15:00 cet (Rostock, Stockholm time zone);
16:00 eet (Helsinki, Tallinn time zone)
Please register via the link at the bottom (just name and e-mail address) to get the participation link.
Oxygen and hypoxia in Shallow Baltic Waters, first insights into the role of upwelling, mixing and transport by internal waves
Abstract:
Coastal hypoxia is a growing global threat to marine ecosystems (Breitburg et al. 2018). While deep waters of the Baltic Sea have been nearly permanently anoxic since the mid-20th century (Carstensen et al. 2014), near-coastal hypoxia—defined here as hypoxic conditions in waters shallower than 20 m—has received less attention. However, over the past decade, evidence suggests an increase in such events in the Baltic Sea (Conley et al. 2011; Lennartz et al. 2014; Piehl et al. 2022).
To better understand the spatial and temporal dynamics, as well as the intensity of coastal hypoxia, we established in the framework of the Shore to Basin (S2B) research initiative a long-term observatories in the western Baltic Sea near Boltenhagen, an area used by both commercial and recreational fishers. Since July 2021, the observatory has continuously measured water velocity, temperature, and bottom oxygen concentrations along a depth gradient (5–25 m) using moored sensors and an acoustic current meter. A major challenge, we had to work with, was the strong biofouling of oxygen sensors during the summer months. These measurements are complemented by regular research cruises employing CTD casts and a velocity shear microstructure probe, as well as numerical modelling of that area.
A striking finding is the occurrence of short-term hypoxic events, even in the shallowest parts of the study area. These events can last less than half a day and reach oxygen levels lethal to higher organisms. Due to their brevity, they are difficult to capture with conventional monitoring and require continuous, high-resolution measurements. The long-term, high-resolution time series now allows for statistical analysis of such events.
Process investigations reveal that these intense hypoxic pulses result from a complex interaction: they are driven by the coastal upwelling of deep hypoxic water, modified by local turbulent mixing and lateral transport, and amplified by significant oxygen consumption at the seafloor and in the water column. Analysis of field data in combination with numerical modelling did additionally show the role of mode-2 internal waves as a transport mechanism for oxygen.
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