Extreme oxygen depletion and nutrient release in coastal waters: New findings and environmental monitoring strategies

Author: Gerald Schernewski (IOW)

Oxygen is an important indicator for assessing the ecological status of coastal waters. Measurements taken as part of current monitoring programs generally show good oxygen levels throughout the water column in almost all coastal waters of Mecklenburg-Western Pomerania. However, mass die-offs of fish and mussels have been observed repeatedly in the Szczecin Lagoon (Oderhaff), presumably due to oxygen depletion. In cooperation with the State Agency for Nature Conservation, Environment and Geology (LUNG), the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) combined long-term observation data from the last 40 years with a spatially and temporally high-resolution ecosystem model to investigate oxygen conditions in the Szczecin Lagoon in detail.

The model shows that oxygen depletion in the Szczecin Lagoon frequently occurs extensively directly above the sediment. These situations are not detected by the standard environmental monitoring measurements, which are taken one meter above the seabed. Besides the mass die-off of organisms, oxygen depletion also leads to a significant release of phosphorus from the sediment, a process known as internal eutrophication. These processes have numerous negative consequences for water quality. However, no stimulation of potentially toxic cyanobacterial blooms was observed. Due to reduced nutrient inputs, the duration and spatial extent of oxygen-depleted areas on the seabed have decreased significantly over the past 20 years.

Based on these findings, improved monitoring approaches were discussed with LUNG. A floating station in the central area of ​​the lagoon will continuously collect data using probes extending to the seabed. Model simulations will be used for the spatial extrapolation of the data. This measurement approach is also transferable to other coastal waters.

The Szczecin Lagoon (a) and examples of mussel and fish deaths that are presumably due to temporary oxygen deprivation.

The results are the result of a collaboration between the DAM projects PrimePrevention and Coastal Futures II.

Publication:

Schernewski, G., Neumann, T., Piehl, S., von Weber, M. (2025): New approaches to unveil the unknown: Oxygen depletion and internal eutrophication in a Baltic lagoon over decades. Front. Environ. Sci., Volume 13, doi: 10.3389/fenvs.2025.1620191