Stratification, hypoxia and ecological impacts in semi-enclosed coastal water bodies : International lessons for Lakes Grevelingen and Veere

This report provides a quick scan literature review of the ecological consequences of stratification and hypoxia in semi-enclosed coastal water bodies around the world that are similar to the Dutch saline lakes Grevelingen and Veere. These lakes, originally estuarine systems, were transformed by hydraulic engineering as part of the Delta Works, resulting in reduced tidal exchange and persistent stratification. Seasonal hypoxia and anoxia threaten benthic habitats, food web stability, and compliance with EU Water Framework Directive and Natura 2000 targets. A particular knowledge gap is the extent to which stratification and hypoxia in deeper water have effect on the benthic ecosystem in shallow areas (< 5 m water depth) and the functioning of the pelagic food web.

To contextualize these challenges, an international comparison was conducted across nine case studies, including the Baltic Sea, Limfjord, Venice Lagoon, Aetoliko Lagoon, Mar Menor, Chesapeake Bay, Swan River, Saemangeum Lake, and Jellyfish Lake. Each system illustrates how stratification and hypoxia emerge from interactions between physical enclosure, nutrient loading, and internal biogeochemical feedback loops. Ecological impacts range from benthic die-offs and trophic collapse to altered community composition and reduced ecological resilience.

Across the nine international case studies, we find indicative evidence that deep-water stratification and hypoxia can propagate effects into shallow zones, e.g. via episodic mixing which contributes to benthic die-offs and weakening benthic–pelagic coupling. In multiple cases, pelagic food webs under stratified low-oxygen regimes show reduced zooplankton grazing and increased jellyfish dominance, implying a shift from top-down to bottom-up control.

Some open questions are not answered with this study, such as whether stratification and oxygen depletion are the main cause of the observed ecological alterations or just one of many effects caused by the closure of the systems, parallel to and interlinked with changes in the lakes’ ecology.

Management strategies across systems vary, from nutrient load reduction and artificial mixing to nature-based solutions and adaptive governance. The findings underscore that single interventions are rarely sufficient. Instead, the case studies suggest that integrated approaches, addressing both external pressures and internal feedbacks, are essential in long-term ecological recovery.