Variable effects of ecosystem restoration in a eutrophic coastal lagoon: reoxygenation by increasing water exchange

Increased anthropogenic activities are affecting water quality, e.g. leading to eutrophication and deoxygenation, culminating in biodiversity loss in coastal ecosystems globally. In the Southwest Delta in the Netherlands, large scale engineering to protect coastal areas against storm surges has turned several tidal inlets and estuaries into coastal lagoons and (marine) lakes. The water quality in these ecosystems has strongly deteriorated as a result of stagnation of bottom waters in combination with eutrophication. One such ecosystem, Lake Veere, showed signs of recovery after restoration of water exchange with the adjacent tidal marine Eastern Scheldt in 2004. In recent years, regular water monitoring has revealed the return of low-oxygen conditions, however, along with other signs of worsening water quality such as fish kills and jellyfish blooms. Here, we assess the role of the sediments in the (re)occurrence of low-oxygen conditions in Lake Veere. During two sampling campaigns in 2022, water column and sediment samples were collected. Geochemical analysis, including direct in-situ flux measurements with a benthic lander, revealed an increasing sedimentary oxygen demand (SOD) from the western (sea-side) part of the lake to the east, from ~10 to >100 mmol O2 m-2 d-1. This gradient in SOD opposes the observed trend in water column deoxygenation, with low-oxygen conditions predominantly prevailing in the central and western part of the lake and not in the east. This indicates that, despite restoration efforts, large parts of the lake are still highly sensitive to deoxygenation. Sediment analyses show the near-absence of iron-oxides, hence little capacity to buffer toxic hydrogen sulfide, which indeed accumulated in pore waters, reaching concentrations of up to 10 mmol L-1. In the central part of the lake, hydrogen sulfide even accumulated in the bottom waters, pointing towards its potential involvement in the observed fish kills in the region. Our results illustrate the difficulty of improving water quality through changes in water exchange alone because of strong legacy effects of eutrophication and deoxygenation in the sediment.