Strongly reducing sediments hinder restoration of a eutrophic coastal marine ecosystem

Abstract Anthropogenic activities are causing deoxygenation of coastal waters leading to degradation of such ecosystems globally. In the Dutch Southwest Delta, coastal management has turned tidal waters into marine lakes. The ensuing stagnation and stratification in combination with eutrophication has caused a decline in water quality, including decreased oxygen (O 2 ) saturation. Restoring such systems is challenging as exemplified by Lake Veere, which was reconnected to the tidal marine Eastern Scheldt, via a culvert, two decades ago. After an initial improvement, water quality strongly declined again in recent years. Here, we investigate the consequences of the restoration effort on coastal deoxygenation by leveraging existing monitoring data of oxygen, temperature, salinity, and benthic macroinvertebrates. In addition, we quantified the O 2 demand of the sediment at selected sites via field measurements. Water quality monitoring data indicate that the restored tidal influence resulted in strong improvements in O 2 saturation of bottom waters. However, improvements were confined to the eastern part nearby the inflow of marine water. Modelled bottom water O 2 concentrations and observed benthic macroinvertebrate densities are related, and show reduced densities under low-O 2 , especially for less mobile infauna. This highlights the detrimental effect of deoxygenation on benthic faunal communities. Geochemical analyses reveal a very high sedimentary O 2 demand, mainly caused by high concentrations of the reduced solutes ammonium and hydrogen sulphide (H 2 S). These solutes are toxic and are formed by anaerobic (re)mineralization of organic matter. Furthermore, low availability of easily reducible iron-oxides in the lake sediments mutes the formation of iron-sulphides, explaining high porewater H 2 S concentrations, and release of H 2 S into bottom waters. Combined, our results illustrate the challenge of restoring a coastal ecosystem by increasing lateral oxygen supply without addressing the legacy effects of eutrophication in the sediments, reemphasizing the importance of nutrient reductions in restoration strategies.