Data and code for: Strongly reducing sediments hinder restoration of a eutrophic coastal marine ecosystem

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 (O2) 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 O2 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 O2 saturation of bottom waters. However, improvements were confined to the eastern part nearby the inflow of marine water. Modelled bottom water O2 concentrations and observed benthic macroinvertebrate densities are related, and show reduced densities under low-O2, especially for less mobile infauna. This highlights the detrimental effect of deoxygenation on benthic faunal communities. Geochemical analyses reveal a very high sedimentary O2 demand, mainly caused by high concentrations of the reduced solutes ammonium and hydrogen sulfide (H2S). 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-sulfides, explaining high porewater H2S concentrations, and release of H2S 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.人类活动正导致近岸水体缺氧，进而在全球范围内引发此类生态系统的退化。 在荷兰西南三角洲地区，海岸管理工程已将潮汐水域改造为海洋湖泊。随之而来的水体停滞与层化现象，叠加富营养化作用，导致水质下降，其中包括氧气（O₂）饱和度降低。 此类生态系统的修复极具挑战，以弗尔湖（Lake Veere）为例：二十年前，该湖通过涵洞与潮汐型海洋环境的东斯海尔德河（Eastern Scheldt）重新连通。在经历初期改善后，近年来水质再次大幅恶化。 本研究依托现有氧气、温度、盐度及底栖大型无脊椎动物（benthic macroinvertebrates）的监测数据，探究此次修复工程对近岸水体缺氧的影响。此外，本研究通过野外实地测量，对选定点位的沉积物需氧量（Sediment Oxygen Demand）进行了量化分析。 水质监测数据显示，修复后引入的潮汐作用使底层水体的氧气饱和度得到显著提升。但这一改善仅局限于靠近海水入流的东部区域。模拟得到的底层水体氧气浓度与实测的底栖大型无脊椎动物密度呈显著相关关系：在低氧环境下，生物密度会出现下降，其中活动能力较弱的底内动物（infauna）受影响尤为显著。这一结果凸显了水体缺氧对底栖动物群落的不利影响。 地球化学分析结果表明，沉积物需氧量极高，这主要是由于还原态溶质——铵盐与硫化氢（H₂S）的浓度较高所致。这些溶质具有毒性，由有机质的厌氧（再）矿化作用生成。此外，湖泊沉积物中易还原氧化铁的含量较低，抑制了铁硫化物的形成，这也解释了孔隙水中高浓度硫化氢的成因，以及硫化氢向底层水体释放的现象。 综合来看，本研究结果表明：若仅通过增加侧向供氧来修复海岸生态系统，却未解决沉积物中富营养化遗留效应，修复工作将面临重重挑战；这也再次强调了在修复策略中削减营养盐输入的重要性。