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We studied how changing human impacts affected phytoplankton dynamics in the freshwater and brackish tidal reaches of the Zeeschelde estuary (Belgium) between 2002 and 2018. Until the early 2000s, the Zeeschelde was heavily polluted due to high wastewater discharges. By 2008, water quality had improved, resulting in lower nutrient concentrations and higher oxygen levels. Since 2009, however, increased dredging activities resulted in altered hydro-dynamics and increased suspended sediment concentration. The combined effects of these environmental changes were reflected in three marked transitions in phytoplankton community composition. Assemblages were dominated by Thalassiosirales and green algae (especially Scenedesmaceae) until 2003. The period 2003-2011 was characterized by the wax and wane of the centric diatoms Actinocyclus and Aulacoseira, while in the period 2012-2018 Thalassiosirales and Cyanobacteria became dominant, the latter mainly imported from the tributaries. Phytoplankton biomass increased sharply in 2003, after which there was a gradual decline until 2018. By 2018, the timing of the growing season had advanced with about one month compared to the start of the study, probably as a consequence of climate warming and intensified zooplankton grazing pressure. Our study shows that de-eutrophication (during the 2000s) and morphological interventions in the estuary (in the 2010s) were dominant drivers of phytoplankton dy-namics but that the main shifts in community composition were triggered by extreme weather events, suggesting sig-nificant resistance of autochthonous communities to gradual changes in the environment.

Title : Compendium voor Kust en Zee = Compendium for Coast and Sea. Vlaams Instituut voor de Zee (VLIZ): Oostende. ISSN 2983-5526; e-ISSN 2983-5534 Volume : 2023 Issue : Pagination : 1-13

The freshwater Scheldt Estuary (Belgium), located in one of the most densely populated areas of Europe (over 500 inh./km²), is a highly eutrophic system currently recovering from hyper-eutrophication thanks to improved waste water treatment efforts. As a result, the ecosystem shifted from a respiration dominated system towards a more autotrophic system, causing a spectacular and rapid improvement of the water quality, especially documented by increased oxygen levels. However, focusing at the biogenic parameters in the water column, the freshwater Scheldt does not meet good water quality standards yet, and for the past 10 years, no important further improvement is recorded. On the contrary, there are indications of a declining quality again. This time, water quality from the catchment is no longer the major problem. Important hydrodynamic and geomorphologic changes caused by dredging, deepening, … what else? Activities in favor of navigation ( ) within the estuary itself seem to influence water quality. Altered residence times and a changing light climate can indeed strongly reduce pelagic primary production.

Zowel de Marker Horizons, als de Surface Elevation Table’s en de Marker Palen geven nog steeds aan dat een sterke opslibbing van het gebied gaande is. De trend die in 2014 reed opgemerkt werd, met name de vermindering van de hoeveelheid jaarlijks afgezette sediment, werd verder doorgetrokken in 2015. De verklaring hiervoor is dat de meeste slikken zich niet ver meer onder de gemiddelde hoogwaterlijn bevinden. Verwacht wordt dat de sedimentatiesnelheden de komende jaren nog verder zullen dalen. Voor de vegetatie is dit positief nieuws. Nu al merken we in het noordwestelijke gedeelte van het gebied, het gebied met de laagste sedimentatiesnelheden, een voorzichtige ontwikkeling van de diverse schorvegetaties op. Andere delen hebben anderzijds nog steeds te lijden onder de hoge sedimentatiesnelheden, waardoor vegetaties niet de kans hebben om de stukken te koloniseren. Een aantal algemene tot schaarse schorrensoorten hebben het gebied ondertussen weten te vinden en zetten de eerste stapjes in het kolonisatieproces. Andere zeldzame, maar te verwachten schorrensoorten, hebben het gebied nog niet ontdekt. Dit is niet geheel onlogisch daar de meeste van deze soorten typisch pas verschijnen in de latere successiestadia van de vegetatie. Verwacht wordt dat het dan ook maar een kwestie van tijd is tot de meeste van deze soorten Burchtse Weel weten te koloniseren.

In this paper, an analysis of continuous SSC measurements in the Sea Scheldt is presented. Information from different projects that focus on the state of the Scheldt Estuary (Belgium-Netherlands) and the management of dredging and disposal activities, is discussed to increase the insight in the effects of mud disposal and the increasing sediment concentrations (SSC) in the Lower Sea Scheldt. A positive feedback loop has been hypothesized and supported by observations in several estuaries, showing that an increase in tidal amplitude may lead to an increase in sediment concentrations, decrease of hydraulic drag resistance and again in tidal amplitude increase. Estuaries that have undergone such transformations are now characterized by hyperturbid conditions, the presence of fluid mud layers and ultimately, a poor ecological state. The feeds to the positive feedback loop appear to be present in the Lower Sea Scheldt as well: increase of tidal amplitudes in the past decades, and more recently changes in SSC. Although the long-term trend may still not be revealed, it is shown that mud disposal in the Low Sea Scheldt influences SSC in a larger area. Increased mud disposal leads to rather immediately elevated SSC values. On the other hand, a decrease of mud disposal also resulted in decreased SSC values. It is therefore suggested that future sediment management choices will partly determine whether SSC will continue to rise in the Sea Scheldt.

Deelstudie 9 volgt de ontwikkeling op in estuariene herstelprojecten. Hierbij kunnen we twee types monitoring onderscheiden: onderzoeksmonitoring en projectmonitoring. In Lippenbroek wordt onderzoeksmonitoring verricht in het kader van OMES. In een zeer gedetailleerde proefopzet wordt de ontwikkeling van het gebied opgevolgd, met als doel kennis te vergaren. Lippenbroek is immers het eerste gecontroleerd overstromingsgebied met gecontroleerd gereduceerd getij. Een tweede type van monitoring vormt de projectmonitoring. Deze monitoring is minder intensief en gaat via een aantal sleutelparameters hoe de gebieden evolueren. Binnen OMES worden Bergenmeersen, Burchtse en Bazel opgevolgd.

Several recent studies have revealed the impacts of the climate variability in the dynamic of zooplankton in different estuarine ecosystems, imposing a need for more and continued global studies. Presently, there is a growing appreciation in international collaborations to compare and contrast estuarine ecosystem response to climate variability across geographical gradients, including long-term changes in zooplankton. We performed a meta-analysis comprising field data from 7 location (Mondego estuary-Portugal, Seine estuary- France, Sheldt estuary- Belgium, Kiel fjord - Germany, Gulf of Riga- Latvia, Gulf of Bothnia – Sweden and Finland Archipelagos). The use of climatic modes (e.g. NAO, ENSO) has proven useful in investigating links between climatic variations and ecological patterns. Therefore, the main focus will be to test the influence of the NAO on abundance of organisms, key species, local environment and whether these relationships are generally positive, whether they are sensitive to methodological differences among studies, between taxonomic group and key species. The knowledge gained will contribute to quantitatively evaluate the multi-scale structure of climate and marine environment and to identify a set of environmental indicators to assess the estuarine ecosystem state and risks for ecological shifts.

Flood Control Areas (FCAs) are one of the measures to protect the Flemish part of the Scheldt estuary against storm surges. Recently, in some of these FCAs a Controlled Reduced Tide (CRT) was introduced by means of simple sluice constructions, aiming at a combination of flood control with nature development and the ecological functions of intertidal areas. This paper gives an overview of the different types of inlet sluices and energy dissipation measures that have been designed, built and/or are under construction: an inlet sluice with an inclined drop, an inlet sluice with a pooled step cascade and a combined inlet-outlet sluice with an internal drop structure.

We report a 40 year record of eutrophication and hypoxia on an estuarine ecosystem and its recovery from hypereutrophication. After decades of high inorganic nutrient concentrations and recurring anoxia and hypoxia, we observe a paradoxical increase in chlorophyll-a concentrations with decreasing nutrient inputs. We hypothesise that algal growth was inhibited due to hypereutrophication, either by elevated ammonium concentrations, severe hypoxia or the production of harmful substances in such a reduced environment. We study the dynamics of a simple but realistic mathematical model, incorporating the assumption of algal growth inhibition. It shows a high algal biomass, net oxygen production equilibrium with low ammonia inputs, and a low algal biomass, net oxygen consumption equilibrium with high ammonia inputs. At intermediate ammonia inputs it displays two alternative stable states. Although not intentional, the numerical output of this model corresponds to observations, giving extra support for assumption of algal growth inhibition. Due to potential algal growth inhibition, the recovery of hypereutrophied systems towards a classical eutrophied state, will need reduction of waste loads below certain thresholds and will be accompanied by large fluctuations in oxygen concentrations. We conclude that also flow-through systems, heavily influenced by external forcings which partly mask internal system dynamics, can display multiple stable states.