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This report is compiled on request of the Maritieme Toegang and has the goal of evaluating a proposed critical nitrogen (N) load to the Scheldt brackish tidal marshes, defined as the N load above which “there is a risk that the quality of the habitat will be significantly affected by the acidifying and/or eutrophicating influence of atmospheric nitrogen deposition”. The present document consists of two main sections: in the first, the proposed nitrogen threshold is evaluated; in the second, a nitrogen mass-balance for brackish tidal marshes is constructed, based on a) available literature from comparable estuaries and b) data obtained from the Scheldt. The magnitude of the pools and fluxes of nitrogen in brackish tidal marshes and its transformations is then evaluated and compared to the proposed critical N load. Most N pools and fluxes in brackish tidal marshes (and in the Scheldt in particular) are in the same order of magnitude or regularly exceed the proposed threshold. This could indicate that estuaries in Europe are already highly impacted by nitrogen pollution, yet it might also mean that the threshold value itself is inappropriate and was not constructed keeping estuaries like the Scheldt in mind. This report discusses both hypotheses.

Managing sediments, especially from dredging, disposal, nourishments and sand mining is important for the management of estuaries and coastal areas. When implemented in the right way, a sediment management strategy can be qualified as a nature-based solution as it uses the physical processes of erosion and sedimentation to create added value. There is a need for an evaluation of sediment strategies and the habitats that are created for a wider range of objectives than only biodiversity and nature conservation. The concept of ecosystem services provides this broader framework.

It is expected that climate change will cause more frequent extreme events of heavy precipitation and drought, changing hydrological conditions in riverine ecosystems, such as flow velocity, evapotranspiration (drought) or runoff (heavy precipitation). This can lead to an increased input of terrestrial organic matter and elevated levels of dissolved organic carbon (DOC) and CO2 due to degradational processes in water. Consequences for submerged macrophytes, as essential organism group, are still poorly understood. The combined effects of changing flow velocity, DOC and CO2 have not been studied before, so this was tested in a racetrack flume experiment on the macrophyte Berula erecta using a trait-based approach. The plants were exposed to two different flow velocities, two DOC concentrations and two CO2 concentrations in a full factorial design. Apart from individual dose-response tests, two climate change scenarios were tested: a wet scenario simulating heavy precipitation and runoff with high flow velocity, high DOC and CO2 concentrations and a dry scenario simulating evapotranspiration with low flow velocity, high DOC and high CO2 concentrations. Growth rate, biomass, morphology, chlorophyll and nutrient content (C, N, and P) were measured. B. erecta responded strongly to both scenarios. Biomass and the relative growth rate increased and stems were shorter, especially in the wet scenario, and vegetative reproduction (the number of stolons) decreased. In both scenarios, the N content was lower and P content higher than in conditions without climate change. It can be concluded that climate change effects, especially shading by DOC, strongly influence macrophytes: macrophyte abundance will probably be negatively affected by climate change, depending on the macrophyte species and abundance of epiphytic algae. This may have consequences for other components of the aquatic ecosystem.

Anurans from the genus Xenopus have long been used as standard testing organisms and occur naturally in tropical and sub-tropical areas where malaria vector control pesticides are actively used. However, literature on the toxic effects of these pesticides is limited. This review analyses the available data pertaining to both Xenopus and the pesticides used for malaria vector control in order to determine the pesticides that have the greatest potential to influence amphibian health while also identifying gaps in literature that need to be addressed. Amphibian diversity has shown the fastest decline of any group, yet there are still voids in our understanding of how this is happening. The lack of basic toxicity data on amphibians with regard to pesticides is an issue that needs to be addressed in order to improve effectiveness of amphibian conservation strategies. Meta-analyses performed in this review show that, at current usage, with the available acute toxicity literature, the pyrethroid pesticide group could hold the highest potential to cause acute toxicity to Xenopus sp. in relation to the other MVCPs discussed, but the lack of data cripples the efficacy with which meta-analyses can be performed and conclusions made from such analyses. Several studies have shown that DDT accumulates in Xenopus sp. from malaria vector control areas, but accumulation of other MVCPs in frogs is still largely unknown. Through this review we hope to encourage future research into the field of amphibian ecotoxicology and to promote the use of the Xenopus standard model in order to build comprehensive datasets that may be used in amphibian conservation.

In 2013 verdwenen onverwachts de macrofyten (ondergedoken waterplanten) in grote delen van de Grote Nete. De Vlaamse Milieumaatschappij (VMM) zocht naar de aanleiding van deze verdwijning en naar mogelijkheden om de waterplantenpopulatie te herstellen. Uit dit vooronderzoek kwamen twee plausibele hypotheses naar voren omtrent de oorzaak van het probleem: (1) de aanwezigheid van grote aantallen invasieve Chinese wolhandkrabben die de waterplanten zouden aantasten en groei verhinderen, en (2) de aanwezigheid van een ecotoxicologische stof die de groei van waterplanten zou inhiberen. Dit eindrapport geeft een overzicht van de resultaten van het onderzoek naar de eerste hypothese.