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As an estuary being restored, the Scheldt (Belgium/The Netherlands) offers an interesting setting to study the response of organisms and ecosystems to changing conditions. In this regard, this study specifically deals with the spatio-temporal distribution and possible genetic differentiation among the species complex Eurytemora affinis (Copepoda, Calanoida). Until the 1990s, E. affinis typically occurred downstream the estuary. In parallel to water quality improvement, E. affinis since recently also occurs upstream in the estuary and in some of the tributaries. In a former study, Gasmi et al. (2014), found little genetic differentiation between the upstream population and downstream one, which is probably at the origin of the upstream population. However, surprisingly, two different genetic entities of the E. affinis complex were present: one in the main stream of the Scheldt estuary and one in its tributaries. This present work aims at understanding how independent tributaries, situated at opposite sides of the estuary, can host a single genetic entity while another one is present only in the main estuary. A dense sampling in the main stream and the two main tributaries was made in May 2014 (12 locations) and four ISSR primers producing 78 polymorphic loci were used to perform a molecular analysis. Bayesian and hierarchical analysis revealed that the results are more mixed and the different genetic entities are less well delimited than reported in our former study based on samples collected in 2012. A genetical mix between tributary and upstream populations is now observed. This suggests that a homogenization process of the E. affinis populations is in progress in parallel to the improvement of water quality. These results are discussed and several explanations are proposed. • Molecular diversity of Eurytemora affinis. • Fast genetic changes in populations of the species complex Eurytemora affinis. • Spatio-distribution of E. affinis genotypes in parallel to an estuary restoration process. [ABSTRACT FROM AUTHOR]

Abstract: The inter-annual variability in phytoplankton summer blooms in the upper reaches of the Schelde estuary was investigated between 1996 and 2005 by monthly sampling at 10 stations. The large inter-annual variations of the chlorophyll a concentration in the freshwater tidal reaches were independent from variations in chlorophyll a in the tributary river Schelde. Summer mean chlorophyll a concentrations were significantly negatively correlated with flushing rate (Spearman correlation: r =−0.67, p =0.05, n =9) but not with temperature, irradiance and suspended particulate matter or dissolved silica (DSi) concentrations. During dry summers, low flushing rates permitted the development of dense phytoplankton populations in the upper part of the estuary, while during wet summers high flushing rates prevented the development of dense phytoplankton blooms. Flushing rate was also found to be important for the phytoplankton community composition. At low flushing rates, the community was dominated by diatoms that developed within the upper estuary. At high flushing rates, chlorophytes imported from the tributary river Schelde became more important in the phytoplankton community. The position of the chlorophyll a maximum shifted from the head of the estuary when flushing rates were low, to more downstream when flushing rates were high. Although DSi concentrations tended to be lower during years of high phytoplankton (mainly diatom) biomass, the relation with flushing rate was not significant. [Copyright &y& Elsevier]

Flux calculations demonstrate that many estuaries are natural filters for trace metals. Yet, the underlying processes are poorly investigated. In the present study, it was hypothesized that intertidal marshes contribute significantly to the contaminant filter function of estuaries. Trace metal concentrations and sediment characteristics were measured along a transect from the subtidal, over an intertidal flat and marsh to a restored marsh with controlled reduced tide. Metal concentrations in the intertidal and restored marsh were found to be a factor two to five higher than values in the subtidal and intertidal flat sediments. High metal concentrations and high accretion rates indicate a high metal accumulation capacity of the intertidal marshes. Overbank sedimentation in the tidal marshes of the entire estuary was calculated to remove 25% to 50% of the riverine metal influx, even though marshes comprise less than 8% of the total surface of the estuary. In addition, the large-scale implementation of planned tidal marsh restoration projects was estimated to almost double the trace metal storage capacity of the present natural tidal marshes in the estuary.