Critical transitions in suspended sediment dynamics in a temperate meso-tidal estuary

Abstract

There is growing consensus that human interventions can fundamentally change fine sediment transport in estuaries. Critical transitions in response to human interventions have been hypothesized based on indirect observational evidence and theoretical understanding. So far direct evidence has been lacking. Based on a 20 year data-set of surface suspended particulate matter (SPM) concentrations, we present empirical evidence of critical transitions in a temperate meso-tidal estuary. In 2008–2009 the SPM dynamics of the Scheldt estuary (Belgium/The Netherlands) changed dramatically. Not only did the total amount of sediment in suspension increase, a new maximum turbidity zone (MTZ) at typical winter discharges appeared. At intermediate and low summer discharges the longitudinal distribution of SPM now flickers between two markedly different states. Our data suggest that a range of human interventions (fairway widening and deepening, dredging and dumping activities) set the scene leading to the observed transitions. Moreover the freshwater MTZ in the Scheldt and in its major tributary exhibit an increasing sensitivity towards freshwater discharge, coinciding with water quality improvements. This suggests large scale impacts of changes in eutrophication status on estuarine sediment dynamics. This has largely been a blind spot in morphodynamic research.

Effects of sea-level rise on tides and sediment dynamics in a Dutch tidal bay

“…With the resonance impacts ruled out, the increased TR due to SLR in the Oosterschelde can result from reduced friction and the amplifying nature of a convergent basin. In a dissipative basin, SLR and channel deepening can reduce the bottom friction and increase tidal amplitude (e.g., de Jonge et al, 2014). With the declining friction, the system can even shift from a damping into amplifying basin, such as the Hudson River (Ralston et al, 2019).…”

“…found in a theoretical study that narrowing and deepening of small convergent estuaries tends to increase tidal amplitude and flood dominance, resulting in a hyper-turbid state. Deepening-induced stronger tidal amplitude is reported in the Ems (de Jonge et al, 2014), Hudson (Ralston et al, 2019), and Loire rivers , as well as the Cape Fear River estuary (Familkhalili and Talke, 2016), especially in the upper reach. Tidal responses to deepening also show intra-and intersystem variability due to the various deepening extents and characteristics of estuaries (Talke and Jay, 2020).…”

“…In surrounding systems such as the Western Scheldt estuary and Rotterdam waterway, south and north to the Oosterschelde, respectively, tides and sediment dynamics have been changing over the last century as a result of sand mining, dredging, and modifications of shorelines and navigation channels van Rijn et al, 2018;Cox et al, 2019;Dijkstra et al, 2019a). The tidal range in the southern North Sea has increased since the 1950s, partially due to the engineering works on the Dutch Delta (Hollebrandse, 2005).…”

“…Shifts in estuarine and coastal tidal regimes including tidal amplitude, residual currents, and tidal asymmetry can potentially modify sediment transport, which further influences shoreline morphology and the accretion (or erosion) of salt marshes and tidal flats (van Goor et al, 2003;Chernetsky et al, 2010;Nnafie et al, 2014). Furthermore, in estuaries and embayments, changes in tidal mixing and currents are expected to have implications for salt intrusion, nutrient transport, primary production, and other ecosystem functions (Nienhuis and Smaal, 1994;Zhang et al, 2010;de Jonge et al, 2014). Therefore, in order to understand and mitigate the SLR hazards in estuaries and coastal bays, it is of high priority to study the SLR impact on tides.…”

Evolution of the long-term and estuary-scale phytoplankton patterns in the Scheldt estuary: the disappearance of net growth in the brackish region

“…On the other hand, the implementation of the wastewater treatment capacity in, for example, Brussels in 2006 (Brion et al, 2015) resulted in a significant improvement of the water quality (Maris and Meire, 2017;Cox et al, 2019). In particular, the oxygen levels significantly increased and did not show critical concentrations for fish abundance below 2.5 mg L −1 (see O 2 in Fig.…”