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

T J S Cox,P Meire,T Maris,T Van Engeland,K Soetaert

doi:10.1038/s41598-019-48978-5

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.

Highlights

One of the key defining characteristics of estuaries is their ability to effectively trap marine and fluvial sediments[1]
We look for interactions of sediment dynamics with the human-induced improvements in water quality, testing the hypothesis that changes in eutrophication status can have system scale impact on estuarine sediment distribution
Lowest concentrations are found in the Westerschelde (WS, 0 km–50 km from the mouth; 1995–2015 mean: 36.9; sd: 17.2)

Summary

Introduction

One of the key defining characteristics of estuaries is their ability to effectively trap marine and fluvial sediments[1]. Over the past years the research community has taken renewed interest in estuarine suspended particulate matter (SPM) dynamics This has been triggered by dramatically changed SPM dynamics resulting in hyper-turbid conditions in certain heavily impacted estuaries[11,12]. At high SPM concentrations, vertical density gradients can result in reduced vertical mixing[18] potentially affecting the flow and introducing further asymmetry and upstream fine sediment transport[19]. The biophysical determinants of aggregation-disaggregation dynamics of cohesive sediments is currently a hot research topic (e.g.22) In estuaries this is important since the settling velocity of aggregates co-determines the location and intensity of the maximum turbidity zone. Water treatment efforts led to rapid changes in water quality, first in the USS (around 200326), and later in the Rupel tributary[28,29]

Methods

Results

Discussion

Conclusion