Early indicators of tidal ecosystem shifts in estuaries

Abstract

Forecasting transitions between tidal ecosystem states, such as between bare tidal flats and vegetated marshes, is crucial because it may imply the irreversible loss of valuable ecosystem services. In this study, we combine geospatial analyses of three European estuaries with a simple numerical model to demonstrate that the development of micro-topographic patterning on tidal flats is an early indicator of marsh establishment. We first show that the development of micro-topographic patterns precedes vegetation establishment, and that patterns tend to form only on tidal flats with a slope of <0.3 degrees. Numerical modelling then provides an explanation for the formation of micro-topography due to the natural concentration of draining surface water over very gentle slopes. We find this early indicator to be robust across three estuaries where anthropogenic deepening and narrowing has occurred in recent decades, which may suggest its broader applicability to other estuaries with similar morphological management.

Cross-shore parallel tidal channel systems formed by alongshore currents

“…The systematic study of parallel tidal channels can provide strong support for coastal management and ecological environment protection in the future. First of all, since the parallel tidal channel system is controlled by hydrodynamic and topographic conditions, managers can judge the local environmental conditions and changes based on the morphology of parallel channels through remote sensing, such as the tidal current regime and ecosystem states 35 , providing help for the overall management of tidal flats, especially in the context of global climate change such as sea level rise. Second, it also provides a way to artificially shape the tidal channel system: by adjusting the strength of the alongshore current and the surrounding terrain, the secondary tidal channel system can be shaped as needed to meet the local demand for water, sand, and material exchanges.…”

Size Distribution and Variation in Surface-Suspended Sediments in the North Passage Estuarine Turbidity Maximum of the Yangtze Estuary

“…In order to match the regular tidal cycle, 25 h of data during spring and neap tides for 2016, 2017, and 2018 were used for analysis (Table 1). Since the Yangtze River estuary is a tidal-dominated estuary, implying that the tidal effect is dominant, we assume that tides have a greater effect on estuaries than waves [45][46][47]. For this reason, the present study focused on the influence of tides on the surface sediment concentration and the particle size distribution during the four tidal periods (flood, high slack water, ebb and low slack water).…”

Abiotic origins of self‐organized ridge‐runnel patterns on tidal flats

“…These patterns mainly occur high in the tidal frame, where twice daily tidal inundation does not always occur, and on tidal flats of very shallow slope, below 0.3 (Fivash et al 2023). The formation of ridge-runnel patterns on the tidal flat facilitates the expansion of marsh vegetation in successive years to areas lower in the tidal frame, where hydrodynamic conditions are harsher, due to the stressamelioration by the patterned topography (Fivash et al 2020(Fivash et al , 2023. Unlike more canonical patterned environments, tidal flat "ridge-runnel" patterns are not created by the marsh vegetation itself, but form either through physical or biophysical mechanisms on the tidal flat that marsh seedlings then merely take advantage of (Fivash et al 2020(Fivash et al , 2021Cao et al 2021).…”

“…The linear drainage features commonly run shore normal, parallel to the direction of the steepest slope. These patterns mainly occur high in the tidal frame, where twice daily tidal inundation does not always occur, and on tidal flats of very shallow slope, below 0.3 (Fivash et al 2023). The formation of ridge-runnel patterns on the tidal flat facilitates the expansion of marsh vegetation in successive years to areas lower in the tidal frame, where hydrodynamic conditions are harsher, due to the stressamelioration by the patterned topography (Fivash et al 2020(Fivash et al , 2023.…”

“…Recent studies demonstrate that the formation of "ridgerunnel" regular patterns in the tidal flat bathymetry can trigger a rapid transition to a vegetated state (van de Vijsel et al 2020Fivash et al 2023;Fig. 1).…”