G.S. Fivash1*, M. Heuner 2, D. Holthusen2, J. Carus2, T.J. Bouma1,
Worldwide, estuaries have been intensively utilized as shipping-lanes to service the global supply chain. To maintain their function in service of port cities, major structural changes are taking place in estuaries that will ultimately impact in their ecological character. The rising and flattening of estuarine morphology, driven by human usage, has been forecasted to drive the evolution of tidal ecosystems toward widening vegetated zones encroaching upon an ever-narrowing band of tidal flats. This future scenario has significant consequences for protected species that rely on intertidal mudflats for food and habitat. In one heavily navigated estuary, the Western Scheldt (servicing the port of Antwerp), vegetation is not only expanding over the rising mudflat shelves, but is also occurring more frequently at lower elevations every year. This is indicative of the presence of additional feedback processes that aid the establishment of vegetation in previously inhospitable areas. In this study, we demonstrate how biogeomorphic feedbacks between vegetation and tidal drainage patterns are responsible for this accelerating terrestrialization process.
This is done through GIS analysis of semi-annual bathymetry and false color data of the Dutch Western Scheldt between 2004 and 2020.
The appearance of small-scale drainage patterns on high intertidal flats commonly precedes the invasion of salt marsh vegetation via seedling-establishment. Here we demonstrate how the formation of these small-scale drainage patterns is becoming more likely as high intertidal mudflats progressively flatten into plateaus. Vegetation establishment is more likely to occur at low elevation in these well-drained patterned landscapes. The establishment of pioneer vegetation then leads to drainage pattern intensification and propagation, expanding the region hospitable to further establishment. This feedback process effectively allows the vegetation to step-stone deeper down the tidal gradient once initial establishment has occurred. Together, this suggests that whilst the raising of estuarine tidal flats in navigated estuaries increases linearly in time, the expansion of the vegetated areas in these environments will occur in the coming decades in sudden self-reinforcing events, as the threshold requirements in bathymetric elevation and slope for biological succession are suddenly met, and the vegetation-drainage feedback loop is set in motion.
Figure: Mudflat channelization followed by vegetation expansion (in red, 2018) on Hoogeplaat in the Western Scheldt. Source: False color images, Rijkswaterstaat.