Modeling estuaries as eco-engineered landscapes : How species shape the morphology of past, present and future estuaries

Abstract

Estuarine morphology evolves from a combination of various drivers, including hydrodynamics, sediment input, human interference and ecology. The latter has been shown to affect local morphology through eco-engineering, meaning the modification of the physical habitat through the presence or activity of the prevailing species. Despite many studies confirming eco-engineering effects on local morphology, our understanding of how eco-engineers determine the morphology of entire estuaries is still limited. This understanding is however crucial to better describe the past formation of tidal systems, to guarantee their sustainable usage, and to protect our coasts against the future impacts of climate change. In this PhD thesis, a novel eco-morphodynamic model is presented to elucidate the effects of various eco-engineers in estuaries. The modelling framework captures the feedback between environmental processes, eco-engineering and changes in habitat through the adapting morphology. The model revealed that eco-engineering by vegetation is one main determinant for the establishment and expansion of saltmarshes in dynamic estuaries. Hereby, this effect depends on the vegetation age marked by increasing plant sizes and resilience to stress, which defines a distinct density zonation. The expansion of the saltmarsh enhances the accumulation of mud on intertidal bars, depending on vegetation properties: The species-specific establishment strategy controls the abundance and thickness of mud layers in estuaries. At the scale of intertidal flats, species-specific establishment, growth, mortality rules control the formation of tidal channel networks, associated levee formation and drainage density. Climate-induced shifts in dominant species affect the channel network, enhancing the capacity to drain marshes and export sediment, reducing their resilience against sea level rise. Macrozoobenthic bioturbators enhance the system-wide erosion of the estuary by resuspending cohesive mud. Through their activity, these species reduce bed elevations, expose the supratidal area and cause lateral expansion of the estuary. Biofilms stabilize mud and morphology, but their effect reduces when macrobenthos grazes on them. A combination of several bioturbators and biofilms results in a restructured species community, solely as a consequence of the eco-engineering effects: The altered habitat by one species allows for the colonization by another, which outcompetes the first species. These ecological interactions lead to an adapted morphology, showing that the prediction of morphological evolution is not trivial and requires the consideration of ecosystem interactions. Sea level rise reduces mud content of the estuary promoting a species shift, where a sand-prone bioturbator thrives and expands its habitat while a mud-prone species almost disappears. This shift of dominating eco-engineers has implications for the net eco-engineering effect, morphology and estuarine resilience to sea level rise. In ancient estuaries of the Palaeozoic, the evolution of plant traits that include root development, increasing plant densities and dimensions and enhanced resilience against environmental stresses, gradually increased estuarine mud content. Here, the onset of arborescent vegetation led to system-scale mud deposition as opposed to previously observed local mud deposition within plant patches. These results indicate that the earliest small plants had the capacity to eco-engineer their environment and that the greening of the continents affected morphologies, which rendered pre-vegetation sedimentary processes non-uniformitarian.