Bio-physical controls on tidal network geomorphology

The copyright of this thesis rests with the author and no quotation from it or information derived from it may be published without the prior written consent of the authorLooking over a tidal wetland, the tidal network characterised by its intricate system of bifurcating, blind-ended tidal courses clearly stands out from the overall landscape. This tidal landform exerts a fundamental control on the morphology and ecology within the tidal environment. With today’s recognition of the ecological, economical and societal values provided by tidal wetlands, which has been notably reflected in the development of restoration management strategies across Europe and USA, there is a need to fully understand the nature and development of tidal networks as well as their relationships with associated landforms and biotic components (e.g. vegetation), to eventually guarantee the success of current and future restoration practices. Accordingly, this research aims to bring further insights into the bio-physical controls on the geomorphology of tidal networks. To this end, a combination of remote sensing, modelling and field activities was employed. A geo-spatial analysis was performed at Queen Mary, University of London (UK), to address the variability of tidal network patterns. A series of network scale morphometric variables was extracted using airborne LiDAR data among selected tidal networks across the UK depicting different planview morphologies, and supplemented with the collection of corresponding marsh scale environmental variables from published sources. Multivariate statistics were then performed to characterise the variability of tidal network patterns and identify the inherent environmental controls. The analysis has revealed that every network type can be characterised based upon measures of network size and complexity, with each network pattern depicting proper morphometric aspects. Particularly, the stream Strahler order and the median depth of the network main channel have the highest discriminating weight on the patterns investigated. High correlation between the latter variable and network main channel width has revealed that linear, linear-dendritic and dendritic networks followed a transitional gradient in their aspect ratio approximated by a power law and thus are seen to depict similar erosional processes. To the contrary, meandering networks clearly depart from this relationship, and show particular segregation in their aspect ratios with respect to dendritic networks. Globally, differentiation on network morphometric properties has been linked to environmental conditions specific to the marsh physiographic setting within which a tidal network develops. Conceptually, tidal networks seem to adapt to marsh environmental conditions by adopting suitable morphologies to drain their tidal basin effectively. An eco-geomorphic modelling framework was developed at University of Trento (Italy), to address tidal network morphological development. In line with current theories as well as modelling advances and challenges in the field of tidal network ontogeny, emphasis was thus placed on the investigation of tidal channel formation and evolution in progressive marsh accretional context. Under these environmental conditions, tidal network development can be ascribed to the combination of two channel-forming processes: channel initiation results from bottom incisions in regions where topographic depressions occur; channel elaboration results from differential deposition, contributing to the deepening of the tidal channels relative to the adjacent marsh platform. Further evolutionary stages including channel reduction proceed from the horizontal progradation of the marsh platform which may lead eventually to channel infilling. Moreover, both qualitative and quantitative results allude to an acceleration of the morphological development of the synthetic tidal networks with increasing sediment supply. These different observations thus emphasise the prevalence of depositional processes in shaping tidal channels. In a second stage, the investigation was extended to the role of the initial tidal flat morphology as an inherent control on tidal network development, by considering different scenarios of topographic perturbations, which has revealed its legacy on tidal network morphological features. Modelling experiments have also acknowledged salt marsh macrophytes as a potential control on network evolution depending on their biomass distribution within the tidal frame. However, tidal channel morphodynamcis appears to be sensitive to the way biomass growth is mathematically parameterised in the model. In view of the current challenges in transcribing mathematically such a dynamic process and the relevance of bio-physical interactions in driving salt marsh and tidal network evolution, a field survey was conducted in a temperate salt marsh in the Netherlands, as part of the mobility to UNESCO-IHE (Netherlands) in partnership with University of Antwerp (Belgium), to assess vegetation distribution and productivity in the tidal frame. Particularly, emphasis was placed on extending investigations on the possible presence of relationships involving vegetation properties in different climatic and ecological conditions from those characterising these previously documented relationships. Regression analysis has revealed that biomass growth can be expressed as a linear function of marsh relative elevation, providing therefore direct empirical validation for corresponding assumptions reported in the literature and used in the present modelling framework; surprisingly, that increase did not correlate with an increase in species richness and diversity. Analysis of likely associations between vegetation morphometrics and total standing biomass yielded only a single linear relationship linking the latter variable to stem height. In truth, these observations may bear reconsiderations on the global validity of the assumptions used in the formulation of some eco-geomorphic processes which are applied in the study and prediction of wetland resiliency facing climate change

Carbon dynamics and CO2 and CH4 exchange in the mangrove dominated Guayas river delta, Ecuador

Although estuaries are considered important pathways in the global carbon cycle, carbon dynamics in tropical estuaries is relatively understudied. Here, the tidal, seasonal and spatial variability of particulate organic carbon (POC), dissolved inorganic carbon (DIC), carbon dioxide (CO2) and methane (CH4), among other biogeochemical variables related to carbon cycling, were studied in the Guayas river delta (Ecuador) to document the sources, processing and fluxes of these carbon forms. All variables were studied during a semi-diurnal (13 hour) tidal cycle and along river transects at low and high tides, all carried out during one dry and rainy season. POC and total suspended matter (TSM) strongly covaried and peaked at high tidal flow velocities during a tidal cycle and at high river discharge during the rainy season, suggesting that resuspension of bottom sediments and/or surface erosion in the river catchment were a dominant source of particulate matter in the water column. The δ13C of POC, (from ~-22‰ to ~-27‰) showed an increasing contribution of marine phytoplankton to the POC pool as moving downstream along the delta during the dry season. Upstream DIC concentrations (~1200 μmol L-1) were high in the Guayas river delta as compared to other tropical estuarine systems, and the δ13C of DIC revealed a shift from a more phytoplankton dominated source in the dry season and downstream (~-4‰) to a relatively more terrestrial source in the rainy season and upstream (~-12.5‰). Both DIC and its δ13C showed slight but consistent deviations from conservative mixing that hint at inputs of 13C depleted DIC from mineralization along the delta. High values of the partial pressure of CO2 (pCO2) observed upstream and in the rainy season (~5250 μatm), associated with O2 undersaturation (~60%) and low δ13CDIC, suggest a strongly heterotrophic system, and resulted in high CO2 efflux to the atmosphere. CH4 concentrations were also higher during the rainy than dry season (93.5±62.5 vs. 61.3±39.5 nmol L-1), but unlike pCO2, showed tidal variations similar to TSM and POC, thus alluding to potential CH4 release from sediments during resuspension events at high tidal flow velocities. This explorative survey revealed complex drivers and biogeochemical processes acting upon various spatio-temporal scales which are necessary to consider for a complete understanding of the carbon biogeochemistry in estuarine systems. Similar surveys on estuarine carbon in data scarce regions are encouraged to constrain uncertainties in coastal zone carbon budgets

Coastal wetland adaptability to sea level rise: The neglected role of semi‐diurnal vs. diurnal tides

Abstract Tidal marshes and mangroves are threatened by relative sea level rise (RSLR) in certain regions on Earth. Elsewhere, these coastal wetlands can adapt through sediment accretion and resulting surface elevation gain. Studies identifying drivers of the global variability in coastal wetland adaptability to RSLR ignored the role of the tidal pattern, varying from semi‐diurnal to diurnal globally. Here, we present a meta‐analysis, including 394 marsh and mangrove sites worldwide, and demonstrate that the tidal pattern explains ~ 25% of the variability in wetland elevation response to RSLR. Using a numerical model, we illustrate that less frequent, diurnal tides trigger lower sediment accretion rates, hence higher wetland vulnerability to RSLR, for various values of RSLR rates, tidal range and sediment supply. Our findings reveal a previously overlooked but relevant driver of coastal wetland adaptability to RSLR and call for new research as tidal patterns may affect other wetland ecosystem functions and services

Supporting data for: Coastal wetland adaptability to sea level rise: the neglected role of semi-diurnal versus diurnal tides

Dataset necessary to reproduce the analyses and results presented in the paper. Belliard, J.-P., Gourgue, O., Bouillon, S., Govers, G., Kirwan, M.L., & Temmerman, S. (2021) Coastal wetland adaptability to sea level rise: the neglected role of semi-diurnal versus diurnal tides, submitted to Geomorphology. The dataset contains: • The source codes of the zero-dimensional numerical model, parametrized for the two model experiments, and the Matlab scripts to generate model input data (pre-processing). • The processed data that are reported and represented in figures in the associated manuscript. • The Matlab scripts to generate the figures of the manuscript based on these processed data

Nature-based shoreline protection by tidal marsh plants depends on trade-offs between avoidance and attenuation of hydrodynamic forces

In face of growing land-flooding and shoreline-erosion risks along coastal and estuarine shorelines, tidal marshes are increasingly proposed as part of nature-based protection strategies. While the effect of plant species traits on their capacity to attenuate waves and currents has been extensively studied, the effect of species traits on their capacity to cope with and grow under wave and current forces has received comparatively less attention. We studied the relationships between species zonation and the associated two-way interactions between species traits and hydrodynamics, by quantifying the effectiveness of avoidance and attenuation of hydrodynamic forces under field conditions. Measurements were done for two pioneer tidal marsh species in the brackish part of the Elbe estuary (Germany). Schoenoplectus tabernaemontani (S. tabernaemontani), which grows as a single stem without leaves and Bolboschoenus maritimus (B. maritimus) which grows as a triangular stem with multiple leaves. Our results reveal that S. tabernaemontani grows more seaward being exposed to stronger hydrodynamic forces than B. maritimus. The stems of S. tabernaemontani have, in comparison to B. maritimus, a lower flexural stiffness and less biomass, which decrease the experienced drag forces, thereby favoring its capacity to avoid hydrodynamic stress. At the same time, these plant traits which favor such avoidance capacity, were shown to also result in a lower capacity to attenuate waves and currents. Hence this implies that there are trade-offs between avoiding and attenuating hydrodynamic forces. Most efficient attenuation of waves and currents is thus only reached when species have the ability to grow under the prevailing hydrodynamic forces. Therefore, we argue that the two-way interaction between plants and hydrodynamics contributes to species zonation. The presence of this species zonation in turn enhances the overall efficiency of nature-based shoreline protection in pioneer tidal marshes