Coastal restoration success via emergent trait-mimicry is context dependent

Coastal ecosystems provide vital ecosystem functions and services, but have been rapidly degrading due to human impacts. Restoration is increasingly considered key to reversing these losses, but is often unsuccessful. Recent work on seagrasses and salt marsh cordgrasses highlights that restoration yields can be greatly enhanced by temporarily mimicking key emergent traits. These traits are not expressed by individual seedlings or small clones, but emerge in clumped individuals or large clones to locally suppress environmental stress, causing establishment thresholds where such density-dependent self-facilitation is important for persistence. It remains unclear, however, to what extent the efficacy of restoration via emergent trait-based mimicry depends on the intensity of stressors. We test this in a restoration experiment with the temperate seagrass Zostera marina at four sites (Finland, Sweden, UK, USA) with contrasting hydrodynamic regimes, where we simulated dense roots mats or vegetation canopies with biodegradable structural mimics. Results show that by mimicking sediment-stabilizing root mats, seagrass transplant survival, growth and expansion was strongly enhanced in hydrodynamically exposed environments. However, these positive effects decreased and turned negative under benign conditions, while mimics insufficiently mitigated physical stress in extremely exposed environments, illustrating upper and lower limits of the application. Furthermore, we found that aboveground structures, designed to mimic stiff rather than flexible vegetation canopies, underperformed compared to belowground mimics. Our findings emphasize the importance of understanding the conditions at the restoration site, species-specific growth requirements, and self-facilitating traits that organisms may express when applying emergent trait-mimicry as a tool to improve restoration success

Bio-morphodynamics of coastal wetlands with mangrove vegetation

Mangroves are valuable coastal ecosystems that support and protect the lifes and lifelihoods of people across the tropics/subtropics. They predominantly grow at muddy shorelines, where their trunks and numerous aerial roots protect the coasts against erosion. Rising sea levels, reduced sediment availability, land-use change and widespread mangrove clearance are the primary threats to not only mangrove coverage but also the stability of the coastal landscape. Since the complexity of underlying mechanisms controlling mangrove ecosystems makes it difficult to predict mangrove evolution and devise management regimes, we are in dire need for improved process-based knowledge. This PhD thesis fulfills this need, through a bio-morphodynamic computer model developed to predict decadal and centennial mangrove- and coastal dynamics in response to changes in the coastal environment. The model shows that the balance of sea-level rise and sediment supply is a main determinant in predicting changes in mangrove coverage and diversity. Different coastal environmental conditions create distinct mangrove behaviors in response to rising sea levels. Inland dikes and barriers may constrain mangrove inland migration under sea-level rise, reducing their coverage and diversity. However, increasing fluvial sediment supply resulting from land-use change upstream stimulates mangrove expansion. Modelling shows that the coastal management strategy of mangrove clearance does not restore previously existing sandy flats but might precipitate an even muddier system. My results suggest that effective coastal management needs to consider a system approach and non-linear interactions between mangrove forests and their coastal environment

Adventures of salt marsh recruits: Mechanistically understanding the journey to dispersal and establishment

Salt marshes are among the most valuable ecosystems, but currently suffering from massive habitat shrinkage. Conservation and restoration practices oriented toward salt marsh ecosystems are in full swing around the world. However, outcomes of such practices were often elusive because of the erratic demographic loss or mortality during the early recruitment stages. Mechanistic insights into the underlying processes that enable or disable vegetation recruitment are thus indispensable cornerstones for decision-making in salt marsh conservation and restoration. Relative to asexual reproduction through tillering and cloning, significant advantages exist in rapidly establishing foundational vegetation and building population genetic diversity through seed-based recruitment strategies. In this study, we provide integrated experimental insights in the relative importance of biotic and abiotic factors in dominating seed-based salt marsh recruitment. After detaching from parent plant, the dispersal potential of seeds is primarily constrained by buoyancy period. In wind- and wave-dominated environments, seed dispersal speed would vary greatly depending on the wave magnitude, wind direction, as well as dispersal units’ type and morphology. The floating seeds would end up in microsites due to loss of buoyancy, interception of landscape elements and burial of sediments, with their subsequent re-movements typically tied to high-energy hydrodynamic disturbance and associated erosion events rather than biotic traits. In spring, however, seeds would adjust their lift-off threshold, driven by regaining buoyancy and increasing the surface-drag forces during germination, thereby greatly increasing the probability of seed entrainment during calm hydrodynamic events. Seedling dislodgement would occur at any point during rooting, depending on species-specific and time-varying entanglement between stochastic disturbance pulses versus seedlings’ resistance, underscoring the significance of Windows of Opportunity (WoO; i.e., disturbance-free/low periods) for recruitment outcomes. In the early stages of establishment, root-shoot antagonism characterizes the growth rate of seedlings’ resistance to dislodgement by hydraulic disturbances. Subsequently, root length seizes the dominance and determines seedlings’ resistance to dislodgement by sediment erosion. Shallow burial by sediments during rooting would disproportionately strengthen the overall resistance of seedlings and amplify the effectiveness of WoO. Seedling displacement due to insufficient resistance does not necessarily purport the end of recruitment. Since dislodged seedlings are positively buoyant and able to sustain growth in water columns, they have the potential to drift with tidal currents over long distances. After arriving at a new settlement site, these dislodged seedlings may repeat similar processes, including retention, (re-)rooting, and survival, under analogous regulatory mechanisms as the initial establishment from seeds. Of particular note, the effectiveness of WoO in promoting rooting here is further regulated by the age of seedlings being dislodged, with seedlings dislodged at a younger age more likely to achieve re-establishment. Overall, this study contributes to the holistic understanding of both opportunities and bottlenecks involved in seed-based salt marsh recruitment. This mechanistic process study can be translated into practical guidelines for effective conservation and restoration of salt marshes through seed-based or seedling-based approaches, thereby contributing to incorporating salt marshes in nature-based applications such as coastal defense, climate stability, or carbon sequestration

Environmental change in a mega-delta: Dynamics of salt intrusion in the Vietnamese Mekong Delta

Natural resources of the transboundary Mekong River are existential to the livelihood of tens of millions of people. Numerous mainstream and tributary impoundments, overexploitation of groundwater resources as well as excessive sand mining are emblematic of rapid economic development in the region. These significantly strain the riverine resources, and reflect in hydrological regime shift, biodiversity decline and sediment starvation. The Mekong Delta, indispensable to Vietnam’s food and economic security, is impacted by both climatic and anthropogenic drivers of change, within and beyond the delta. Global sea level rise, river discharge anomalies, land subsidence, riverbed/bank and coastal erosion and saline water intrusion are among the most pressing challenges. Among them, increased saline water intrusion, not only has cost the delta millions of dollars yearly in freshwater shortage and crop loss, but is also identified as the key to its strategic land use planning. The present research aims to demonstrate the combined and isolated effects of climatic and anthropogenic drivers of change on past, present, and future dynamics of salt intrusion in the world’s 3rd largest delta. To fill the knowledge gaps, we addressed five topics in this research: 1) we developed a 1D-2D numerical model of the delta to study flow division and barotropic tidal dynamics within the multi-channel estuarine system; 2) we studied the historical trends of tides and salinity in the delta; 3) we developed the first 3D numerical model of the entire Mekong Delta and identified the physical processes leading to increased salinity in detail; 4) we used the 3D model to project saline water intrusion over the next three decades for the delta; 5) we expanded on policy implications of the main findings for the delta and the basin, applicable to similar systems worldwide

Morphodynamics of channel networks in tide-influenced deltas

Deltas formed and change by erosion and sedimentation of sand and mud driven by river flow, ebb and flood. Deltas often have channel networks where the upstream river channel splits in several places, known as bifurcations, into two downstream channels. At bifurcations, river flow and the tides distribute sediment over the channel network and determine the development of the entire delta landscape. In most situations in river bifurcations, one of the two downstream branches fills in and is abandoned. For bifurcations with a large influence of the tides, the distribution of flow and sediment is complex and less well understood than in river-dominated bifurcation. This thesis improves our understanding of the morphodynamics of tide-influenced bifurcations. A novel one-dimensional (1D) numerical model was developed to simulate morphodynamics in tide-influenced river networks. The results of the 1D model are validated by a well-established two-dimensional model. The outcomes show that: (1) with tides, channels are less likely to fill in and be abandoned than in river-dominated bifurcation system, and (2) in channel networks with multiple-bifurcations, the final morphological equilibrium strongly depends on the initial conditions, meaning that the delta development is a complex result of the river- and coastal processes and the history of the system

Building and raising land: meandering rivers and filling estuaries

Estuaries form in drowned landscapes where rivers transition into the sea and come in a broad range of shapes and sizes. Previous efforts to categorise these tidal systems used the relative importance of rivers, waves and tides. However, this left differences due to the history of these systems largely unexplained, particularly the degree of estuary filling. Until now, only two kinds of steady-state estuaries are recognised in the literature, namely an unfilled lagoonal estuary with limited sediment supply and dynamics and a completely filled estuary with an ideal upstream-converging shape. A mechanism that could explain the diversity in estuary filling is well-known in rivers: floodplain formation. In this thesis, the parallels of muddy, vegetated floodplain formation in rivers and estuaries are explored. A recurring research question is to what extent these tidal floodplains influence the steady state of entire estuaries. By means of estuary experiments, it was studied to what extent a balance in floodplain formation and destruction also influences the steady states of estuaries. With only sand, the estuary filled sufficiently to form a multi-channel pattern with intertidal bars. Mud filled accommodation in the intertidal-to-supratidal reach. Compared to the experiments without vegetation, the vegetation channelised flow, which increased sediment transport in the upstream direction. Mud and vegetation narrowed the subtidal channels in the upstream direction, where the extent of intertidal and supratidal areas was governed by the balance between floodplain formation and destruction. This suggests that a range of steady states exist between the end-members of an unfilled and a completely filled estuary. Fast sea-level rise was applied in a new filling estuary experiment with otherwise the same conditions and caused sediment deposition closer to its marine and fluvial sources. As a result, the central part of the estuary became sediment-starved and subsequently drowned with fewer locations available for vegetation establishment. Next, four polders were added to the Western Scheldt in a numerical model and tested for different opening sequences and inlet widths. Transitional polders opened in an upstream-to-downstream sequence resulted in stronger shallowing of the estuary and hence in smaller tidal ranges than a downstream-to-upstream sequence, suggesting the opening sequence can play a part in managing flood risk. Polders opened later in a sequence temporarily experienced a lag in mud deposition, and net sand import was smaller for polders with a wide foreshore due to deeper inlet erosion. This thesis proposes that tidal floodplains promote the filling of estuaries, where the balance between floodplain formation and destruction determines the steady state of an estuary. A stronger tendency to form floodplains results in a narrower estuary, similarly to how more extensive floodplains transform rivers from braided to meandering. Fast sea-level rise will probably enhance floodplain destruction in estuaries, which asks for effective strategies such as transitional polders to retain sediments, raise land and maintain ecologically valuable intertidal areas

A Holocene flood record of the Lower Rhine

Severe floods caused extensive damage and life-loss throughout Europe over the last decades. The magnitude and the short recurrence interval of large events raised questions about the actual safety standards for flood protection. This was reason to raise the 1,250-yr design flood for river dikes in the Netherlands (Waterwet, 2009) from 15,000 to ~16,000 m3s-1. A major problem in calculating the magnitude of the design flood is uncertainty, which mainly originates from limited data availability for predicting the recurrence time of extreme events; a 110-year interval of discharge data presumably poorly represents the distribution of extremes through time. Non-stationarity of the flooding regime further complicates the use of short discharge records for flood frequency analysis, as it is not expected that the distribution and magnitudes of floods is fixed in time. During the Holocene, climate variability and growing human influence have exerted perturbations to the fluvial system, which translates to gradual changes in flood probabilities. In this research available data was increased by harvesting information from sedimentary records from oxbow lake fills of the Lower Rhine. These records were cross-validated with instrumental records, historical records of the last six centuries, and between overlapping sedimentary archives. This resulted in a flood record that stretches back to 8,200 years BP. To arrive at such a record and to use it for flood risk assessments, methodological advances were necessary. First, it was shown how alternative observational information from multiple stations and historical records can be used to extend the discharge record at Lobith. Based on this data it was already demonstrated that floods of the last decades are more rare than previously considered. Moreover, this data clearly indicated multi-decadal non-stationarity of the flooding regime. Next, various types of infilling abandoned channels were studied to target the best sites for retrieving sedimentary flood records; it was concluded that especially oxbow channel fills yield long and subtle flood deposits registration with high preservation potential and recording of flood magnitudes in the coarsest admixed grains in flood beds. Sedimentary characteristics of channel fills and flood beds were correlated with contemporaneous discharge records (since 1770 AD) and major geomorphological changes in the Lower Rhine floodplain. Besides the possibility to use largest events as stratigraphic markers, the correspondence of general fill properties to accumulation rates allowed to refine standard linear age-depth modelling. Discharges of palaeofloods were calculated from the established regression between grain-size characteristics in flood deposits and measured discharges, and hydraulic modelling based on the elevation of slackwater deposits on high terrace levels in the Lower Rhine Valley. It was found that around 4700 years ago, an extreme must have occurred of at least 13,250 m3s-1, larger than any measured discharge. Considering anthropogenic adjustments, this discharge corresponds to at least 14,000 m3s-1 in the present situation, thus reaching similar values as current flood protection levels. Other floods of similar size (millennium floods) occurred around 784 and 1374 AD, and 4500 and 6200 years ago. The 8200-year record also indicates strong temporal variability in the flood regime, especially strong during the Little Ice Age

Impact of small-scale transverse bed slope effects on large-scale morphology: Experimental and modelling studies

Morphodynamic models are essential tools to predict the development of fluvial and tidal systems in scientific and engineering studies, and are increasingly used for decision making regarding climate change mitigation, flood control, navigation and engineering works. However, many existing morphodynamic models predict unrealistically high channel incision, which is masked by increasing downward sediment transport on the side-slopes of channels up to two orders of magnitude higher than the default value. The formulation for this slope effect is overly simplistic and based on a few experiments with a small range of flow conditions and fine sediments. In this thesis, firstly the slope effect was quantified for a large range in flow conditions and sediment sizes by executing over 250 experiments. Secondly, model simulations were conducted for various scales and environments to identify possible explanations for the severe incision and to study the effect on large-scale morphology. Experimental results showed a new relation, but are still in the same order as the default value for the slope parameter in morphodynamic models. Model results show that the severe incision in morphodynamic models depends on grid size and the amount of suspended sediment. An arbitrary bed slope calibration to obtain realistic morphology may cause an order of magnitude error in rate of morphological change, channel depth and bar dimensions. Furthermore, it is impossible to calibrate a model on both sediment transport magnitude and morphology, which has major implications for calibrated models that are used for decision making, and suggests a critical knowledge gap

Washover inundation and barrier island accretion

The overall aim of this PhD-thesis was to improve the understanding of the role of the washover openings on the Wadden Islands on the storm-induced and cross-shore directed sediment transport on the short- (event scale) as well as medium-term (annual to decadal time scale). The Wadden Islands form a chain of barrier islands, located in the North Sea along the coast of the Netherlands, Germany and Denmark. Wadden Islands in a natural state often are characterized by the presence of several washover openings. In the past though many of these openings were closed off by artificial sand-drift dikes. This measure prevents the area behind the (artificial) dunes from flooding, but has as side effect that potential onshore-directed and storm-induced sediment transport is blocked. Coastal zone management organizations in the Netherlands consider the re-opening of the washovers by removing parts of the sand-drift dikes and it is hypothesized that this might lead to vertical accretion behind the dunes. The influence of wave, tide and storm surge conditions on sediment transport across the beach of a typical mesotidal Wadden Island during inundation was explored with a morphostatic 1D XBeach model study (Chapter 2).The medium-term sediment transport across the barrier island was studied. Six representative inundation classes were distinguished, ranging from frequently occurring, low-energy events to infrequent, high-energy events, and the hydrodynamics and sediment transport during these events were simulated. An analysis of the model simulations showed that larger storm events cause larger cross-shore sediment transport due to stronger currents and larger waves. However, the net onshore directed sediment transport during a storm levels off or even becomes smaller for the largest inundation classes because it is counteracted by larger mean water levels in the Wadden Sea (back-barrier). This opposes or even reverses sediment transport during inundation. The effects of the geometry of the washover openings on hydrodynamics and sediment transport during inundation were investigated with a morphostatic 2D XBeach model study (Chapter 3). Based on the XBeach simulations, two important effects of washover width were identified: firstly, for narrow openings flow contraction is important, causing relatively larger sediment exchange rates per unit width. Secondly, in a wider opening sediment is transported over a larger width, resulting in larger sediment mass exchange rates. These effects combined lead to a larger sediment load that is transported through a washover opening when the opening is wider. Morphological developments (25 years) were explored in a morphodynamic 2D XBeach model study (Chapter 4). Results showed that inundation events erode the washover openings with values of more than 0.5 m in total and deposit this sediment onshore. When a high rate of sea-level rise is taken into account (10 mm/y), deposition values increase with approximately 60%, and likewise vertical accretion rates increase as well. However, the accommodation space created by sea-level rise is not completely compensated by washover deposits. The natural evolution of a barrier island in absence of coastal management, and specifically the evolution of washover openings and the landward supply of sediment was investigated in a case study at the island of Rottumeroog (Chapter 5). The island is characterized by a natural washover, interrupting the dune system. The first years since 2005 were characterized by strong erosion of the beach and foreshore, followed by the development of a 600 m wide breach in the dunes. This breach developed into a washover and has been active since then. Bed level elevation in the washover increased with values up to 0.8 m, reflecting cross-shore supplies of sediment in a single storm season of about 62 m^3/m (volume per meter coastline)

Global chronostratigraphical correlation table for the last 2.7 million years, version 2019 QI-500

A substantially updated version of the correlation table showing chronostratigraphical subdivisions of late Cenozoic geological time, spanning the last 2.7 million years is presented. It provides scientists, students, professionals and the general public with a ready reference to stratigraphical terms and schemes in use in different areas for similar periods. The updates comprised the status of Quaternary chronostratigraphic subdivision, the combined age-modelled geomagnetic and isotope records from ocean drilling records, and revised regional correlation schemes, notably for eastern Europe. The paper describes the chart in its 2019 QI-500 form and contains sections on its types of usage and formal subdivision status, besides reference and description of the contents of the various columns. The paper also describes and discusses the resolution of correlations in younger and older parts of the last 2.7 Ma