No description available.

No description available.

Coastal saltmarshes are vital ecosystems because (a) they physically buffer the land against storms and flooding from the sea and (b) ecologically they are high-productivity systems in estuaries and marine coastlines that shelter and support fish and bird populations. Saltmarshes are highly sensitive to sea level change. Any saltmarshes are now threatened by rising sea level, but how they will respond and at what rate is unclear. Managing saltmarshes is therefore necessary, but requires a good understanding of their development in order to predict how they might respond to sea level change. Current management practice in the UK is mainly managed realignment landward and future scenarios are predicted with computer models. Both use the hypothesis of facilitation succession, whereby saltmarsh progrades seawards. An alternative hypothesis is saltmarsh development by transgression landward due to rising sea level. This thesis critically examines how saltmarshes have developed under different sea-level change regimes in order to gain an insight into how they are likely to be affected by future sea level rise. Using established micropalaeontological techniques, Foraminifera tests and Ostracoda shells were extracted from sediment cores taken from saltmarshes representing a range of sea level change histories during the Holocene. Sampling of modern environments allowed saltmarsh vegetation zones to be characterised by foraminiferal and ostracod assemblages which were then used to reconstruct the development of saltmarshes over time as preserved in the cores. Sediment layers in the cores were dated using three techniques: Optically Stimulated Luminescence (OSL), radio carbon (14C) and 137Cs / 210Pb. The latter hypothesis is supported in southern England where marine transgression caused saltmarshes to migrate landwards, in contradiction to the facilitation succession hypothesis. In Scotland saltmarshes advanced seawards but due to a marine regression. Future studies should explore the applicability of these findings to saltmarshes outside the British Isles.

Global climate change threatens the survival of microtidal wetlands by altering fundamental hydrological aspects such as precipitation patterns and tidal exchange. The combination of these stressors results in increased flooding period and soil salinity in coastal wetlands. In this study, we combined the use of detailed hydrological measurements (wetland water level and salinity), LIDAR elevation models, and water stable isotopes tracers (δD, δ18O) to study the balance between freshwater and saltwater inputs on two microtidal wetlands: a saltmarsh in Matagorda, Texas and a freshwater-forested wetland in Humacao, Puerto Rico. In Matagorda, Texas, we described the process of connectivity between different hydrologic units (isolated and connected ponds) within the saltmarsh. Pond connectivity only occurred when water levels in major water bodies adjacent to our study site reached a threshold elevation of 0.39 m. Connections events were correlated to rainfall and— to a lesser extent— wind speed and direction. We conclude that connectivity within the saltmarsh is driven by the combined effect of tidal influence and rainfall inputs, factors that will be altered by sea level rise and climate change-related changes in long term weather patterns. In Humacao, Puerto Rico, we gathered a detailed dataset of changes in salinity and water level in a freshwater forested wetland dominated by the endangered salt intolerant species Pterocarpus officinalis. In addition, we studied tree water use and identified important water sources to the wetland using stable isotope tracers. Firstly, we provide evidence that recent hydrological alterations have effectively transformed the system from mostly freshwater, to a saltwater wedge estuary. Salinity inputs travel via a tidal creek channel that allows the progression of a saltwater wedge to the inland parts of the forest. Our results suggest that inland progression of the saltwater wedge is influenced by amplitude of tidal exchange in the middle portions of the tidal creek and by extended dry periods in the headmost part of the tidal creek. Isotope data showed that surface standing water was influenced by tidal water sources during the dry season, although the spatial extent of this influence was constrained to areas of the forest that had been previously deforested. The isotopic content of groundwater samples taken at increasing distances from the tidal creek revealed that— although surface waters are dominated by freshwater inputs (rainfall and runoff) during the wet season— the influence of tidal water sources at soil depths greater than 60 cm persists throughout the year. Nonetheless, isotopic content of Pterocapus officinalis stem water samples suggest that tree water uptake is constrained to very shallow, unsaturated parts of the soil. We conclude from both case studies that the long term vulnerability of microtidal wetlands to climate change is determined by the interaction of increased annual variability of freshwater inputs along with a steady increase in mean sea levels, and aggravated by extreme climatic events.

Nature-based solutions (NBS) are increasingly popular infrastructure protection options, particularly in coastal engineering. These systems have shown the ability to provide similar coastal protection services to traditional hard schemes while providing other ecological and economic benefits, and a capacity to adapt to changing contexts. One prominent example of coastal NBS are saltmarshes: fields of flexible or semi-flexible vegetation, which have been found to significantly reduce damage to local communities under daily and storm conditions. Scientific study of these complex, multi-faceted structures is growing in volume, but there remain many knowledge gaps in the field. Numerical modelling is a powerful tool for investigating both large- and small-scale behaviours of saltmarshes. Numerical models provide a controlled, repeatable, and easily variable method for testing how a marsh impacts local hydrodynamic climates and how incident flow or wave conditions affect the behaviour of their constitutive vegetation. Small-scale plant behaviour is the focus of this thesis. Literature on the subject has been chiefly limited to greatly simplified vegetation modelling, reducing plants' behaviour to that of straightforward rigid cylinders. While this can be effective, it requires significant calibration to measured data and may not provide an accurate picture of the intricate flow dynamics surrounding an individual plant, let alone a full marsh system. Recently, numerical models capable of modelling flexible structures have been developed and used by researchers. However, studies applying these tools have focussed on replicating the more significant hydrodynamic effects of marshes, such as mixing or wave attenuation. By doing so, the calibration requirements of the rigid-type models remain, and the way the plants themselves are modelled loses physical meaning beyond their hydrodynamic impacts. The work presented in this thesis aims to expand on current flexible plant modelling research by evaluating a new numerical modelling tool in the open-source software REEF3D for replicating in situ saltmarsh plant behaviour in terms of drag force and motion response to hydrodynamic forcing. Three experimental programs were designed and conducted in order to thoroughly evaluate both aspects of the model. The first, based on a flume study performed by Paul et al. (2016), tested the drag force response to regular wave action. The second, based on the work of Tschisgale & Fröhlich (2020), further investigated the drag force response using closed- and open-channel flow, as well as solitary waves. The third, based on a flume study performed with live vegetation by Markov et al. (2023), evaluated the accuracy of the motion response to irregular waves. Consistent through all three programs was an overestimation of the examined behaviour and, in the third case, persistent model breakdown. These results demonstrate that, as tested, the evaluated tool is unsuitable for this purpose. It is suggested that this is due to the foundational assumptions of the model, namely that the material of the flexible structure is of a linearly viscoelastic type, whereas a nonlinear elastic material would be more appropriate for this application. These results highlight the difficulty of numerically modelling these systems and the need for further research developing and applying practical modelling tools for marshes.

Saltmarshes in SE England are eroding rapidly and one potential impact is the loss of habitat for fishes. Saltmarshes have been created by setting back the existing line of flood defence through managed realignment. The use by fishes of natural and managed realignment habitats at Tollesbury, Abbotts Hall and Orplands was examined (2005-07). Three seasonal groups were apparent in the fish assemblages of the managed realignment sites: February-April (Pomatoschistus microps and Sprattus sprattus), May-September (Dicentrarchus labrax and Atherina presbyter) and October-January (Liza aurata and Liza ramada). The sites were used mainly by 0- and 1-group fishes and adult P. microps. The mean abundance (July - August 2007) was 558 0.1 ha' (range 76 - 2699 0.1 ha'). In summer, small (< 30 mm) zooplanktivorous D. labrax fed successfully at all sites. Larger (30-59 mm) D. labrax consumed more macroinvertebrates in the Tollesbury managed realignment and two established marshes than at Abbotts Hall and Orplands. By autumn there were no site-specific differences in gut fullness of D. labrax. Stable isotope ratio analysis and gut contents analysis revealed that small (< 50 mm) D. labrax, S. sprattus and A. presbyter assimilated zooplankton which eat detritus, resuspended microphytobenthos and some phytoplankton. L. aurata assimilated zooplankton and microphytobenthos. P. microps (20-50 mm) and A. presbyter (80-99 mm) assimilated benthic meiofauna. Larger (50-230 mm) D. labrax assimilated macroinvertebrates which eat microphytobenthos, Ulva spp., C3 plants and detritus. Some recommendations for saltmarsh restoration are provided with an estimate of the economic value of bass in saltmarshes,to highlight further areas of research.