Physical-biological interactions limit the resilience of coastal wetlands to sea level rise

<p>Predictions of the effects of sea-level rise over the next century on coastal wetlands vary widely due to uncertainties on environmental variables, but also due to simplifications on the simulation methodologies used. Here, we investigate how accretion and migration processes affect wetland response to sea level rise (SLR) using a computational framework that includes all relevant hydrodynamic, sediment transport and vegetation dynamics mechanisms that affect wetland evolution, and it is efficient enough computationally to allow the simulation of long time periods. We apply this framework to different settings typically found in coastal wetlands around the world, comprising different vegetation types, different sediment loads, obstructions to flow and drainage structures, both natural and man-made. We find that the vast majority of wetland settings analysed are unable to cope with high SLR rates and disappear before the end of the century. Our findings are consistent with paleo-records that indicate limits on the accretion capacity of coastal wetlands during periods of high SLR rates.</p>

Accretion, retreat and transgression of coastal wetlands experiencing sea-level rise

The vulnerability of coastal wetlands to future sea-level rise (SLR) has been extensively studied in recent years, and models of coastal wetland evolution have been developed to assess and quantify the expected impacts. Coastal wetlands respond to SLR by vertical accretion and landward migration. Wetlands accrete due to their capacity to trap sediments and to incorporate dead leaves, branches stems and roots into the soil, and they migrate driven by the preferred inundation conditions in terms of salinity and oxygen availability. Accretion and migration strongly interact and they both depend on water flow and sediment distribution within the wetland, so wetlands under the same external flow and sediment forcing but with different configurations will respond differently to SLR. Analyses of wetland response to SLR that do not incorporate realistic consideration of flow and sediment distribution, like the bathtub approach, are likely to result in poor estimates of wetland resilience. Here, we investigate how accretion and migration processes affect wetland response to SLR using a computational framework that includes all relevant hydrodynamic and sediment transport mechanisms that affect vegetation and landscape dynamics, and it is efficient enough computationally to allow the simulation of long time periods. Our framework incorporates two vegetation species, mangrove and saltmarsh, and accounts for the effects of natural and manmade features like inner channels, embankments and flow constrictions due to culverts. We apply our model to simplified domains that represent four different settings found in coastal wetlands, including a case of a tidal flat free from obstructions or drainage features and three other cases incorporating an inner channel, an embankment with a culvert, and a combination of inner channel, embankment and culvert. We use conditions typical of SE Australia in terms of vegetation, tidal range and sediment load, but we also analyse situations with three times the sediment load to assess the potential of biophysical feedbacks to produce increased accretion rates. We find that all wetland settings are unable to cope with SLR and disappear by the end of the century, even for the case of increased sediment load. Wetlands with good drainage that improves tidal flushing are more resilient than wetlands with obstacles that result in tidal attenuation, and can delay wetland submergence by 20 years. Results from a bathtub model reveals systematic overprediction of wetland resilience to SLR: by the end of the century, half of the wetland survives with a typical sediment load, while the entire wetland survives with increased sediment load.

Spatial–Temporal Wetland Landcover Changes of Poyang Lake Derived from Landsat and HJ-1A/B Data in the Dry Season from 1973–2019

As China’s largest freshwater lake and an important wintering ground for white cranes in Asia, the Poyang Lake wetland has unique ecological value. However, wetland cover types have changed dynamically and have attracted the attention of society and researchers over the past few decades. To obtain detailed knowledge and understanding of the long-term landcover dynamics of Poyang Lake and the associated driving forces, Landsat and HJ-1A/B images (31 images) were used to acquire classification and frequency maps of Poyang Lake in the dry season from 1973–2019 based on the random forest (RF) algorithm. In addition, the driving forces were discussed according to the Geodetector model. The results showed that the coverage of water and mudflat showed opposite trends from 1987–2019. Water and vegetation exhibited a significant decreasing trend from 1981–2003 and from 1996–2004 (p < 0.01), respectively. A phenomenon of vegetation expanding from west to east was found, and the expansion areas were mainly concentrated in the central zone of Poyang Lake, while vegetation in the northern mountainous area of Songmen (region 1) and eastern Songmen Mountain (region 2), showed a significantly expanded trend (R2 > 0.6, p < 0.01) during the five-decade period. The year-long dominant distribution of water occurred mainly in the two deltas formed by the Raohe and Tongjin rivers and the Fuhe and Xinjiang rivers, with deep water. In the 1973–2003 and 2003–2019 periods, a total of 313.522 km2 of water turned into swamp and mudflat and 478.453 km2 of swamp and mudflat transitioned into vegetation, respectively. Elevation and temperature appeared to be the main factors affecting the regional wetland evolution in the dry season and should be considered in the management of Poyang Lake. The findings of this work provide detailed information for spatial–temporal landcover changes of Poyang Lake, which could help policymakers to formulate scientific and appropriate policies and achieve restoration of the Poyang Lake wetland.

A catchment to coast framework for the evolution of a coastal mangove wetland

&lt;p&gt;Coastal wetlands are at the interface between land and sea, receiving water, sediment and nutrients from upstream catchments and also being subject to tides, wave and changing sea levels. Analysis of their future evolution requires the analysis of the entire catchment to coast system, including the effects of climate variability and change and land use changes. We have developed a modelling framework that is able to include both catchment and coastal processes into the evolution of coastal wetlands by coupling an ecogeomorphological wetland evolution model with a hydrosedimentological catchment model to include both tidal and catchment runoff inputs. We drive the model with storm events and sea-level variations and analyse scenarios of future climate and land use for a catchment in Vanua Levu, Fiji that includes a mangrove wetland at the catchment outlet. We inform our model with field, remote sensing and historical data on land use, tides, sediment and nutrient transport and cyclone activity.&lt;/p&gt;

Understanding the effects of dynamic sediment inputs on the prediction of coastal wetland evolution

&lt;p&gt;Over the last two decades, there have been important advances in eco-geomorphological modelling of coastal wetlands to predict their evolution. Different features have been incorporated into models, bust most applications still assume a constant or static sediment concentration as input representing average conditions. Such imposition is related to many constraints in obtaining a time series of total suspended matter (TSM). However, with the increasing availability of multispectral satellite products and the development of artificial intelligence algorithms, TSM data can be estimated through remote sensing. This work aims to assess the effect of using a dynamic time varying condition for the TSM input when simulating eco-geomorphological processes. We implemented a modelling framework adapted to conditions found in SE Australian estuaries, which includes hydrodynamic and sediment transport processes. Many scenarios where simulated encompassing different levels of average TSM and water levels. Our findings suggest that under low water levels and low sediment concentration, a static TSM input results in more accretion than a dynamic input. However, at higher levels and concentration, the dynamic input led to higher accretion. Predictions of vegetation distribution were not particularly sensitive to changes in TSM over time.&lt;/p&gt;

Large-Scale Marsh Loss Reconstructed from Satellite Data in the Small Sanjiang Plain since 1965: Process, Pattern and Driving Force

Monitoring wetland dynamics and related land-use changes over long-time periods is essential to understanding wetland evolution and supporting knowledge-based conservation policies. Combining multi-source remote sensing images, this study identifies the dynamics of marshes, a core part of wetlands, in the Small Sanjiang Plain (SSP), from 1965 to 2015. The influence of human activities on marsh patterns is estimated quantitatively by the trajectory analysis method. The results indicate that the marsh area decreased drastically by 53.17% of the total SSP area during the study period, which covered the last five decades. The marsh mostly transformed to paddy field and dry farmland in the SSP from 1965 to 2015, indicating that agricultural encroachment was the dominant contributor to marsh degradation in the area. Analysis of the landscape indexes indicates that marsh fragmentation was aggravated during the past five decades in the SSP. Trajectory analysis also indicated that human activities have acted as the primary driving force of marsh changes in the SSP since 1965. This study provides scientific information to better understand the evolution of the wetland and to implement ecological conservation and sustainable management of the wetlands in the future.

Temporary wetland evolution in the upper Chinchiná river basin and its relationship with ecosystem dynamics

A study was performed regarding high Andean wetland degradation in a paramo area between the municipalities of Villamaría and Manizales, Colombia, by way of multi-temporal analysis, using satellite images from optical sensors, such as LANDSAT and RAPIDEYE, as well as images from RADAR sensors (ALOS PALSAR, SENTINEL 1), and analysis of anthropic and natural factors. As a result, the wetlands have begun a significant, linear decline with 67.9% water mirror loss in a nine-year period. There is also a direct relationship between wetland loss, and decreases in precipitation, and anthropization processes. It was determined, from the anthropic factor analysis, that that livestock and agricultural land use are those which cause the greatest negative effect on wetland decline in the studied area.