Coastal Landform Constrains Dispersal in Mangroves

Mangrove forests are dynamic ecosystems found along low-lying coastal plains along tropical, subtropical, and some warm-temperate coasts, predominantly on tidal flats fringing deltas, estuaries, bays, and oceanic atolls. These landforms present varied hydrodynamic and geomorphological settings for mangroves to persist and could influence the extent of within-site propagule transport and subsequent local regeneration. In this study, we examined how different landform characteristics may influence local genetic diversity, kinship, and neighborhood structure of mangrove populations. To do so, we considered independent populations of Avicennia marina, one of the most abundant and widespread mangrove species, located in estuarine and coastal bay environments spread across the Western Indian Ocean region. A transect approach was considered to estimate kinship-based fine-scale spatial genetic structure using 15 polymorphic microsatellite markers in 475 adult A. marina trees from 14 populations. Elevated kinship values and significant fine-scale structure up to 30, 60, or 90 m distances were detected in sheltered systems void of river discharge, suggesting a setting suitable for very local propagule retention and establishment within a neighborhood. Slopes of a linear regression over restricted distance within 150 m were significantly declining in each sheltered transect. Contrastingly, such a spatial structure has not been detected for A. marina transects bordering rivers in the estuarine systems considered, or alongside partially sheltered creeks, suggesting that recruitment here is governed by unrelated carried-away mixed-origin propagules. South African populations showed strong inbreeding levels. In general, we have shown that A. marina populations can locally experience different modes of propagule movement, explained from their position in different coastal landforms. Thus, the resilience of mangroves through natural regeneration is achieved by different responses in coastal landforms characterized by different hydrodynamic conditions, which can be important information for their management and protection within the variety of coastal environments.

Between Natural and Anthropogenic Coastal Landforms: Insights from Ground Penetrating Radar and Sediment Analysis

Both natural and anthropogenic coastal landforms characterize Penang Island. As years have passed it is a challenge to differentiate the genuineness of landmasses created by natural geological formations or by coastal reclamation projects. An account is given of the environmental impact of solid wastes used for reclaiming land in coastal areas of Penang and of the impact of a major sewage outfall in the western channel. Leaching of heavy metals was shown to be one of the main sources of contamination from solid wastes. This paper presents eight lines of ground penetrating radar (GPR) surveys and sediment analysis to identify the anthropogenic interventions that shaped the urban landscape of Penang Island by excavations, filling, and embankment construction along the coastline and differentiate it from the natural one. The surveys were implemented in two locations, the Batu Ferringhi area, representing the natural coastline, and Persiaran Bayan Indah (the Queensbay Mall area), representing the anthropogenic coastal landform. The apparent depth of penetration that was achieved using a 250-MHz antenna is limited (less than 5 m). The results show between natural and anthropogenic sediment recorded different radar facies. In complement mode, mean grain size distribution, sorting, skewness, and kurtosis graphics of sediment samples from both sites correspond with the GPR data. This technique can likely be applied to the developing coast, where natural and anthropogenic coastal landform data is incomplete, considering future coastline development.

Structure-from-Motion-Derived Digital Surface Models from Historical Aerial Photographs: A New 3D Application for Coastal Dune Monitoring

Recent advances in structure-from-motion (SfM) techniques have proliferated the use of unmanned aerial vehicles (UAVs) in the monitoring of coastal landform changes, particularly when applied in the reconstruction of 3D surface models from historical aerial photographs. Here, we explore a number of depth map filtering and point cloud cleaning methods using the commercial software Agisoft Metashape Pro to determine the optimal methodology to build reliable digital surface models (DSMs). Twelve different aerial photography-derived DSMs are validated and compared against light detection and ranging (LiDAR)- and UAV-derived DSMs of a vegetated coastal dune system that has undergone several decades of coastline retreat. The different studied methods showed an average vertical error (root mean square error, RMSE) of approximately 1 m, with the best method resulting in an error value of 0.93 m. In our case, the best method resulted from the removal of confidence values in the range of 0–3 from the dense point cloud (DPC), with no filter applied to the depth maps. Differences among the methods examined were associated with the reconstruction of the dune slipface. The application of the modern SfM methodology to the analysis of historical aerial (vertical) photography is a novel (and reliable) new approach that can be used to better quantify coastal dune volume changes. DSMs derived from suitable historical aerial photographs, therefore, represent dependable sources of 3D data that can be used to better analyse long-term geomorphic changes in coastal dune areas that have undergone retreat.

Multiple sand bar dynamics in the macrotidal Shinduri beach, west coast of Korea

<p>Extensive tidal flats and sandy beaches are a typical coastal landform along the macrotidal west coast of Korea. Macrotidal beaches typically with long stretches and the gentle slope, contain often a series of sand bar, parallel to the coastline. Shinduri beach under macrotidal condition in a semi-closed embayment, western coast of Korea, has three to four lines of sand bars on about 500 m stretches of intertidal zone. Interesting feature is the dynamic behaviour of the multiple bars. These bars appear only in summer and disappear dramatically all in winter, which is opposite pattern in common beaches. In order to understand the seasonal dynamics of multiple bars on a macrotidal beach, three years of topographic survey using a VRS-GPS system have been conducted at three-months interval on six transects placed on the beach. In addition, an ADV/ADCP was deployed to collect wave data on a bar. Shinduri beach is a 4 km in length and about 500 m in width. Topographic survey reveals that three to four bars occur only during summer, and disappear suddenly during winter. In response to bar growth and destruction, beach slopes become steeper in winter and gentler in summer. Mean grain sizes show generally shoreward coarsening trend, ranging from 2.0 phi to 2.75 phi. Sediments get coarser in summer, but finer in winter, which are opposite compared to other beaches in the west coast of Korea. Wave data show strong seasonality, high waves in winter and much gentler waves in summer, suggesting the study area experienced by monsoon climate. The opposite pattern of multiple bar dynamics, growth in summer and destruction in winter, is likely associated with strong winter waves, destroying the bars and hence filling the trough of bars, thereby the beaches becoming flat in topography. From spring the bars start to form under normal wave condition. This signifies that local wave condition is more important for maintaining patterns of multiple bars.</p>

Geological Evidence of Shoreline Erosion and Mitigation Challenges

A survey was carried out along parts of the coastal stretch of Accra, the capital of Ghana, to determine the existence andthe extent of erosion. The survey was also to determine the effectiveness of the engineering solutions that have been put inplace. Coastal land forms as evidence of erosion were identified during the field survey. These include landslides of steepcoastal cliffs, coastal caves and arches, retreat of coastal cliffs, headlands, stacks and sand dunes. These pieces of evidenceconfirm earlier findings that the coastal shoreline of Accra is being eroded. The extent of erosion, however, is high in soft rocks and low in hard rocks. Several coastal protection measures have been put in place to address the effects of erosion by the sea waves and currents. These include revetments, jetties, ripraps and beach nourishment. Some of the mitigation measures however have shown signs of failure. The signs include rusted metal basket supporting cobbles of gabions, jetties causing down-current erosion of shoreline cliffs, and reduction in coconut population along the shoreline owing to human activities, such as sand winning and diseases attacks. Sand winning along the shore is lowering shoreline morphology and enhancing sea transgression and the destruction of coastal structures. Keywords: Erosion; Coastal landform; Coastal protection; Revetments; Shoreline morphology

Sea Surface-Visible Aquaculture Spatial-Temporal Distribution Remote Sensing: A Case Study in Liaoning Province, China from 2000 to 2018

Aquaculture plays an important role in providing food and reducing poverty but it affects environmental change and coastal ecosystems. Remote sensing is a technology that is helpful in the spatial-temporal dynamic monitoring of aquaculture, coastal management, and environmental monitoring. Most research focuses on inland and coastal areas, and little attention is paid to the extensive distribution of marine aquaculture. As an example, we use the freely available Landsat data of the developed marine aquaculture Liaoning Province of China and use the object-oriented automatic extraction method to analyze the spatial and temporal distribution information of marine aquaculture from 2000 to 2018. The accuracy evaluation from the randomly distributed sample points in high-resolution remote sensing images shows that the extraction accuracy for all of the five individual years of aquaculture area was higher than 82%. The results showed that (1) in the past 19 years, the area of marine aquaculture in Liaoning Province showed an increasing trend, which increased from 35.41 km2 in 2000 to 201.83 km2 in 2018, approximately 5.7 times increase in total area, but the growth rate decreased slightly due to government policy and the environmental quality of the sea area. (2) The centroid of offshore aquaculture in Liaoning Province shows a migration pattern to the northeast, in general, extending from the Dalian Bay sea area to the eastern sea area of the Dalian Chengshantou National Nature Reserve of Coastal Landform in the northeastern direction, and the migration distance reached 48.78 km. Moreover, the migration distance between 2005 and 2010 was the largest of all of the periods, reaching 35.43 km. The new marine aquaculture areas are mainly concentrated in the eastern direction of Xiaoyao Bay, the Changshan Islands, and Guanglu Island in Changhai County. (3) The landscape pattern of marine aquaculture in Liaoning Province is split, large-scale aquaculture and small-scale aquaculture are symbiotic, and landscape ecological activities are active. For local managers, this study can provide valuable supporting data for the assessment of marine aquaculture yield in this region, comprehensive control and management of the marine environment, and stability of the marine ecosystem. For other countries or regions, this work provides a great reference value for monitoring the dynamic spatial distribution of marine aquaculture.