Salt marshes are very valuable and threatened ecosystems, and are challenging to study due to their difficulty of access and the alterable nature of their soft soil. Remote sensing methods in unmanned aerial vehicles (UAVs) offer a great opportunity to improve our knowledge in this type of complex habitat. However, further analysis of UAV technology performance is still required to standardize the application of these methods in salt marshes. This work evaluates and tunes UAV-photogrammetry and UAV-LiDAR techniques for high-resolution applications in salt marsh habitats, and also analyzes the best sensor configuration to collect reliable data and generate the best results. The performance is evaluated through the accuracy assessment of the corresponding generated products. UAV-photogrammetry yields the highest spatial resolution (1.25 cm/pixel) orthomosaics and digital models, but at the cost of large files that require long processing times, making it applicable only for small areas. On the other hand, UAV-LiDAR has proven to be a promising tool for coastal research, providing high-resolution orthomosaics (2.7 cm/pixel) and high-accuracy digital elevation models from lighter datasets, with less time required to process them. One issue with UAV-LiDAR application in salt marshes is the limited effectiveness of the autoclassification of bare ground and vegetated surfaces, since the scattering of the LiDAR point clouds for both salt marsh surfaces is similar. Fortunately, when LiDAR and multispectral data are combined, the efficiency of this step improves significantly. The correlation between LiDAR measurements and field values improves from R² values of 0.79 to 0.94 when stable reference points (i.e., a few additional GCPs in rigid infrastructures) are also included as control points. According to our results, the most reliable LiDAR sensor configuration for salt marsh applications is the nadir non-repetitive combination. This configuration has the best balance between dataset size, spatial resolution, and processing time. Nevertheless, further research is still needed to develop accurate canopy height models. The present work demonstrates that UAV-LiDAR technology offers a suitable solution for coastal research applications where high spatial and temporal resolutions are required.

During the last decades many salt marshes worldwide have suffered important losses in their extent and associated ecosystem services. The salt marshes of San Vicente de la Barquera estuary (N Spain) are a clear example of this, with a drastic reduction in vegetation surface over the last 60 years. This paper provides insights into the main factors controlling salt marsh functioning in sheltered estuarine areas. Regional and local factors have been disaggregated to identify the main drivers controlling the functioning of the salt marsh to develop appropriate management measures according to the evolution of the system. These factors have been studied in their spatial context through detailed maps of change in vegetation cover combined with topographic data obtained from UAV and RTK-DGPS surveys. The results demonstrate that in this estuary the salt marsh area is declining following a fragmentation process. No clear pattern of vegetation loss/gain with elevation has been identified. However, the results point to increased hydrodynamic stress in the area, with stronger currents inside the estuary. This is probably the major factor responsible for the decline of the salt marshes in the San Vicente de la Barquera estuary. Furthermore, several human interventions during the 20th century (local drivers) have also probably contributed to a lower resilience against SLR (regional driver). This work demonstrates that both natural and human drivers of change need to be considered when characterizing the evolution of salt marshes, wherever efficient management strategies need to be designed.

Geomorphological changes in recent decades in three estuaries along the Iberian coast were analysed using aerial orthophotographs. A hierarchical classification scheme, based on a literature review representing 26 estuarine eco-geomorphological features relevant to estuarine dynamics and functioning, is described. The estuaries selected were San Vicente de la Barquera (N Spain), Guadiana River (SW border between Spain and Portugal) and the Ebro River Delta mouth (NE Spain). For these systems, a 60-year time series of high-resolution maps was developed, analysing the changes in feature surfaces. The main subsystems analysed were beach, dunes, saltmarshes and the drainage network. The results of the cartographies showed general behaviour common to all transitional systems, relationships among main subsystems and processes inherent to each one. This work illustrates how beaches and dunes serve as a protective barrier for the tidal flats, acting as a sediment buffer for the entire system. The subsystems are connected by the drainage network responsible for the exchange of matter and energy between them. Furthermore, an accuracy assessment was performed in one of the study zones to identify the limitations of mapping with aerial photographs. The results explain the changes with time but also the processes and relationships between the estuarine features at a long-term scale. This work adds an important perspective towards a general understanding of their dependence on intrinsic and boundary conditions.

The present work focuses on the recent morphological changes recorded in the Ebro River delta, by using aerial photographs in the last six decades and by analyzing changes in the main constitutive features through geomorphological maps. Geomorphological maps of the years 1956 and 2017 are here presented. The results obtained give very valuable clues about the recent trends of the river delta mouth, which can be used to predict future coastal changes to be expected in the following decades. In this sense, by analysing changes in the surface by means of the geomorphological maps together with a shoreline analysis, a differential behavior has been described at both sides of the river mouth: the left side, El Garxal wetland, shows an accretionary trend with rates reaching +40 m/year, while the right side, San Antonio Island, shows erosive trends of more than -20 m/year. This last side also presents a surface reduction of more than 50 ha in the last 60 years. These results suggest that, approximately by the year 2050, the emerged San Antonio Island may disappear if shoreline retreat trends are maintained, making El Garxal exposed to easterly storms, the main erosive dynamic processes in this zone and, therefore changing the entire configuration of the Ebro River delta mouth in the upcoming years. Despite possible solutions have been described in recent works, they may not contribute to a total recovery of the most natural part of the Ebro Delta. Urgent management plans are required to attempting to slow or reverse these trends, otherwise one of the most valuable ecosystems of the delta could disappear.

Aranda, M.; Gracia, F.J.; Peralta, G., and Flor-Blanco, G., 2020. The application of high-resolution mapping for the analysis of recent eco-geomorphological changes in San Vicente de la Barquera estuary (North Spain). In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 341–345. Coconut Creek (Florida), ISSN 0749-0208.Geomorphological changes in recent decades in the estuarine system of San Vicente de la Barquera (Cantabria, Spain) are analyzed. For this analysis, two high-resolution geomorphological maps are generated from visual interpretation of digital orthophotos (1956 and 2017). The Habitat Digitizer Tool for ArcGIS was used as an interface to rapidly identify and assign attributes to the polygons created. Settings are conditioned at a spatial scale of 1:2500 with a minimum mapping unit (MMU) of 50 m2. The main results show remarkable changes in the studied saltmarshes, with a drastic 85% reduction in the occupied surface (from 25.65% of the estuary in 1956, to 3.62% in 2017). The results here presented suggest that the geomorphological changes and recent evolution observed in the saltmarshes of San Vicente de la Barquera may be related to: (1) a decrease on river sedimentary discharge and a higher coastal sediment transport inside the estuary, (2) human interventions like dredging of the main channel or shore urbanization and, finally, (3) changes on mean sea level rise in the last century, which significantly affect low-lying areas which have not potential area to migrate. These factors have altered the estuarine dynamics, reducing the area occupied by some geomorphological units and, therefore, causing habitat changes and disappearance. This work illustrates the importance of fine-resolution mapping to estimate trends and the development of proper management measures in order to restore estuarine habitats and their associated ecosystem services.

The main objective of this work is to present a novel methodology to assess the environmental, ecological, and conservation status of mid-latitude estuaries by means of an index-based method, EstuarIndex. The EstuarIndex is an integration of four sub-indexes, assessing the status of the main subsystems present on estuaries: sandy environments, dunes, tidal flats, and the drainage network. Each sub-index includes variables combining three types of factors: morphosedimentary and oceanographic factors, ecological factors, and management and protection factors. The environmental status is finally evaluated in five possible classes: (1) Very low, (2) Low, (3) Medium, (4) High, and (5) Very High, according to EU requirements for classifying the conservation status of habitats. Three pilot zones along the Spanish coast have been chosen for testing the method: San Vicente de la Barquera estuary (N Spain), Guadiana estuary (SW Spain), and Ebro River delta mouth (E Spain). They present different oceanographic and geomorphological conditions. The results obtained have proven that EstuarIndex is a suitable methodology for the application to other estuaries. The systematic application of EstuarIndex on broad time scales would allow evaluating the system trends, what seems key for implementation of more realistic restoration strategies, and it has a great potential as a tool for environmental management in natural protected areas. Furthermore, it may help in the detection of the most relevant site-specific vulnerabilities for long-term sustainability in response to both natural and artificial drivers.