Assessing salt marsh loss and degradation by combining long‐term LANDSAT imagery and numerical modelling

Abstract

The precise evaluation of the conservation status of salt marshes, the understanding of the mechanisms behind loss and degradation and the anticipation of future threats are fundamental to assist management solutions able to reverse the declining trend observed in these environments. Previous methods combining remote sensing and hydrodynamic modelling have been revealed to be effective in assessing degradation in restricted marshes, but unable to characterise dynamics of marshes at the estuarine scale. This study proposes an alternative methodology to identify priority areas for conservation by combining LANDSAT satellite remote sensing and numerical modelling. Using the Ria de Aveiro as case-study, the main objectives of this work are to: (1) identify vegetation loss and degradation in estuarine marshes; (2) determine the mechanisms behind marsh deterioration; (3) anticipate future threats posed by climate-induced mean sea level rise and; (4) provide management solutions aiming to protect and recover degraded salt marshes. LANDSAT imagery reveals that the most deteriorated salt marshes are located on the central and upper lagoon regions, and the numerical results support that marsh loss and degradation result from the increased tidal action triggered by the deepening of the lagoon channels. The increase in tidal currents promoted the erosion of marsh shoreline, whereas the changes in the inundation patterns over the intertidal areas triggered interior drowning. Climate-induced mean sea level rise will likely exacerbate marsh deterioration, increasing the urgency to implement a management plan aiming to protect marshes from erosion and promote sedimentation in submerged marshes, to ensure their sustainability. K E Y W O R D Smarsh drowning, marsh shoreline erosion, Ria de Aveiro, salt marsh management, tidal dynamics | INTRODUCTIONSalt marshes are considered among the most valuable and productive ecosystems in the world, providing a broad range of services, such as nutrient regeneration, primary production, habitat for marine species and coastal defence (Boorman, 1999). Despite their multiple values, salt marsh area and condition are declining globally, due to growing threats posed by both socioeconomic ('human') and biogeophysical ('natural') actions, exacerbated by climate changes impacts (Mariotti & Carr, 2014). In recognition to their value and to reverse this tendency, management and conservation policies for these habitats have been developed and implemented all over the world; however, these proved to be insufficient in many systems, which continue to degrade (Hansen & Reiss, 2015). Mariotti (2016) summarised three major

Coastal Floods Induced by Mean Sea Level Rise—Ecological and Socioeconomic Impacts on a Mesotidal Lagoon

“…However, a recent study [16] concluded that the lagoon deepened since 1987, because of dredging activities performed in the late 1990s and due to the ebb dominance of the inlet and central lagoon area, that promotes sediments exportation from these areas to the adjacent ocean. In turn, [19] verified that the salt marshes of the lagoon have been degraded mainly after the lagoon deepening. These findings suggest that future inundation extents can be even higher than those projected in this study if this evolutionary trend continues in the future.…”

“…On the one hand, when these regions change to subtidal, these habitats tend to disappear, making it impossible for several species to survive in the new conditions, as showed by [40]. On the other hand, the general increase in the MSSE on the intertidal areas and salt marshes can indirectly affect the local vegetation, since their roots will possibly be flooded during longer periods and consequently less oxygenated, which, according to previous studies [19,40], may have adverse consequences on the salt marsh extent and, therefore, on the local ecosystems.…”

“…In fact, some of the regions adjacent to the lagoon are, today, often flooded by salt water during the equinoctial tides as well as under storm surge events, posing a risk to residential areas and agricultural activities, among others [18]. The frequency of these episodes increased during the last years in response to the lagoon deepening, which intensified the tidal action in the system [17,19]. In addition, large extents of salt marshes have degraded or disappeared since the end of the nineties, decreasing the natural protection of the system against inundations [19].…”

“…The frequency of these episodes increased during the last years in response to the lagoon deepening, which intensified the tidal action in the system [17,19]. In addition, large extents of salt marshes have degraded or disappeared since the end of the nineties, decreasing the natural protection of the system against inundations [19]. Previous studies further evidence that, in the absence of coastal protection works, the magnitude of inundations may well be intensified in the future with MSL rise, further increasing socioeconomic and environmental losses [16,20].…”

Remote sensing‐based retrieval of soil moisture content using stacking ensemble learning models

“…Remote sensing retrieval of SMC outperforms traditional measurement methods because it provides the electromagnetic spectrum reflected from the soil surface quickly and over a long time. Moreover, remote sensing methods are characterized by their wide monitoring range, high processing speed, and low cost (Lopes et al, 2021; Tripathi & Tiwari, 2021). There are several remote sensing methods, such as reflectance and vegetation index methods based on visible near‐infrared (Przeździecki & Zawadzki, 2020; Tian et al, 2021; Zhou et al, 2022), thermal inertia method based on thermal infrared (Gao et al, 2022; Nguyen et al, 2022), temperature dryness vegetation index method (Wei et al, 2020; Xu et al, 2022), and soil moisture retrieval based on active and passive microwaves (Rigden et al, 2020; Song et al, 2021; Sun et al, 2021).…”