Mapping the structure of mixed seagrass meadows in the Mexican Caribbean

Abstract

The physical and ecological importance of seagrass meadows in coastal processes is widely recognized, and the development of tools facilitating characterization of their structure and distribution is important for improving our understanding of these processes. Mixed (multi-specific) meadows in a Mexican Caribbean reef lagoon were mapped employing a multiparameter approach, using PlanetScope remote sensing images, and supervised classification based on parameters related to the structure of the seagrasses meadows, including the cover percentages of seagrass/algae/sediment, algae thalli and seagrass shoot densities, canopy heights and estimated leaf area index (LAI). The cover, seagrass and algae densities, and seagrass canopy heights were obtained using ground truth sampling, while the LAI was estimated using data obtained from long-term monitoring programs. The maps do not show the differentiation of seagrass species, but ground truthing contemplated characterization of the density of Thalassia testudinum, Syringodium filiforme and Halodule wrightii and their respective LAIs. S. filiforme was the dominant species in terms of shoot density, and T. testudinum was dominant in terms of LAI. In the multiparameter-based map four classes were defined, based on the cover and structural characteristics, and its overall accuracy was very high (~90%). Maps based on sediment cover and LAI alone also had 4 classes, but they were less accurate than the multiparameter-based map (~70% and ~80%, respectively). The multiparameter-based seagrass map provided spatially-explicit data on the abundance and structure of seagrasses, useful for future monitoring of the changes in the meadows, and also for studies of that require data of large-scale meadow structure, such as inventories of associated biota, blue carbon storage, or modelling of the local hydrodynamics.

Beyond the Beach: Multi-Parameter Interpretation of Shore Deterioration in a Caribbean Reef System

“…The reef lagoon has an average depth of 3-4 m and maximum depth of 8 m. Most of the bottom is covered by calcareous sand, stabilized by mixed seagrass meadows dominated by the species Thalassia testudinum and Syringodium filiforme, and, to a lesser extent, Halodule wrightii, in association with rhizophytic algae [26].…”

“…[6,43]. These stressors have caused changes in the vegetative community of the reef lagoon, including the loss of near-shore seagrasses [6,26] and the loss of key reef-building species, with a consequential decrease in rugosity [38,44,45]. This degradation of ecosystems is signaled as one of the causes of the changes on the shore, such as the erosion of beaches and foredunes [4].…”

“…In 2021, two field campaigns were conducted to assess the characteristics of the seagrass meadows. The first campaign took place in the northern area, between April and June 2021 (105 points evaluated, details in [26]), while the second was conducted in the central area, from September to November, 2021 (137 points evaluated) (Figure 1). For the southern area, the study in [52] was used (146 points evaluated September 2019-January 2020).…”

“…For the central area, an unsupervised classification (12 classes and K-means cluster analysis) was used to help to define the class of each point sampled. Difficulties encountered in preparing the classification map of the central area are presented in [56], the main one being the difficulty in differentiating meadows dominated by seagrass from those dominated by macroalgae. These classified points, transformed into shapefile polygons of 4-6 pixels, named seed pixels were used as input and validation data for the supervised classification of the satellite images.…”

“…Most studies conducted in the Caribbean evaluated parameters on coastal ecosystems and on the shore separately [5,[26][27][28][29][30]. Identification of the main problem of a shore using key parameters and an integrated analysis of them would enable a better understanding of the entire coastal environment, diagnosis of the shore, and identification of critical areas in terms of shore deterioration.…”