Evaluating Seagrass Meadow Dynamics by Integrating Field-Based and Remote Sensing Techniques

Abstract

Marine phanerogams are considered biological sentinels or indicators since any modification in seagrass meadow distribution and coverage signals negative changes in the marine environment. In recent decades, seagrass meadows have undergone global losses at accelerating rates, and almost one-third of their coverage has disappeared globally. This study focused on the dynamics of seagrass meadows in the northern Adriatic Sea, which is one of the most anthropogenically affected areas in the Mediterranean Sea. Seagrass distribution data and remote sensing products were utilized to identify the stable and dynamic parts of the seagrass ecosystem. Different seagrass species could not be distinguished with the Sentinel-2 (BOA) satellite image. However, results revealed a generally stable seagrass meadow (283.5 Ha) but, on the other hand, a stochastic behavior in seagrass meadow retraction (90.8 Ha) linked to local environmental processes associated with anthropogenic activities or climate change. If systemized, this proposed approach to monitoring seagrass meadow dynamics could be developed as a spatial decision support system for the entire Mediterranean basin. Such a tool could serve as a key element for decision makers in marine protected areas and would potentially support more effective conservation and management actions in these highly productive and important environments.

Keywords: Adriatic Sea; Cimodocea nodosa; Sentinel-2; change analysis; image classifiers.

Figure 1

The 2014 baseline seagrass status and the 2020 mapping in the study area (a). Spatial distribution of C. nodosa, P. oceanica and Z. noltei (b).

Figure 2

Identified pixels for infinitely deep-water reflectance estimation (a) and the location of random points for diffuse attenuation coefficients determination (b).

Figure 3

Bathymetry versus log-transformed water surface reflectance for bands 2 (R_w = 490), 3 (R_w = 560) and 4 (R_w = 665) (a) and spectral dependence of diffuse attenuation coefficient for water surface reflectance values for bands 2 (R_w = 490), 3 (R_w = 560) and 4 (R_w = 665) (b). Values of attenuation coefficient are in meters because they are depth-specific, since reflectance values are unit-less.

Figure 4

SVM classified pre-processed Sentinel-2 image showing: (1) a–seagrass cover in 2020 (red polygons are classified seagrasses, yellow dots are accuracy assessment points), and (2) b–seagrass dynamics along the Slovenian coast between 2014 and 2020.