On the possibility to map submerged aquatic vegetation cover with Sentinel-2 in low-transparency waters

Abstract

.Modifications in submerged aquatic vegetation (SAV) spatial and temporal abundance patterns indicate changes in marine environmental conditions or physical disturbances and need to be monitored. Vegetation percent cover (%cover) is recognized as one of the key parameters in SAV monitoring. Coastal waters of the Baltic Sea are often turbid and contain high amount of colored dissolved organic matter. These factors significantly reduce the water depth, where benthic parameters can be detected by remote sensing. Field campaigns were carried out in a low-transparency Pärnu Bay area to assess to what extent multispectral Sentinel-2 (S2) satellite can be used for SAV %cover mapping in such waters. An average depth restriction for S2 benthic vegetation detection remained near 1.5 to 2.0 m. Empirical and physics-based methods were applied to S2 imagery to compare their performance for SAV %cover retrieval. Both methods identified similar %cover patterns. Model validation results showed that R2 of the best-performing models remained between 0.56 and 0.66 and root-mean-square error between 22.11 and 28.06. As physics-based inversion models do not require extensive set of training data for model calibration, those can be used for retrospective time series analysis across multitemporal images.

Assessing Seasonal and Inter-Annual Changes in the Total Cover of Submerged Aquatic Vegetation Using Sentinel-2 Imagery

“…High-accuracy atmospheric correction is required for physicsbased bio-optical inversion models [25]. Our previous studies have shown that high-quality water surface spectra are retrieved as a result of DWC atmospheric correction [19,26]. The data visualization was performed in ENVI ® image analysis and processing software.…”

“…The ODW mask was created based on the 12th of May 2021 S2 image using a supervised classification approach [19] and used on all the collected images (Table 1). We are aware that water transparency differences can occur between different dates, so some areas could be classified either as ODW or optically shallow water (OSW) depending on the water conditions on the specific time.…”

“…Additional information about the field studies, model parameterization and validation activities can be found in Ref. [19].…”

“…Additional information about the field studies, model parameterization and validation activities can be found in Ref. [19]. The first step of IDA modeling includes the forward modeling, where physics-based equations and measured water column parameters (concentrations of CDOM and phytoplankton and particulate backscatter) are used to model a large number of water surface reflectance spectra (Figure 2).…”

“…This study builds on our previous research [19], where the performances of empirical and physics-based models for SAV percent cover (%cover) predictions were assessed in optically complex Baltic Sea waters. Given study concluded that a physics-based model is able to produce comparable and even higher accuracy predictions of the SAV %cover than empirical methods.…”