Hindcast and Near Real-Time Monitoring of Green Macroalgae Blooms in Shallow Coral Reef Lagoons Using Sentinel-2: A New-Caledonia Case Study

Abstract

Despite the necessary trade-offs between spatial and temporal resolution, remote sensing is an effective approach to monitor macroalgae blooms, understand their origins and anticipate their developments. Monitoring of small tropical lagoons is challenging because they require high resolutions. Since 2017, the Sentinel-2 satellites has provided new perspectives, and the feasibility of monitoring green algae blooms was investigated in this study. In the Poé-Gouaro-Déva lagoon, New Caledonia, recent Ulva blooms are the cause of significant nuisances when beaching. Spectral indices using the blue and green spectral bands were confronted with field observations of algal abundances using images concurrent with fieldwork. Depending on seabed compositions and types of correction applied to reflectance data, the spectral indices explained between 1 and 64.9% of variance. The models providing the best statistical fit were used to revisit the algal dynamics using Sentinel-2 data from January 2017 to December 2019, through two image segmentation approaches: unsupervised and supervised. The latter accurately reproduced the two algal blooms that occurred in the area in 2018. This paper demonstrates that Sentinel-2 data can be an effective source to hindcast and monitor the dynamics of green algae in shallow lagoons.

Shallow Water Bathymetry Retrieval Using a Band-Optimization Iterative Approach: Application to New Caledonia Coral Reef Lagoons Using Sentinel-2 Data

“…Conversely, predictions based on shorter wavelength ratios do not saturate within the interval from 0 to 25 m, but they have very low sensitivity in the shallower area, while the deeper areas undergo a light but systematic underestimation. Moreover, the presence of phytoplankton or green vegetation (as in Brisset et al [61] during a green tide) at a shallow depth likely triggers an increase in the green reflectance. Using this band as the denominator brings the risk of a non-depth-related decrease in the ratio, which can bias shallow estimations.…”

Mapping of Coral Reefs with Multispectral Satellites: A Review of Recent Papers

“…Sentinel-2 [62], a European Space Agency's satellite, can be compared to Landsat satellites in terms of spatial and spectral resolution. Sentinel-2 was initially designed for land monitoring [63] but has been used for monitoring oceans (and more specifically coral reefs bleaching) and mapping benthic habitat [64][65][66][67][68][69][70]. Specific spectral bands of Sentinel-2, such as SWIR-cirrus and water vapor bands, are especially useful for cloud detection and removal algorithms [71][72][73][74][75].…”