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Assessment of Sentinel-2 to monitor highly dynamic small water bodies: The case of Louro lagoon (Galicia, NW Spain)

Casal Gema

Oceanologia

2022

No. 64 (1)

88--102

Coastal lagoons have been providing ecological, economic and cultural benefits for many centuries. Despite their importance, the monitoring of coastal lagoons poses numerous challenges related to their complex environmental processes, their large variability in size and their remote location, inhibiting effective management programmes. This study demonstrates the effectiveness of Sentinel-2 satellites to map highly dynamic morphological and hydrological changes in the Louro lagoon, a small choked lagoon located on the Galician coast (NW Spain). For this purpose, a simple methodology using the Normalised Difference Water Index (NDWI) has been evaluated, which allows to monitor the sand barrier changes and the inlet formation. The results show that the sand barrier's opening and closing might take only a very short period, and the recovery of the lagoon to its full water level can happen in less than a month. Sentinel-2 images also reveal drastic changes in the water level once the sand barrier is broken. A water surface area of 0.24 km2 was estimated on 04/11/2019, while this surface was reduced to 0.10 km2 on 04/12/2019. Monitoring these changes is critical to understand the different processes ongoing in these valuable environments and making informed decisions for their management and protection.

Seasonal pattern of the chlorophyll-a in a coastal lagoon from the southern Baja California (Mexico), described with in situ observations and MODIS-Aqua imagery

Jiménez-Quiroz María del Carmen, Martell-Dubois Raúl, Cervantes-Duarte Rafael, Cerdeira-Estrada Sergio

Oceanologia

2021

No. 63 (3)

329--342

This study aims to estimate, with a climatology perspective, the average seasonal pattern of phytoplankton biomass (SP-PBavg) and its distribution in Bahia Magdalena (Mexico) as a baseline to evaluate PB changes in future studies. This lagoon is in a semi-arid region, lacks river discharges, and channels with vegetation are limited at the north and south zones. SP-PBavg was estimated with chlorophyll-a (chl-a) data obtained in 21 sites on daily MODIS-Aqua imagery (2002-2013; n = 2,418) from a ready-to-use public database. The first step was to establish criteria to use imagery and validate with in situ observations taken in 14 sites (2002-2011; n = 312). MODIS-Aqua overestimated chl-a (mean ± confidence interval95: 5.09 ± 0.97 mg m−3; n = 225); with differences among sites. There were no differences near the inlet lagoon (p < 0.05), where the water characteristics are Case-1 while values were significantly higher in the eastern shore and two or three times higher in the mouth of north and south channels, whose water characteristics are similar to Case-2. Multivariate statistical methods allow defining zones into the lagoon and describe their SP-PBavg with both in situ and MODIS Aqua data, but the former's sample size was small, and the patterns were only delineated. In the inlet surroundings, chl-a peaks from March/April to June/July. On the eastern shore, where MODIS Aqua and in situ data were correlated, despite concentration differences, chl-a is higher from March/April to October, with peaks in June and September. In the mouth of internal channels, chl-a was higher than other sites and during a longer period; however, the very high MODIS-Aqua values suggest that the satellite also detects organic matter supplied by phytoplankton and other vegetables, which explain the high lagoon's productivity. These results validate the use of MODIS Aqua imagery to describe the chl-a seasonal patterns in the sea's vicinity.