Abstract
Coastal lagoons are transitional ecosystems with complex spatial and temporal variability. Remote sensing tools are essential for monitoring and unveiling their variability. Turbidity is a water quality parameter used for studying eutrophication and sediment transport. The objective of this research is to analyze the monthly turbidity pattern in a shallow coastal lagoon along two years with different precipitation regimes. The selected study area is the Albufera de Valencia lagoon (Spain). For this purpose, we used Sentinel 2 images and in situ data from the monitoring program of the Environment General Subdivision of the regional government. We obtained Sentinel 2A and 2B images for years 2017 and 2018 and processed them with SNAP software. The results of the correlation analysis between satellite and in situ data, corroborate that the reflectance of band 5 (705 nm) is suitable for the analysis of turbidity patterns in shallow lagoons (average depth 1 m), such as the Albufera lagoon, even in eutrophic conditions. Turbidity patterns in the Albufera lagoon show a similar trend in wet and dry years, which is mainly linked to the irrigation practice of rice paddies. High turbidity periods are linked to higher water residence time and closed floodgates. However, precipitation and wind also play an important role in the spatial distribution of turbidity. During storm events, phytoplankton and sediments are discharged to the sea, if the floodgates remain open. Fortunately, the rice harvesting season, when the floodgates are open, coincides with the beginning of the rainy period. Nevertheless, this is a lucky coincidence. It is important to develop conscious management of floodgates, because having them closed during rain events can have several negative effects both for the lagoon and for the receiving coastal waters and ecosystem. Non-discharged solids may accumulate in the lagoon worsening the clogging problems, and the beaches next to the receiving coastal waters will not receive an important load of solids to nourish them.
Highlights
Coastal lagoons are transitional ecosystems between inland and coastal waters
We applied the standardized anomalies approach to the analysis of spatial and temporal patterns of turbidity. This methodology allows comparing variables measured with different units, such as suspended particulated matter (SPM) and Secchi disk depth (SDD) in this study, and detecting deviations from a baseline
Turbidity patterns in the Albufera lagoon show a similar trend in wet and dry years, which is mainly linked to the irrigation practice of rice paddies
Summary
Coastal lagoons are transitional ecosystems between inland and coastal waters They are shallow water bodies separated from the ocean by a barrier and connected, at least intermittently, to the ocean by one or more restricted inlets [1]. Given these characteristics, they exhibit complex spatial and temporal variability. The Copernicus Sentinel-2 mission of the European Space Agency (ESA) comprises a constellation of two polar-orbiting satellites, and the first one, Sentinel-2A is operational since June 2015 This mission combines both a high spatial (10–60 m) and a high temporal resolution (5 days) that are necessary to monitor coastal lagoons [4,5]
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