Calibration and validation of an algorithm for remote sensing of turbidity over La Plata river estuary, Argentina

The La Plata River, located at 35°S on the Atlantic coast, is one of the largest waterways of South America. It carries a large amount of suspended particulate and dissolved organic matter, and is considered among the most turbid waters of the world. Very high values of total suspended matter have been reported in this region, with mean values ranging from 100 to 300 g m-3 and extreme concentrations up to 400 g m-3. Satellite sensors have shown to be the best tools available to map river plumes and to study their influence on the adjacent ocean. However, global algorithms for remotely estimating sediment concentration are not currently available. Moreover, such high sediment loads represent a challenge to atmospheric correction algorithms which usually rely on the assumption of zero water-leaving reflectance in the near infrared or short wave infrared part of the spectrum (black pixel assumption). In the extremely turbid waters of La Plata Estuary such assumptions are not valid. A two band algorithm to estimate turbidity using near infrared and the short wave infrared bands (858 nm and 1240 nm) of the MODIS-Aqua sensor is presented. The model is calibrated using in situ reflectance and turbidity measurements from turbid waters of the Southern North Sea and Scheldt River (Belgium) and then applied to MODIS imagery of La Plata River estuary (Argentina). A good correlation was found between modelled and in situ turbidity values when the algorithm was applied to concurrent MODIS imagery. Moreover, satellite-derived turbidity maps show a spatial distribution of sediment consistent with patterns and characteristic features of the estuary

Variability of La Plata River extremely turbid waters using MODIS-Aqua images and its relation to fish habitat selection

La Plata River drains the second largest basin in South America after the Amazon River and is considered one of the most turbid rivers in the world. It carries high amounts of nutrients, suspended particulate and dissolved organic matter to the adjacent shelf waters which impact the ocean’s physical, chemical and biological properties. The estuary is an area of high ecological importance. The outer region, where the freshwater (rich in nutrients) interacts with the coastal water, is the spawning and nursery area of many coastal species. The objective of the present study is to analyze the variability and offshore export of La Plata turbid waters into the adjacent continental shelf and its relation to the habitat selection of the whitemouth croaker Micropogonias furnieri. This fish is one of the most important species in terms of biomass that supports the traditional fisheries of the Argentinean, Brasilian and Uruguayan coastal region and its spatial distribution is highly influenced by salinity and turbidity differentially according to the life stage. Satellite ocean color data were used to present the synoptic quantification of turbidity variability on seasonal and interannual timescales for La Plata river plume area. Eight years (2002-2010) of MODIS-Aqua local area coverage were analyzed. Standard ocean colour products are not valid in these extremely turbid waters. Therefore, turbidity maps were generated using near infrared (NIR) and short wave infrared (SWIR) bands with a modified atmospheric correction algorithm which takes into account non-zero reflectance in the SWIR bands. Results show that the spatial distribution pattern of age-classes of Micropogonias furnieri is highly related to specific turbidity ranges. The role of forcing such as river discharge and wind-driven circulation in the redistribution of the sediment plume is also analyzed both at seasonal and interannual time scales using in situ and scatterometer wind data

Improving water reflectance retrieval from MODIS imagery in the highly turbid waters of La Plata river

The accurate retrieval of marine reflectance from remotely sensed data depends on the effective removal of the contribution of the atmosphere to the total signal that reaches the sensor at the top of the atmosphere, i.e. the atmospheric correction process. This is particularly important in highly turbid waters where assumptions made by traditional atmospheric correction algorithms for open ocean waters are often invalid. We have analyzed three different atmospheric correction algorithms in the challenging turbid waters of La Plata River estuary located in the western Atlantic coast at ~35° S. Three algorithms were applied to Moderate Resolution Imaging Spectroradiometer (MODIS) images of the region, the standard near-infrared (NIR) algorithm and two algorithms that use the short wave infrared (SWIR) bands. The two SWIR algorithms differ in the way the aerosol model is selected. The standard NIR atmospheric correction completely failed in retrieving water reflectance in the river plume waters mainly due to sensor saturation and an incorrect estimation of the marine contribution in the NIR. The standard SWIR approach showed better results, but unphysical correlations between marine features and atmospheric products, such as aerosol reflectance, in the most turbid part of the estuary were clearly identified. The use of an iterative SWIR-based atmospheric correction approach that accounts for non-zero water reflectance in the SWIR bands seems to be a good alternative for retrieving accurate marine reflectance. The difference in the derived water reflectance between the two SWIR approaches showed a spectral dependence, being higher in the shorter wavelengths and lower in the NIR. A comparison between MODIS-derived turbidity values from the different atmospheric correction approaches and in situ data showed no significant differences mainly because the one-band turbidity algorithm applied uses the 859 nm band where differences between the approaches are lower

Spatio-temporal variations in chlorophyll-a concentration in the Patagonic continental shelf: an example of satellite time series processing with GRASS GIS temporal modules

The planktonic microscopic and photosynthesizing organisms inhabiting oceans, globally called phytoplankton, constitute the basis of all oceanic food webs and play a fundamental role in carbon cycling and biogeochemical cycles. Changes in phytoplankton biomass have significant impacts on all the biological, physical and geochemical processes occurring in the aquatic system. Therefore, there has been a rising interest in studying, monitoring, and understanding these variations and their timing at the regional and global scales. Since algae have photosynthetic pigments, such as chlorophyll-a, that absorb and reflect sun light mainly in the visible part of the electromagnetic spectrum, phytoplankton is capable of changing the optical properties of surrounding water, that is the ocean color. This principle allows to estimate chlorophyll-a concentration (proxy for phytoplankton biomass) through measures of the reflected radiation from the upper ocean layer by means of remote sensing. The objective of this study was to analyze the spatio-temporal variations of satellite chlorophyll-a concentration (chl-a) and phytoplankton blooms in the continental shelf and shelf break of the Argentinian patagonic region using the newly implemented temporal modules of GRASS GIS

Monitoring Turbidity in San Francisco Estuary and Sacramento–San Joaquin Delta Using Satellite Remote Sensing

This study utilizes satellite data to investigate water quality conditions in the San Francisco Estuary and its upstream delta, the Sacramento-San Joaquin River Delta. To do this, this study derives turbidity from the European Space Agency satellite Sentinel-2 acquired from September 2015 to June 2019 and conducts a rigorous validation with in situ measurements of turbidity from optical sensors at continuous monitoring stations. This validation includes 965 matchup comparisons between satellite and in situ sensor data across 22 stations, yielding R 2 = 0.63 and 0.75 for Nephelometric Turbidity Unit and Formazin Nephelometric Unit (FNU) stations, respectively. This study then applies remote sensing to evaluate patterns in turbidity during the Suisun Marsh Salinity Control Gates Action ("Gates action"), a pilot study designed to increase habitat access and quality for the endangered Delta Smelt. The basic strategy was to direct more freshwater into Suisun Marsh, creating more low salinity habitat that would then have higher (and more suitable) turbidity than upstream river channels. For all seven acquisitions considered from June 29 to September 27, 2018, turbidity conditions in Bays and Sloughs subregions were consistently higher (and more suitable) (26-47 FNU) than what was observed in the upstream River region (13-25 FNU). This overall pattern was observed when comparing images acquired during similar tidal stages and heights