Project Description
Above-water hyperspectral sensors collect, just above the water surface, irradiance and radiance spectra at a very fine spectral resolution for the computation of the water reflectance or, also called, the water colour. When mounted on autonomous pointing systems, such as the PANTHYR1 and HYPSTAR2, these sensors provide continuous measurements of the water colour. Since the colour of the water varies with the constituents within the water column and their concentrations, these measurements are very helpful in monitoring the quality of the water. For instance, pigments of Chlorophyll-a (Chla), a compound related to the phytoplankton biomass, significantly increase blue (between 400-500 nm) and red (between 600-700 nm) light absorption and backscatter a small amount of visible green (between 500-600 nm) light and at the red-edge (~700 nm).
Figure 1. The PANTHYR instrument at the Blankaart reservoir in September 2019 taking measurements of the water colour with relatively high Chla concentrations (left) and the HYPSTAR instrument at the Blankaart in January 2021 measuring the colour of relatively clear water (lower Chla concentrations).
With their hyperspectral measurements, the HYPSTAR and PANTHYR can also depict very small spectral features (i.e., absorption and backscattering), allowing the detection of other pigments which are characteristic of certain algal groups. For instance, the PANTHYR was able to catch the presence of Phaeocystis and diatoms in the Southern Bight of the North Sea (Figure 2). Phaeocystis species is a genus of algae that form large blooms during spring. Although it is not toxic, it is important to monitor them as they alter the standard trophic chain, indicate the presence of unbalance nitrate to silicate ratio (i.e., due to anthropogenic nitrate excesses), and may lead to the deposition of important foam on beaches which presents some inconveniences for touristic and economics activities.
Figure 2. PANTHYR data (collected in the frame of the HYPERMAQ project https://odnature.naturalsciences.be/hypermaq/) allowing to differentiate Phaeocystis species (identified as P. Globosa) from diatoms through hyperspectral derivative analysis (Lavigne, H., et al. Monitoring of high biomass Phaeocystis globosa blooms in the Southern North Sea by in situ and future spaceborne hyperspectral radiometry, In. Prep.)
Within CALLISTO, the HYPSTAR and PANTHYR data will be used to monitor algal groups over two basins, the water reservoir at Blankaart (Belgium), and, the lagoon basin in Turin (Italy). These basins act as the drinking water buffer and first biologic treatment step before intake into the water production facilities. Extensive droughts and intensive industrial and agricultural activities in the surroundings, have affected the quality of the water over the last decades, with increasing levels of eutrophication and more severe algal blooms. This results in operational problems, higher maintenance costs, extended downtime of several systems and taste, odour and other quality problems in the final drinking water. Figure 3 shows how the HYPSTAR measurements (deployed since January 2021 at the Blankaart reservoir) aid in monitoring the water quality. Indeed, the red Chla absorption maximum at 665 nm and the reflectance peak around 710 nm are used as proxy for Chla concentrations. After validation of the algorithms, with pigment concentrations and algal biomass measured from grab samples, the HYPSTAR and PANTHYR will allow continuous monitoring of the water quality.
Figure 3. HYPSTAR data taken at Blankaart in May 2021 where we expect different concentrations of Chla (as seen by the differences in absorption and reflectance features at 670 and 705 nm, respectively) (left), and, Chla from the grab samples (Southern, S, and Northern, N, side of the basin) with the reflectance ratio 705-670 nm (right).
References
Project Details
- DateJanuary 29, 2022
- WriterClémence Goyens and Héloise Lavigne, RBINS
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