Climate change accentuates heat waves in The Netherlands. This, in combination with high nutrient concentrations in Dutch surface waters, is intensifying the proliferation of toxic blue-green algae (cyanobacteria), a threat for the health of the population and ecosystems. Lakes are resources for recreational activities, such as bathing, sailing and fishing. Dutch water managers struggle with lack of high spatial resolution information and updated/real-time alarm systems. Algal concentration data and its spatial distribution is important to support and calibrate (early warning) forecasting models.
This study focuses on collecting and comparing simultaneous in-situ and remote sensing data of chlorophyll-a and phycocyanin, and assessing the suitability of each method to characterize spatial (and with depth) variations of algae concentrations. The in-situ measurements were performed using a portable water quality spectrometer (WISP-3) and a hand-held fluorescence algae sensor (TriLux). Remote-sensing data was derived from Sentinel-2 multi-spectral earth observation satellite using existing algorithms (WISP-3 measurements were used to force atmospheric corrections). The data collection campaigns took place during overpasses of the satellite and when days were cloud-free, and between 10am and 15pm.The spatial variations of algal concentrations were investigated using autonomous unmanned vehicles equipped with the algae sensor to perform depth profiles and multiple transects along the canals and the lake. The study sites for this research were Lake Paterswoldsemeer and its main tributary channels. This region is managed by the Water Authority Noorderzijlvest that was also involved in this monitoring campaign.