Investigating early warning system for algal blooms : -a study by in situ measurement of water quality using the EXO2 probe, remote sensing and water sampling in Stora Ullfjärden, Håbo kommun

Algal blooms can pose a serious threat to humans and animals in lakes used for drinking water and recreation in the bloom contains toxin producing cyanobacteria. Early warning of impending algal blooms requires systems with high temporal resolution. Cyanoalert from Brookman Geomatics is such a system that uses remote sensing satellites from the Eu Copernicus program to supervise amongst others chlorophyll in water systems.Measurements of chlorophyll are studied in this project to identify water quality parameters and conditions affecting the reliability. Biweekly sampling is conducted during the spring of 2021 obtaining chlorophyll data from profile measurements using water sampling a calibrated multiparameter probe. Water samples are then used as accurate reference for satellite measurement comparison.The water samples showed low levels of chlorophyll, turbidity, and pH in the beginning of the sampling in March. During the sampling period the levels rose until reaching a maximum during algae bloom in the end of May. The bloom was preceded by rising water temperatures and the formation of a thermocline and rising pH levels that could be used as suitable indicators for imminent bloom formation.Obtained data and governing effects were analyzed by multivariate regression (PLSR). VIP-plots showing that turbidity had the largest influence on the projected chlorophyll and thereby the measured chlorophyll levels. Also, Secci depth was identified as having strong influence. This indicates that high turbidity is a factor causing accuracy of chlorophyll measurements to deteriorate.A proposed warning system for algal blooms should use remote sensing satellites for detecting chlorophyll and turbidity combined with in situ sensors for the surveillance of water temperature, phycocyanin and pH