Two decades of cyanobacterial bloom dynamics in a shallow eutrophic lake: remote sensing methods in combination with light microscopy

Cyanobacterial blooms are widespread phenomena, expected to increase in frequency, magnitude, and duration due to a changing climate and increasing temperatures. We looked at remote sensing (optical, microwave, thermal) possibilities to enhance our understanding of bloom dynamics and its parameters, retrievable from satellite data in the shallow, eutrophic, transboundary lake Peipsi. Sentinel-3/OLCI (Ocean_and_Land_Colour_Instrument) and ENVISAT/MERIS (Medium_Resolution_Imaging_Spectrometer) data were used for Chlorophyll a (Chl a) detection, from which bloom parameters were retrieved. Pixels were categorized as “bloom” when Chl a concentration exceeded the long-term average by at least 5%. Maximal bloom coverage was about 100% in all parts of the lake, but on average, the blooms covered 96.5% of Lämmijärv, 85.0% of Peipsi sensu stricto (s.s.), and 84.5% of Lake Pihkva. On average, the blooms lasted 101 days in Peipsi s.s., 78 days in Lämmijärv, and 69 days in Lake Pihkva. Higher water levels resulted in significantly shorter blooms in Lämmijärv and Lake Pihkva. Traditional microscopy gave an overview of the main bloom-formers and their changes over a 20-years period. The importance of Gloeotrichia echinulata P.G.Richter has decreased in the last 10 years in Peipsi s.s and the importance of Aphanizomenon has increased in the entire lake.