|Paper title||Cyanobacterial bloom parameters in large eutrophic Lake Peipsi: a view from Sentinel 3|
|Form of presentation||Poster|
Cyanobacteria are successfully growing in many waterbodies, causing potentially toxic surface blooms, hampering the recreational activities, impeding the water usage and causing problems for lake biota. Lake Peipsi is the largest transboundary waterbody in Europe, which consists of three parts: Lake Peipsi s.s., Lämmijärv and Lake Pihkva. Naturally occurring cyanobacterial blooms are a characteristical feature for this eutrophic lake, dominated by Gloeotrichia echinulata, Aphanizomenon, Dolichospermum and lately Microcystis with increasing abundance, especially in L. Lämmijärv. Regular national in situ monitoring covers Estonian side of the lake once per month from minimum 7 locations during vegetation period, but with in situ methods is complicated to give an overview of the bloom dynamics, its onset, the length of the bloom presence and its spatial extent. Remote sensing methods give complement information more frequently and allow better overview to the bloom on spatial scale. We used Sentinel 3 A and B/OLCI FR L1 images with MCI and regional conversion factors for Chlorophyll a (Chl a) concentration assessment, whereas time period of 2016-2021 was studied. Chl a values in Peipsi s.s. were generally lower (below 40 µg/L) in comparison to Lake Lämmijärv and Lake Pihkva, where higher values were present (> 75 µg/L and > 100 µg/L, respectively) during 2019-2021.
The threshold for cyanobacterial bloom presence/absence may be difficult to set, for example according to World Health Organisation the bloom starts already from 10 µg/L of Chl a, but for Peipsi this is not suitable, since majority of values measured during vegetation period are higher. As a lake-specific solution the presence of cyanobacterial blooms was assessed via taking lake-part specific long-term median value of Chl a from historical records (1984-2015) of in situ measurements for the period of June to September + 5%. Cyanobacterial bloom duration and extent differed in lake parts and between different years. Bloom generally started in Peipsi s.s. earlier than in other lake parts, and bloom duration was there longest, lasting >100 days with maximal coverage 68±19 % of the total lake area. Cyanobacterial concentration was higher in Lämmijärv, during the maximum extent of the bloom Lämmijärv was nearly entirely covered by cyanobacteria, with the exception of 2018, when coverage remained below 76%. During 2018 bloom coverage was also lowest in L. Pihkva ( < 30%). In general, the bloom duration in L. Pihkva was similar or shorter than in Lämmijärv, but with higher cyanobacterial biomass.