POSTER Session 1

The optical water type-based variability of phytoplankton community composition in Estonian lakes

Phytoplankton is an essential basis for the aquatic food-webs. Knowledge about phytoplankton community and biomass is vital for assessing the lake’s ecological status and characterizing the effects of eutrophication and climate change. Cyanobacteria tend to receive the widest public attention due to bloom formation and potential toxicity. 30 Estonian lakes were investigated over three years (2018-2020), with concurrent measurements of radiometric data, absorption parameters (absorption by phytoplankton, non-algal particles, colored dissolved organic matter (aCDOM), and phytoplankton community composition via light microscopy. Pigment composition data via High-Performance Liquid Chromatography were retrieved for 2019. Transparency in lakes was between 0.1-7 m (average 2.07), and the amount of in-water substances varied: Chl_a concentration from 1.6 to 105.7 mg/m³ (average 16.4 mg/m³), the amount of suspended matter from 0.8 to 143.3 g/m³ (average 5.63), and aCDOM from 0.5 to 47.3 m–1. The studied lakes were divided into 5 optical water types (OWTs) (Clear, Moderate, Brown, Turbid, and Very_Turbid), based on the features in their remote sensing reflectance spectra (measured with above-water or below-water TriOS RAMSES radiometer sets), and phytoplankton community composition dynamics in each lake type were followed. Cyanobacterial biomass was low in OWTs Clear, Moderate, and Brown, except in one dark-water lake, where the biomass of Microcystis was around 50 g/m³ due to past fertilization. Cyanobacterial biomass had the highest values and largest variability in Very_Turbid class. Zeaxanthin was used as a marker pigment for cyanobacteria, with better results in Clear and Moderate OWT.

Kersti Kangro, University of Tartu, Estonia, https://orcid.org/0000-0001-6143-9330

Ian-Andreas Rahn, University of Tartu, Tartu Observatory, Estonia

Martin Ligi, University of Tartu, Estonian Marine Institute, Estonia, https://orcid.org/0000-0003-2442-5719

Ilmar Ansko, University of Tartu, Tartu Observatory, Estonia

Rene Freiberg, Estonian University of Life Sciences, Estonia

Krista Alikas, University of Tartu, Tartu Observatory, Estonia, https://orcid.org/0000-0003-3855-6525

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