Poster Session 1, Monday,  October  3, 15:40–18:00

Poster 49

Chlorophyll a detection from absorption measurements for various lake types

Phytoplankton pigment absorption (aph) is an important parameter in models for remote sensing retrieval of chlorophyll a (Chl_a), which is still a challenge in turbid inland and coastal waters. Absorption parameters (aph, absorption of non-algal particles (anap) and absorption of coloured dissolved organic matter (aCDOM)) were studied three times over the vegetation period of three years (2018-2020) from various Estonian lakes (30 altogether). From the implementation of Water Framework Directive (WFD) in 2002 Estonian lakes are divided into 8 types according to their surface area, alkalinity, colour, depth, and salinity. This study focused on lakes from WFD types II-VII. Transparency in lakes was between 0.1-7 m (average 2.07) and the amount of in-water substances varied: Chl_a concentration ranged from 1.6 to 105.7 mg/m 3 (average 16.4); the amount of suspended matter was from 0.8 to 143.3 g/m 3 (average 5.63). Absorption parameters varied: aCDOM_443 was between 0.5-47.3 m -1 (average 5.7) and aph_443 was 0.02-3.5 m -1 (average 0.2). CDOM was the main absorber, and in dark-water lakes (WFD_Type_III) CDOM absorption accounted for more than 94%. The highest average contribution by anap (15%) was found in shallow, well-mixed lakes (WFD_type_II). The use of derived conversion factors from aph_443 to Chl_a allowed enhanced retrieval of Chl_a from Sentinel3/OLCI data. However, Chl_a was more accurately detected from a ph at wavelengths around 670, leading to higher correlation coefficient in comparison to a ph at 443 nm, thus aph_670 should be preferred for Chl_a detection from hyperspectral sensors in turbid waters.

Kersti Kangro, University of Tartu, [email protected], 0000-0001-6143-9330 

Krista Alikas, University of Tartu, [email protected] 

Martin Ligi, University of Tartu, [email protected] 

Ilmar Ansko, University of Tartu, [email protected]

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