Day 4

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Paper title Consistency of six in situ, in vitro and satellite-based methods to derive Chlorophyll a in two optically different lakes
Authors
  1. Krista Alikas Tartu Observatory, University of Tartu, Estonia Speaker
  2. Kersti Kangro Tartu Observatory, University of Tartu
  3. Kerttu-Liis Koks
  4. Marju Tamm
  5. Rene Freiberg
  6. Alo Laas Estonian University of Life Sciences
Form of presentation Poster
Topics
  • A7. Hydrology and Water Cycle
    • A7.06 EO for monitoring water quality and ecological status in inland waters
Abstract text Phytoplankton and its most common pigment chlorophyll a (Chl a) are important parameters in characterizing lake ecosystems. We compared six methods to detect Chl a in two optically different lakes in boreal region: stratified clear-water Lake Saadjärv and non-stratified turbid Lake Võrtsjärv. Chl a was measured: in vitro with a spectrophotometer and high-performance liquid chromatography; in situ with automated high-frequency measuring (AHFM) buoys as fluorescence, and with a high-frequency optical hyperspectral above-water radiometer (WISPStation); and with various algorithms, applied on data from satellites Sentinel-3 OLCI and Sentinel-2 MSI.

The agreement between all the methods was from weak (R2=0.1) to strong (R2=0.96), while the consistency was better in turbid lake compared to the clear-water lake where the vertical and temporal variability of the Chl a was larger. The agreement between the methods depends on multiple factors. The radiometric measurements are highly dependent on the environmental and illumination conditions, resulting in higher variability in the recorded signal towards autumn. The effect of non-photochemical quenching (NPQ) correction increases with increased PAR and also is highly depended on the underwater light level, which resulted in up to 15% change in the chlorophyll fluorescence in more turbid conditions compared to 81% in clear water lake Saadjärv. Additionally, calibration datasets and applied correction methods required to account for the variability within phytoplankton amount and composition together with the background turbidity also had an effect on the the consistency of the final Chl a estimation.

Synergistic use of data from various sources allows to get a complex overview about a lake in horizontal and vertical scale but prior to merging the data, the method-based factors should be accounted for. These factors can have high impact on the results and lead to poor management decisions while switching approaches to analyze the Chl a patterns e.g. extending time series for estimating the status of the water body based on Chl a according to EU Water Framework Directive.