Satellite-based water quality monitoring in Lake Vanern, Sweden

摘要

Lake Vanern, Sweden, is one of Europe's largest lakes and has a historical, cultural, ecological as well as economic importance. Lake water quality monitoring is required by national and international legislations and directives, but present programmes are insufficient to meet the requirements. To complement in situ based monitoring, the possibility to obtain reliable information about spatial and temporal water quality trends in Lake Vanern from the ENVISAT mission's MERIS instrument was evaluated. The complete archive (2002-2012) of MERIS (Medium Resolution Imaging Spectrometer) full resolution data was processed using the water processor developed by Free University Berlin (FUB) to derive aerosol optical thickness (AOT), remote-sensing reflectance (R-rs) and water quality parameters: chlorophyll-a (chl-a) concentration, coloured dissolved organic matter absorption at 443nm (CDOM), and total suspended matter (TSM) concentration. The objective was to investigate if, either, FUB reflectance products in combination with potential lake-specific band ratio algorithms for water quality estimation, or directly, FUB water quality products, could complement the existing monitoring programme.Application of lake-specific band ratio algorithms requires high-quality reflectance products based on correctly estimated AOT. The FUB reflectance and AOT products were evaluated using Aerosol Robotic Network - Ocean Color (AERONET-OC) match-up data measured at station Palgrunden in Lake Vanern. The mean absolute percentage differences (MAPDs) of the final reflectance retrievals at 413, 443, 490, 555, and 665nm were 510%, 48%, 33%, 34%, and 33%, respectively, corresponding to a large positive bias in 413nm, positive bias in 443-555nm, and a negative bias in 665nm. AOT was strongly overestimated in all bands.The FUB water quality products were evaluated using match-up in situ data of chl-a, filtered absorbance (Abs(F)(420)) and turbidity as Abs(F)(420) is related to CDOM and turbidity is strongly related to TSM. The in situ data was collected within the Swedish national and regional monitoring programmes. In order to widen the range of water constituents and add more data to the analysis, data from four large Swedish lakes (Vanern, Vattern, Malaren, and Hjalmaren) was included in the analysis. High correlation (r0.85) between in situ data and MERIS FUB derived water quality estimates were obtained, but the absolute levels were over- (chl-a) or under- (CDOM) estimated. TSM was retrieved without bias.Calibration algorithms were established for chl-a and CDOM based on the match-up data from all four lakes. After calibration of the MERIS FUB data, realistic time series could be derived that were well in line with in situ measurements. The MAPDs of the final retrievals of chl-a, Abs(F)(420) and Turbidity in Lake Vanern were 37%, 15%, and 35%, respectively, corresponding to mean absolute differences (MADs) of 0.9 mu gl(-1), 0.17m(-1), and 0.32mgl(-1) in absolute values.The partly inaccurate reflectance estimations in combination with both positive and negative bias imply that successful application of band ratio algorithms is unlikely. The high correlation between MERIS FUB water quality products and in situ data, on the other hand, shows a potential to complement present water quality monitoring programmes and improve the understanding and representability of the temporally and spatially sparse in situ observations.