Perialpine lake basins are essential fresh water resources for Central Europe. Their anthropogenic eutrophication due to phosphorous and nitrogen inputs in the 1970s-1990s and corresponding algal bloom events have exceeded the magnitude of natural variations and jeopardized their manifold societal values. Consequently, the European Union as well as concerned cantons in Switzerland established water quality monitoring protocols defining periodical in situ sampling programs. A recent study demonstrates how Chlorophyll-a concentrations measured by an earth observation satellite can effectively support such programs.
The data used were acquired by the MEdium Resolution Imaging Spectrometer (MERIS) onboard ENVISAT. MERIS is a wide-swath ocean color instrument, providing global coverage every three days. Several methods have been developed to retrieve water constituent concentrations from the 15 narrow spectral bands measured by MERIS. However, most methods are only applicable to the open ocean, where algae growth determines not only the occurrence of chlorophyll, but also the scattering of light by suspended particles (i. e. detritus) or the concentration of dissolved organic matter.
The optical conditions for perialpine inland waters are much more complex. First of all, feeder rivers may cause random mixtures of water constituents. Second, clear, oligotrophic lakes such as Lake Maggiore are among the darkest natural targets on Earth’s surface, leading to a very low signal at a remote sensor (cf. SNR). And third, the brighter land surfaces surrounding the dark water cause intense atmospheric scattering effects over the lake, so-called adjacency effects.
Two specific algorithms specifically designed to meet these challenges were applied to more than 250 images of the alpine area in the years 2003-2008. The results were compared to chlorophyll concentrations routinely measured by several environmental agencies and limnologists. It was found that remote sensing achieves a high agreement with in situ measurements of the top 5 m water column, such as those available for Lake Geneva, Lake Zug and Lake Zurich. However, when the in situ measured reference concentrations represent the top 20 m water layer, such as in Lake Constance or Lake Biel, the correlation decreases drastically.
Clearly, aquatic remote sensing measurements are limited in their vertical representation, and their accuracy decreases with very low chlorophyll concentrations. But they outperform in situ measurements in horizontal and temporal resolution, especially in sites of very high water constituent variations, such as near the inflow of River Sarca in Lake Garda, where the MERIS images revealed very high short-term variations that are not visible in the in situ time series.