Remote sensing reflectance of Pomeranian lakes and the Baltic*

The remote sensing reflectance Rrs, concentrations of chlorophyll a and other pigments Ci, suspended particulate matter concentrations CSPM and coloured dissolved organic matter absorption coefficient aCDOM(λ) were measured in the euphotic zones of 15 Pomeranian lakes in 2007–2010. On the basis of 235 sets of data points obtained from simultaneous estimates of these quantities, we classified the lake waters into three types. The first one, with the lowest aCDOM(440 nm) (usually between 0.1 and 1.3 m−1 and chlorophyll a concentrations 1.3 < Ca < 33 mg m−3), displays a broad peak on the reflectance spectrum at 560–580 nm and resembles the shape of the remote sensing reflectance spectra usually observed in the Baltic Proper. A set of Rrs spectra from the Baltic Proper is given for comparison. The second lake water type has a very high CDOM absorption coefficient (usually aCDOM(440 nm) > 10 m−1, up to 17.4 m−1 in Lake Pyszne; it has a relatively low reflectance (Rrs < 0.001 sr−1) over the entire spectral range, and two visible reflectance spectra peaks at ca 650 and 690–710 nm. The third type of lake water represents waters with a lower CDOM absorption coefficient (usually aCDOM (440 nm)< 5 m−1) and a high chlorophyll a concentration (usually Ca > 4 mg m−3, up to 336 mg m−3 in Lake Gardno). The remote sensing reflectance spectra in these waters always exhibit three peaks (Rrs > 0.005 sr−1): a broad one at 560–580 nm, a smaller one at ca 650 nm and a well-pronounced one at 690–720 nm. These Rrs(λ) peaks correspond to the relatively low absorption of light by the various optically active components of the lake water and the considerable scattering (over the entire spectral range investigated) due to the high SPM concentrations there. The remote sensing maximum at λ ≈ 690–720 nm is higher still as a result of the natural fluorescence of chlorophyll a. Empirical relationships between the spectral reflectance band ratios at selected wavelengths and the various optically active components for these lake waters are also established: for example, the chlorophyll a concentration in surface water layer Ca = 6.432 e4.556X, where X = [max Rrs(695 ≤ λ ≤ 720) – Rrs(λ = 670)]/max Rrs(695 ≤ λ ≤ 720), and the coefficient of determination R2 = 0.95.