Subsurface irradiance reflectance spectra of inland waters differing in morphometry and hydrology

A database has been established for relating subsurface irradiance reflectance, i.e. water ‘colour’, to the optical properties and water quality parameters of more than 120 Dutch inland water bodies. The concentrations of total chlorophyll-a (TChl-a = chlorophyll-a plus phaeopigment), total suspended matter and the Secchi-disc depth varied between 0.6 and 468 mg m-3, 0.8 and 98 g m-3, and 0.15 and 5.50 m, respectively. The water bodies represented very different water types based on morphometry and hydrology. The depth ranged from 0.4 to 40 m, and the surface area from 0.25 ha (fens in the moorland Peel) to 1200 km2 (Lake IJsselmeer). The mean specific phytoplankton absorption coefficient at 676 nm was 0.013 ± 0.003 m2 (mg TChl-a)-1. The absorption by tripton and dissolved humic substances at 440 nm varied between 0.1 and 16.4 m-1, and 0.1 and 65.5 m-1, respectively. By using the spectral position and magnitude of the subsurface irradiance reflectance peak alone, it was possible to distinguish groups of water bodies according to a classical typology based on morphometry and hydrology.