Optical closure in lakes with contrasting extremes of reflectance

We measured inherent and apparent optical properties and computed 'optical closure' in four lakes on the South Island of New Zealand, ranging very widely in reflectance. Optical properties were measured by modern instruments deployed from boats, with supporting laboratory measurements on water samples. The radiative transfer model, Hydrolight, was used with measured inherent optical properties to predict characteristics of the light field for comparison with field measurements. Two glacier-fed lakes were strongly light-scattering and weakly light-absorbing, with optical properties dominated by glacial flour. In marked contrast, two humic-stained wetland lakes were strongly light-absorbing and weakly light-scattering, with optical properties dominated by colored dissolved organic matter. Very good optical 'closure' (i.e., agreement of modeled with measured characteristics of the light field) was obtained for all apparent optical properties examined in all but the most strongly light-scattering lake. Closure was achieved when systematic spectral bias in absorption in this lake, possibly caused by multiple scattering in the field absorption meter, was corrected using laboratory absorption measurements. The highest and lowest measured and modeled water-leaving radiances exceeded the previous range for such studies by a factor of 5. The good optical closure observed over a very wide (200-fold) range of reflectance increases confidence in our ability to specify inputs for radiative transfer modeling while highlighting the enduring challenge of accurately measuring absorption in highly scattering waters and backscattering in highly absorbing waters.