A comparison of methods for estimating euphotic depths in the Great Lakes from satellite observations

Abstract

Because it defines the vertical extent of the zone of active photosynthesis, an estimate of the euphotic depth (Zeu), the depth in the water column at which the intensity of photosynthetically available radiation (PAR) is reduced to 1% of the intensity just below the surface, is a critical component of both depth resolved and vertically integrated numerical models of aquatic primary production. In recent years these models have become more detailed in both space and time and it is necessary to make estimates of Zeu at the same spatial and temporal scales. In large bodies of water such as the Great Lakes, satellite remote sensing is the only practical method available to provide such estimates at the needed resolution, but the algorithms used for estimating Zeu from satellite observations have not been rigorously tested in the Great Lakes. To assess these methods we used measurements of the vertical profile of PAR collected as part of the U.S. EPA’s, Great Lakes National Program Office (GLNPO) annual monitoring program to estimate Zeu. We compared the field measurements to satellite-derived estimates of Zeu obtained from a variety of algorithms ranging from simple empirical relationships to more complicated methods based on the inherent optical properties of the water. We found that an algorithm based on a quasi-analytic determination of absorption and backscatter coefficients that also incorporates a depth-dependent attenuation coefficient performed best.