Satellite ocean color algorithms: a review of applications to the Great Lakes

We review the literature relating to the retrieval of chlorophyll concentrations in the Great Lakes from satellite observations. Most studies show that the satellite estimates of lake chlorophyll concentration are linearly related to the observed concentrations, though they tend to overestimate concentrations at lower values and underestimate them at higher values. Deviations from a consistent, accurate, linear relationship can be attributed to temporal and spatial variations in the inherent optical properties of the color producing agents in the water as well as to varying concentrations of interfering substances such as suspended non-algal particles and colored dissolved organic matter. We confirmed these results by using a simple optical model to examine the sensitivity of the retrieved chlorophyll values to the concentrations of interfering substances and to differences in model parameters. Because the spatial and temporal optical properties of the Great Lakes are unpredictable, no retrieval method is likely to produce accurate results all the time. The papers we reviewed show that simple band ratio algorithms can provide chlorophyll estimates that are proportional to in situ concentrations. The bulk of the literature suggests that the band ratio methods will be of most value in regions where the concentrations of interfering substances such as dissolved organic material or suspended sediments are minimal. Because of these limitations we recommend that future papers presenting chlorophyll analysis based on satellite data provide confirming field observations that include measurements of suspended particles and dissolved organic carbon. We also recommend that Great Lakes scientists explore novel methods for retrieving chlorophyll concentrations from satellite observations that have proven useful in other optically complex waters.