Abstract
We review the literature dealing with retrievals of chlorophyll concentrations in the Great Lakes from satellite observations. Most studies show that the satellite estimates of chlorophyll concentrations 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 non-algal substances that interfere with the retrievals. 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 published literature suggests that the band ratio methods will be of most value in regions where the concentrations of non-algal interfering substances 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 chlorophyll, 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.
Highlights
The number of studies that have used satellite observations to explore biological features of the Great Lakes has increased dramatically in recent years
Modeling the performance of the empirical algorithms for different combinations of interfering substances and properties shows that the empirical band ratio retrieval algorithms can produce estimates of chlorophyll concentrations that are proportional to model input values
This finding is consistent with numerous studies reporting significant linear relationships between chlorophyll determined from satellite observations using band ratio algorithms and that measured in situ
Summary
The number of studies that have used satellite observations to explore biological features of the Great Lakes has increased dramatically in recent years. Surface radiometers, profiling spectral radiometers, and profiling absorption and attenuation meters were used to measure the optical properties of the water column at each sampling station These optical observations provided direct measurements of the remote sensing reflectance values that were input to the retrieval algorithms, avoiding the uncertainty associated with atmospheric correction models and the spatial and temporal mismatch between satellite and biological sampling. The main aim of their research was to examine how resuspended sediments alter the optical conditions in the water, they tested the predictive ability of seven empirical chlorophyll retrieval algorithms and reported the results in a single paragraph at the end of their paper These authors state that “there was a strong correlation between measured and calculated chlorophyll concentration at most stations.”. The authors attribute this to the low ratio of chlorophyll- a to dissolved organic material in the coastal Lake Superior waters they sampled. Budd and Warrington (2004) speculated that CDOM would negatively affect the retrievals though they did not present any CDOM measurements
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