MERIS/MODIS prediction of chlorophyll in Lake Winnipeg
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This report describes results of an investigation of the water quality information content of MERIS and MODIS data by comparing relationships between selected MERIS and MODIS band-ratios and chlorophyll indices, simulated using 400-800 nm water-leaving and surface reflectance data, and paired in situ water quality data from Lake Winnipeg surveys. It shows blue /green band ratios traditionally used in chlorophyll investigations of Case I waters are very poor predictors of chlorophyll a concentration (chl a) in the Case II waters (high, and highly variable inorganic suspended solids and dissolved organic carbon concentrations) characterizing Lake Winnipeg. Among band ratios and indices tested, MERIS MCI (maximum chlorophyll index) is the best overall predictor of chl a (r2 = 0.84 by polynomial regression on MERIS MCI simulated from surface reflectance data) with a root mean square error of prediction of 8.3 mg m-3 over the range 5 < chl a < 32 mg m-3. MERIS FLH, although at r2 = 0.75 not as strongly correlated with chlorophyll as MCI, nonetheless predicts chl a in Lake Winnipeg with a root mean square error of prediction of 7.2 mg m-3 over the same range. MODIS FLH is only weakly correlated with chl a (r2 = 0.48) in Lake Winnipeg. It greatly under-predicted chl a in comparison with concentrations determined by Fluoroprobe along a test transect where chl a ranged from 2 – 80 ug l-1. Along the same transect, MERIS MCI-derived chl a followed the Fluoroprobe-derived data, tending to underestimate only the highest values, and MERIS FLH more generally under-predicted high chl a. The strong performance of FLH in predicting chl a is fortuitous in the sense that cyanophytes do not show the expected fluorescence peak at 685 nm. The FLH – chl a relationship is inverse, and is due to a local reflectance minimum caused by strong absorption by phycoeritherin at 675 nm. This effect appears to be strongest in cyanophyte-dominated algal assemblages characteristic of summer and autumn blooms on Lake Winnipeg, and weaker in bacillariophyte-cryptophyte assemblages, which dominate the early open water season community.