Some Thoughts about CDOM and Remote Sensing Patrick Brezonik Department of Civil Engineering University of Minnesota
CDOM has no uniquely identifying absorbance peaks. Instead, absorbance increases exponentially with decreasing wavelength in the blue and UV regions. ( A) UV ‐ visible absorbance spectra of aquatic humic matter: A, XAD8 extract from Bog S2, Marcell Forest, MN; B, IHSS Suwannee River Fulvic Acid; C, Upper St. Johns River, FL; D, Lake Itasca, MN; E, St. Croix River, MN ‐ WI. (B) Natural logarithm of absorbance vs. wavelength tends to linearize spectra. Trend lines not shown, but spectral slopes, b , are 0.014 ‐ 0.018, r 2 > 0.99, except E ( b = 0.0127 and r 2 = 0.985).
In addition, absorbance spectra for chlorophyll and accessory pigments show overlap with absorbance spectra of AH in the region 400 ‐ 450 nm
Based on results from Menken et al. (2006), reflectance at a single wavelength in the blue region (top two regressions) might work for relatively high CDOM and low chlorophyll but not for all lakes. Correcting for chlorophyll using measured or reflectance ‐ predicted values improves the predictions, especially for low chl a values (bottom three regressions). Statistical relationships between color (C 440 ) and reflectance (R) Regressions involving MODIS wavelengths Low reflectance Lakes with chl a < 10 mg m ‐ 3 Independent All lakes lakes variable(s) (n = 15) (n = 6) (n = 10) r 2 or R 2 r 2 or R 2 r 2 or R 2 ________________________________________________________________ R 412 0.04 0.59 0.41 R 443 0.05 0.56 0.43 R 412 and R 488 /R 551 0.04 0.61 0.49 R 412 and R 700 /R 670 0.45 0.70 0.71 R 412 and chl a 0.36 0.72 0.57 Menken et al.’s overall best predictive relationship was nonlinear, similar to what Kutser et al. (2009) reported, but this is somewhat troublesome because CDOM absorbs negligibly at these wavelengths: 3 . 37 R 2 670 Color 175 ; R 0.88; n 13 440 R 571
Some Early Relationships between Color (CDOM), Absorbance, and CDOM Concentration According to Zepp and Schlotzhauer (1981), absorptivity in humic ‐ rich waters fits an exponential relationship: e a exp( b ( 450 ) e λ is the light absorption coefficient (m ‐ 1 ) at wavelength λ ; a is 0.60 + 0.28 and the spectral slope b = 0.0145 + 0.0006 At least one study (Nyquist 1979) showed that aquatic humus in seawater (i.e., CDOM, in mg/L) is related to absorbance by the approximate equation: [AH] (mg/L) 4 . 72 e exp{ 0 . 140 ( 450 ) This suggests that there is a relationship between CDOM measured optically (by absorptivity or reflectance) and CDOM expressed in mass concentration (mg/L), which presumably is the ultimate goal for carbon cycling studies.
How Do Scientists Report CDOM Values? Marine scientists report the absorptivity of CDOM (A/ ℓ , m ‐ 1 ) at wavelengths of 412 or 440 nm directly. This information can be extracted from multispectral remote sensing data (e.g., Zhu et al., JGR Oceans 116 : C02011 (2011). They generally do not convert absorbance values to equivalent CDOM concentrations in mg/L . They also use fluorescence (excitation at ~330 ‐ 340 nm; emission at 450 nm) to quantify CDOM. Results are expressed in relative units based on calibration to a standard fluorescing compound like quinine. Freshwater scientists have not developed a standard way to report CDOM values.
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