Post-calibration of Fluorescence Chlorophyll

Brought to you by the “Gang of N” (For All Your Data Analysis Needs)

Bill Romano - MD DNR

Elgin Perry - Statistics Consultant

Marcia Olson - NOAA/CBP

Beth Ebersole - MD DNR

Mike Lane - ODU

Stations monitored in 2002

• Turville Creek

• Bishopville Prong

• Shelltown

• Cedar Hall Wharf

• Rehobeth

• Ben Oaks

• Sherwood Forest

• Whitehurst

• Stonington

• Drawbridge

Why Post-calibrate Data

• YSI chlorophyll probe measures any compound in the water sample that fluoresces when irradiated with blue light (470 nm).

• Most fluorescence is usually due to chlorophyll in phytoplankton, but could be a result of other compounds in the water.

• The proposed chlorophyll water quality criteria will be in terms of chlorophyll-a, not “total chlorophyll.”

Mean Differences

Station

Fluorescence

- Extractive p-value n

Ben Oaks 2.68 0.0747 27

Bishopville -6.14 0.2774 20

Cedar Hall Wharf 2.39 0.2069 17

Drawbridge 4.18 0.1973 23

Rehobeth 6.32 0.0004* 17

Shelltown 6.20 0.0032* 17

Sherwood -0.825 0.2556 26

Stonington 7.45 0.0027* 27

Turville 9.611 <0.0001* 27

Whitehurst -0.213 0.7203 26

We Tried Three Methods

• Least squares regression method.

• YSI’s mean ratio method.

• Gang of N’s log ratio method (preferred method).

Post-calibration Methods(Least Squares Regression)

• Regress extractive (CE) on fluorescence (CF) (probe-measured) chlorophyll.

• CE (predicted) = ß0 + ß1* CF + ∊

• Violates assumption that independent variable is measured without error.

Post-calibration Methods(YSI’s Mean Ratio Method)

• Multiply fluorescence chlorophyll (CF) by the mean ratio of time-matched extractive chlorophyll (CE) to CF.

• CE (predicted) = CE/CF * CF.

Sample Date Time ExtractiveProbe Ratio

BEN101 5/1/2002 14:30:00 17.24 24.3 0.709465

BEN102 5/8/2002 10:15:00 44.98 49 0.917959

BEN103 5/15/2002 10:00:00 117.71 202.9 0.580138

BEN104 5/22/2002 10:15:00 51.87 63.5 0.81685

BEN105 5/29/2002 12:30:00 25.01 25.1 0.996414

BEN106 6/5/2002 12:15:00 41.42 55.4 0.747653

BEN107 6/11/2002 10:15:00 46.89 47.1 0.995541

BEN108 6/19/2002 12:15:00 45.88 69.5 0.660144

BEN109 6/25/2002 10:15:00 41.13 38.5 1.068312

BEN110 7/1/2002 13:30:00 60.67 58.5 1.037094

BEN111 7/9/2002 10:45:00 46.84 30.8 1.520779

BEN112 7/18/2002 10:45:00 45.55 60.7 0.750412

BEN113 7/23/2002 9:00:00 84.35 45.7 1.845733

BEN114 7/30/2002 11:30:00 69.46 5.5 12.62909

BEN115 8/6/2002 11:00:00 38.23 13.1 2.918321

BEN116 8/13/2002 10:30:00 18.87 26.1 0.722989

BEN117 8/20/2002 11:30:00 20.69 32.4 0.63858

BEN118 8/27/2002 11:45:00 10.68 13.2 0.809091

BEN119 9/4/2002 10:30:00 3.65 12.6 0.289683

BEN120 9/12/2002 12:00:00 27.75 38.9 0.713368

BEN121 9/17/2002 11:15:00 10.24 11.7 0.875214

BEN122 9/24/2002 9:30:00 20.82 21.3 0.977465

BEN123 10/1/2002 14:15:00 24.03 23 1.044783

BEN124 10/8/2002 11:15:00 11.24 19.4 0.579381

BEN125 10/15/2002 12:00:00 2.18 5.2 0.419231

BEN126 10/22/2002 10:15:00 6.41 9.1 0.704396

BEN127 10/28/2002 12:45:00 16.79 20.5 0.819024

log(x/y)=log x - log y

LNRAT = LNCHL_E – LNCHL_F

Post-calibration Methods(Gang of N’s Log Ratio Method)

• Log(CE/CF) = ß0 + ß1* X + ∊.

• Moves CF to left side of equation, so error is incorporated in the error term.

• Working with logs helps conform to assumptions of regression analysis.

Application of Log Ratio Method(Temperature Effect)

• “… the fluorescence of phytoplankton suspensions can show significant temperature dependence” (YSI).

• Laboratory tests by YSI indicated that apparent chlorophyll concentration increased when temperature dropped, even though no change in phytoplankton content took place.

Temperature Effect

• Regression of log ratio on temperature was significant (p<0.05) for three of ten stations.

• Slope estimate for all models was negative, supporting what appears in YSI manual.

• Sign of slope was not consistent with temperature and turbidity model.

• Turbidity not significant with temperature in the model.

Recommended Adjustment to Fluorescence Using Log Ratio Method

• Estimate coefficients by regressing time-matched log10(extractive chlorophyll) – log10(fluorescence chlorophyll) on temperature.

• Predicted log ratio = INT + 0.013 * TEMP.• Predicted ratio = 10**PLR. • CHLA(predicted) = PR * fluorescence

chlorophyll.