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Post-calibration of YSI chlorophyll A Gang of N Production 27 July 2005 Bill Romano – MD DNR Elgin Perry – Statistics consultant Beth Ebersole – MD DNR Marcia Olson - NOAA

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Elgin and I considered four methods Arithmetic mean ratio First subtracting 0.03 x turbidity from YSI chlorophyll then using a temperature adjustment (VIMS) Using temperature and turbidity Temperature only

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Data used in the analyses 522 station and time matched pairs of extractive and YSI chlorophyll data Data set was split into calibration and validation data sets using a random number SAS function The first 261 records were assigned to the calibration data set

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Arithmetic mean ratio RatioAdj = Extractive chl-a / YSI chlorophyll Calculate the mean ratio for each station Multiply the station mean ratio by YSI chlorophyll from the validation data set Calculate the root mean square error using the difference between post calibrated and extractive chlorophyll from the validation data set RMSE across all stations is 21.17

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Turbidity adjusted data Subtract the YSI recommended turbidity interference factor (0.03 μg/L per NTU) from each chlorophyll reading Calculate the log ratio (logCE – logCFtc) Model the log ratio as a function of temperature Post calibrate using predicted ratio and turbidity adjusted YSI chlorophyll RMSE across all stations is 20.79

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Temperature and turbidity adjustment Calculate the log ratio (logCE – logCF) Model the log ratio as a function of temperature and turbidity Post-calibrate using the predicted ratio and YSI chlorophyll RMSE across all stations is 20.43

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Temperature only adjustment Calculate the log ratio (logCE – logCF) Model the log ratio as a function of temperature Post-calibrate using the predicted ratio and YSI chlorophyll RMSE across all stations is 20.96

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Does turbidity contribute that much to the model? The p-value (type I and type III sums of squares) is 0.17 for the turbidity coefficient in the temperature and turbidity model The turbidity coefficient in: CHLa pred = x (chl YSI ) – x (turb), is half what YSI recommends (-0.03) Should we use it?

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How do extractive and YSI chlorophyll compare? A comparison of log ratio (CF – CE) chlorophyll values using the Wilcoxon test indicates that many stations are significantly different Of the twenty-four station differences, only one was positive, so extractive chla exceeds YSI chlorophyll One would expect the opposite result, because the sonde provides a measure of total chlorophyll

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Mean of extractive and YSI chlorophyll StationYSI chlchlap-value CCM CHE CTT FRG MDR MTI PXT PXT SEV0116 (+) TRQ TUV WXT

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Mean of extractive and YSI chlorophyll StationYSI chlchlap-value XCF XCH XDE XDM XED < XHE < XHF XIH XJF XJG < XJG XJG <0.0001

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Simple ratio adjustment factors differ from station to station StationRatio CCM CHE CTT FRG MDR MTI PXT PXT SEV TRQ TUV WXT (see LSD output)

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Simple ratio adjustment factors (continued) StationRatio XCF XCH XDE XDM XED XHE XHF XIH XJF XJG XJG XJG (see LSD output)

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Observations with RSTUDENT greater in absolute value than 2 may need some attention. Are some values too influential?

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Large values of DFFITS indicate influential observations. A general cutoff to consider is 2. Influential data points?

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- Ratio Method

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