Post-calibration of YSI chlorophyll

1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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: CHLapred = 3.230 + 1.132 x (chlYSI) – 0.015 x (turb), is half what YSI recommends (-0.03) • Should we use it?

9. 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

11. Mean of extractive and YSI chlorophyll

12. Mean of extractive and YSI chlorophyll

13. Simple ratio adjustment factors differ from station to station (see LSD output)

14. Simple ratio adjustment factors(continued) (see LSD output)

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

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

21. - Ratio Method

29. - Ratio Method

37. - Ratio Method

45. - Ratio Method