Alternate Title

Historic Trends in the Secchi Disk Transparency of Lake Pontchartrain

Document Type



A major environmental concern about Lake Pontchartrain is an assumed long-term increase in turbidity based on Secchi disk transparency observations. Regression of the available data on Secchi disk transparency versus time (1953 through 1990) reveals a statistically significant decrease in transparency of about 40%. However, the data set is biased in that it does not adequately represent the seasonal effects of salinity and wind speed. Two analytical procedures were undertaken to determine the extent to which the apparent long-term decrease in transparency was dependent on the seasonal bias. One procedure involved seasonal adjustment of the data for the effects of salinity and wind speed. The other procedure was to remove the seasonal bias by constructing unbiased data sets. Seasonal adjustment for the effects of salinity and wind speed reduced the level of significance for the relationship between Secchi disk transparency and time from about 1% to about 10%. This result indicates that some of the apparent decrease in transparency in the original data is the result of inadequate representation of seasonal effects in the biased data set. In most years data are not available for all months with the result that the seasonal effects of salinity and wind speed are not adequately represented. When the bias was removed by constructing unbiased data sets, the data no longer supported the conclusion of a statistically significant change in Secchi disk transparency from 1953 to 1990; p > 0.5.

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