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Barr Engineering conducted a statistical analysis of the available water quality data for Snail Lake, Lake Owasso, Lake Wabasso, Bennett Lake, Lake Emily and Lake Judy. The Theil distribution-free test was used to determine if the trend for each water quality parameter was significant over time (Hollander and Wolfe, 1973). This statistical method is used when data are not normally distributed, and variances are not equal, such as is the case for these water quality data. Significant differences from a slope of zero (no trend in water quality over time) were determined at the 95% confidence level. The significance is reported as a "p-value", so that a p<0.05 would be a significant trend over time (i.e., trend-line slope different from zero), and a p>0.05 would indicate no significant trend (i.e., trend-line is not significantly different from zero). The following paragraphs discuss the results of trend analyses for all water bodies in GLWMO for which water quality data are available. Lake ass The Secchi disk depth trend in Lake Owasso indicates significant (p<0.05) improvement since 1977 (Figure LO1). Secchi disk depths have been in the range expected for eutrophic lakes with a significant improvement in transparency since the late 1980s. Secchi disk transparency indicates a mesotrophic system during the most recent sampling period. The trend for improved water quality is reflected in chlorophyll concentrations and total phosphorus concentrations as well. The decline in chlorophyll a concentrations was not significant, however, over the period of record (Figure L02). Chlorophyll concentrations in Lake Owasso have been consistently in the range expected for eutrophic systems. Total phosphorus concentrations in Lake Owasso have significantly improved over the period of record (p<0.05, Figure L03). Concentrations during recent sampling years are still indicative of a highly productive eutrophic lake, however. La a aSS Lake Wabasso Secchi disk depth improved significantly (p<0.05) over the period of record, although the long term trend does not indicate a change in trophic status (Figure LWl). Summer average transparency ranged from oligotrophic to eutrophic during this time. There was also a significant decline (p<0.05) in chlorophyll a concentration over the period of record. Summer average chlorophyll concentrations fell within the range expected of mesotrophic systems during the late 1980s and early 1990s with few exceptions (Figure LW2). The decline in chlorophyll concentrations grass_lake_intro.doc Page 4 09/27/99