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The Temporally Integrated Monitoring of Ecosystems (TIME) project utilizes a hybrid sampling approach to achieve its goal of assessing whether emissions controls, mandated by the Clean Air Act Amendments of 1990, have had their intended effect on lakes and streams. A randomly placed triangular grid is used to select, with known probability, lakes and streams for sampling on either an annual basis (in the case of acid-sensitive sites) or on a 4-year rotation. Data from these sites will be used to detect trends in regional characteristics of the target populations. The ability of TIME to detect trends in acid-neutralizing capacity (ANC) and SO42− is dependent on the amount of variability (year-to-year, site-to-site, within-season, and site-by-year interaction) exhibited by the sites. In particular, high levels of year-to-year variability lead to very low power to detect trends. One method to minimize year-to-year variability is to group together sites with similar characteristics. By performing trends tests on seven separate subpopulations the TIME design for northeastern U.S. lakes is predicted to be able to detect trends of the expected magnitudes in ANC (0.5 μeq L−1 yr−1) and SO42− (1.2 μeq L−1 yr−1 with power at or above the 0.90 level and α=0.10. |
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