REGIONAL-SCALE TREND MONITORING OF INDICATORS OF TROPHIC CONDITION OFLAKES

The U.S. Environmental Protection Agency has proposed a sample survey design to answer questions about the ecological condition and trends i n condition of U.S. ecological resources. To meet the objectives, the design relies on a probability sample of the resource population of in terest (e.g., a random sample of lakes) each year on which measurement s are made during an index period. Natural spatial and temporal variab ility and variability in the sampling process all affect the ability t o describe the status of a population and the sensitivity for trend de tection. We describe the important components of variance and estimate their magnitude for indicators of trophic condition of lakes to illus trate the process. We also describe models for trend detection and use them to demonstrate the sensitivity of the proposed design to detect trends. if the variance structure that develops during the probability surveys is like that synthesized from available databases and the lit erature, then the trends in common indicators of trophic condition of the specified magnitude should be detectable within about a decade for Secchi disk transparency (0.5-1 percent/year) and total phosphorus (2 -3 percent/year), but not for chlorophyll-a (> 3-4 percent/year), whic h will take longer.