Regional scale trend monitoring of indicators of trophic condition of lakes
Larsen, D.P., Urquhart, N.S. and Kugler, D.L. (1995)
Water Resources Bulletin, 31, 117-40.
The U.S. Environmental Protection Agency has proposed a sample survey design to
answer questions about the ecological condition and trends in condition of U.S.
ecological resources. To meet the objectives, the design relies on a
probability sample of the resource population of interest (e.g., a random
sample of lakes) each year on which measurements are made during an index
period. Natural spatial and temporal variability and variability in the
sampling process all affect the ability to describe the status of a population
and the sensitivity for trend detection. We describe the important components
of variance and estimate their magnitude for indicators of trophic condition of
lakes to illustrate 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 literature, 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), which will take longer.