Estimating trophic state proportions of a regional lake population: Are larger samples always better?

During the summers of 1991-1994, the Environmental Monitoring and Assessmen t Program (EMAP) sampled 344 lakes throughout the northeastern United State s using a proportional stratified sampling design based on lake size. Appro ximately one-quarter of the 344 lakes were sampled each year (4 years) for total phosphorus to determine the proportion (and associated 95% confidence intervals) of the northeast lake population greater than or equal to 1 ha (11,076 +/- 1,699 lakes) that was in oligotrophic, mesotrophic, eutrophic, or heupereutropic (4 classes) condition according to the total phosphorus c riteria of the North America Lake Manegement Society. Estimates for the sec ond, third, and fourth yr were developed as cumulative of the previous yr s amples and the current yr samples for the northeast as a whole and for each of its three ecoregions (4 regions). New confidence intervals were compute d for each cumulative yr condition estimate. This produced a total (4 years x 4 classes x 4 regions) of 64 cumulative yr tropic condition estimates. C onfidence intervals for 21% of these estimates did not shorten with increas ed sample size. This phenomena raised questions about the accuracy of estim ates based on cumulative sampling procedures. We explain why and how the ph enomenon comes about with both straight random and proportional random samp ling. Further, we present an example of the effects this phenomenon has on lake tropic state condition estimates in the northeastern United States.