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

• Published in: Environmental monitoring and assessment 2000-05, Vol.62 (1), p.71-89
• Main Authors: PETERSON, S. A, URQUHART, N. S
• Format: Article
• Language: English
• Subjects: Analysis methods; Applied sciences; Confidence intervals; Continental surface waters; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental monitoring; Estimates; Eutrophication; Exact sciences and technology; Lakes; Natural water pollution; Pollution; Pollution, environment geology; Studies; USA, Northeast; Water treatment and pollution
• ISSN: 0167-6369; 1573-2959
• DOI: 10.1023/A:1006298230353

During the summers of 1991-1994, the Environmental Monitoringand Assessment Program (EMAP) sampled 344 lakes throughout thenortheastern United States using a proportional stratified sampling design based on lake size. Approximately one-quarter ofthe 344 lakes were sampled each year (4 years) for totalphosphorus to determine the proportion (and associated95% confidence intervals) of the northeast lake population ≥ 1ha (11,076 ± 1,699 lakes) that was in oligotrophic,mesotrophic, eutrophic, or heupereutropic (4 classes) conditionaccording to the total phosphorus criteria of the North AmericaLake Manegement Society. Estimates for the second, third, andfourth yr were developed as cumulative of the previous yrsamples and the current yr samples for the northeast as a wholeand for each of its three ecoregions (4 regions). New confidence intervals were computed for each cumulative yrcondition estimate. This produced a total (4 years × 4classes × 4 regions) of 64 cumulative yr tropic conditionestimates. Confidence intervals for 21% of these estimates didnot shorten with increased sample size. This phenomena raisedquestions about the accuracy of estimates based on cumulativesampling procedures. We explain why and how the phenomenon comesabout with both straight random and proportional randomsampling. Further, we present an example of the effects thisphenomenon has on lake tropic state condition estimates in thenortheastern United States.[PUBLICATION ABSTRACT]