摘要：One of the first steps in assessing long-term changes in aquatic plant communities is to establish how much variability is inherent in sampling the same community on two or more occasions and how much variability is caused by sampling at different times during the same growing season. To better define seasonal and sampling variability, this study examined the maximum depth of plant growth, the proportion of open area in the littoral zone, species number, and diversity from early and late sampling dates during the same growing season for 46 Wisconsin lakes. Floral similarity and community similarity were also examined. Criteria based on the 75% quartile and the lower confidence limit suggest that changes greater than 0.8 m in maximum depth of growth, 13% in proportion of open area, 4 for species number, and 0.04 for diversity are more than expected from seasonal and sampling variability. Floral and community similarities less than 0.70 are also more than expected. These criteria were used to assess changes in a limited number of lakes where plant sampling was done at a three-year interval.
