What is a reasonable plot size for sampling aquatic vegetation

The question of the efficient plot size for sampling vegetation has been discussed many times, but aquatic vegetation is rarely considered. Here we examine a dataset of aquatic (267) and littoral (456) vegetation samples from 60 sandpits across the Czech Republic to determine appropriate plot size while trying to maximise both efficiency and accuracy of the sampling effort. A dataset of littoral vegetation samples was used for comparison in some analyses. The cover of vascular plant species was estimated in nested plots of 1, 4 and 16 m2. Age of sampled plots ranged from 1 to 63 years. Besides species identity itself, basic life-history species traits were considered in the comparison of different plot sizes. Effect of plot size on different ordination patterns was evaluated with Procrustean analyses and the Monte Carlo permutation test. The size categories differed in number of species and species dominance (Simpson index). No significant differences were found between plot sizes in ordinations of aquatic vegetation, neither for species composition nor for the composition of species traits. Our study indicates that aquatic vegetation samples of different size between 1 and 16 m2 are comparable, especially if species functional groups are used instead of species identity. Analyses of successional changes and the influence of environmental variables should not be significantly affected by the plot size used for sampling the vegetation when cover data are used. The different plot sizes did not affect the description of successional trends.