The use of rarefaction and extrapolation as methods of estimating the effects of river eutrophication on macrophyte diversity

A study was carried out to estimate the effect of trophic degradation on the diversity of aquatic plants in rivers, with the application of rarefaction and extrapolation. The purpose of rarefaction was to standardise the uneven number of samples, while extrapolation enabled prediction of the real diversity considering the expected number of species undetected by the sampling effort. Both methods were based on three Hill's numbers: q=0 (species richness), q=1 (Shannon index) and q=2 (inverse Simpson index). The macrophyte survey was carried out at 96 river sites of a uniform abiotic type (medium lowland rivers with sandy substrate). Macrophyte diversity was evaluated based on survey data (the reference sample) as well as using a dataset standardised by rarefaction and extrapolation. Trends in species richness along the trophic gradient were depicted differently by analysis based on the reference sample and on the extrapolated dataset. Based on the reference sample, the increases of river trophy led to diminishing macrophyte richness, whereas use of the extrapolated dataset revealed that the highest plant diversity can be found in mesotrophic rivers. The extrapolated dataset showed that most oligotrophic pristine rivers were poorer in species than mesotrophic watercourses, and the most limited pool of macrophytes was found in highly eutrophic watercourses. Rarefaction/extrapolation methods enable the precise comparison of plant diversity across sites, by predicting the occurrence of rare species. Moreover, the extrapolation method allows assesses of the total biodiversity basing on a smaller number of trials (survey samples).