Eutrophication-Driven Spatial and Temporal Changes in Macrophyte Diversity in Lake Peipsi

We examined spatial and temporal changes of the macrophyte species richness in Lake Peipsi by comparing the frequency of 76 taxa in earlier (1970 and 1980; 49 stations) and recent (1997-2014; 52 stations) data sets. About 35% of these taxa appeared or became largely distributed later than 1970. Significant changes in frequency during the study period were observed for 53 taxa. The period of rapid eutrophication since the 1970s coincided with a clear increase in the species number in the southern, recently hypertrophic lake part. Changes in Shannon's diversity index were analogous to the dynamics of species number, but species evenness did not change significantly. Species that appeared and/or increased their frequency after the 1970s were common hygrophytes, helophytes, and amphibious plants inhabiting the overgrowing littoral. A remarkable growth was observed in the frequency of Phalaris arundinacea, Glyceria maxima, Sium latifolium, Agrostis stolonifera, and Rorippa amphibia while among hydrophytes mainly plants of sheltered habitats such as Spirodela, Hydrocharis, Lemna, Nuphar, Ceratophyllum, Sparganium, Stratiotes, and Elodea increased their frequency. In repeatedly studied 22 stations filamentous algae, Potamogeton gramineus, P. pectinatus, and Stratiotes had appeared by 1980. In these 22 stations the average species number per year in 1997-2014 was similar to or lower than in 1980, and the total average had decreased. Using cluster analyses of 243 observations in 52 stations, six contemporary characteristic littoral vegetation types for L. Peipsi were identified, among them species- rich small landing places and wide monodominant reeds. Our results indicate that anthropogenic eutrophication increased the species number of macrophytes at its beginning, but in L. Peipsi the hump-backed curve is not clearly expressed. The number of species in the lake stays stable due to large oscillations in the water level, removal of reeds, and cleaning of boat canals.