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Due to the rapid and common deterioration of aquatic ecosystems, scientists and environmental protection organizations acutely need means capable of producing quantitative estimates for structural deformations of natural communities. Recently, very common biomass size spectra ignore community taxonomic composition, i.e., one of the most important kinds of biological information. Therefore, another very old, but rare in planktonology, method – the traditional taxonomic size spectrum (TTSS) – can be helpful. TTSS, a specific form of size-frequency distribution of taxonomic units, reveals repeating patterns of deep subalpine Lago Maggiore (Italy) phytoplankton taxonomic structure. The general TTSS pattern was safeguarded during 22 annual cycles (1984-2005), when many principal environmental characteristics were changed considerably during the lake oligotrophication. At the same time, the fine structure deformations of this pattern helped us divide the total oligotrophication process into several stages characterized by notable changes of TTSS peaks' proportions. These peak-height alterations were caused by pronounced changes in the species list and overall taxonomic diversity of the lake phytoplankton. The average cell volume decline was found. It was significantly correlated with the total phosphorus descending trend. This cell volume decline was produced by the addition of numerous species into the medium-and-small size fractions. Typical patterns of the stable and transitory stages were differentiated, which could be valuable for environmental protection and diagnostic applications. The central peak height difference between the stable and the transitory periods was statistically significant. Oligotrophication process decomposition into several more homogenous groups of years was supported by quantitative estimators produced by hierarchical cluster analysis. The highest level of the similarity measure (Pearson r) in pairs of annual TTSS was close to the respective estimates found for other lakes. Concomitantly, its minimal level, produced by a specific pair of abnormal years at the beginning and end of the studied process, was found previously only for pairs taken from two different ecosystems (Lakes Kinneret and Tahoe). This way, TTSS can be applied as a quantitative analysis means of the integral natural community structural evolution. Such tools are acutely needed for environmental management, monitoring, and theoretical ecology.
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