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This paper reports the results of a two-year study on the chemical characteristics and phytoplankton assemblages in the western (Como) and eastern (Abbadia Lariana) sub-basins of Lake Como. The data were collected from September 1997 to August 1999 in the frame of an investigation financed by the Provincial Administration of Como and Lecco. Over the period of study TP concentrations ranged between 6-46 μg P l-1 at Abbadia, and 5-50 μg P l-1 at Como. Reactive phosphorus ranged between 6-31 μg P l-1 at Como, and 6-25 μg P l-1 at Abbadia. By the end of the study, 85 taxa belonging to six algal groups were identified. Chlorophyta (Chlorophyceae and Conjugatophyceae) (30) were the best represented, followed by Bacillariophyceae (23), Cyanoprokaryota (8), Cryptophyceae (6), Chrysophyceae (6) and Dinophyceae (6). The time variation of chlorophyll a (0.5-9.72 μg l-1), phytoplankton density (177-64925 cell. ml-1) and biovolume (21-6665 mm3 m-3) showed the same trend, with the lowest and maximum values recorded during winter and from spring to summer, respectively. In order to compare the seasonal succession in the two sub-basins, cluster analysis and non-metric multidimensional scaling ordination were used jointly. Multivariate analysis showed i) a substantial homogeneity in the time seasonality of the dominant phytoplankton assemblages in the two sub-basins and ii) a clear differentiation of summer phytoplankton from the other seasonal phases. From a purely quantitative point of view, there were dissimilarities between the two stations. The western sub-basin showed a greater algal production than Abbadia, especially in spring and summer. Despite the strong reduction of TP since the 1980s, the results of investigations carried out during the 1990s indicated a substantial stability of phytoplankton . It is emphasised that a reduction in biomass and shift in species composition will be possible only by assuring a severe control of the nutrient loads.
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