| Abstract: | Abstract Researches about the cycle of the phytoplankton in the lake of Piediluco (Umbria). — The Authors studied the phytoplanktonic productivity within an year (1960–1961) in the lake of Piediluco (Central Italy); they have tried also to bring into evidence eventual changes in the phytocoenosis of the lake, basing themselves on some phytoplanktonic analysis made by Pasquini (1924) and by Marchesoni (1940) on samples collected before that Nera River was put in the lake for hydroelectric purpose. The lake of Piediluco, which is a relict of the ancient Pleistocenic lake (Velino lake), is prevàilingly alluvial in origin and it is the second lake of Umbria in extension, after Trasimeno lake, having a surface of Kmq. 1,52. It is situated in the plain of Rieti at 368 m. level s. l. m. and it is 19 m. deep (Fig. 1). In 1932 this lake was exploited to have hydroelectric energy and the « Società Elettrica Terni » built the Stoney dike along Velino River and digged also a canal connecting the lake with the river; another derivation canal, which puts Nera River into the lake of Piediluco, was also digged. Every month, from October 1960 to October 1961 in the eulimnic zone of the lake were collected qualitative (surface and vertical) and quantitative samples (at 1, 3, 5, 10, 15 and 19 m.) and some environmental factors as: temperature, contents in oxygen, in organic matter, in N-NH4, in N-NO3 and in P-PO4 were studied. After the putting of Nera River in the lake, the thermic regime changed: in summer there was a sinking and in winter a rise in temperature. As we can see (Table I), the lake is of a tropical type and in winter it is in homothermal conditions; in summer there is a thermal stratification, which is of brief duration. The thermocline is evident only in June and July (Fig. 2). The Table II, in which the quantities of oxygen dissolved in the water are reported, puts in evidence that the lake of Piediluco has an holomictic regime. After the period of June, July and August, in which the oxygen contents of the deep waters is very low, there is in the late autumn the turnover which produces a complete mixing of superficial and deep waters (Fig. 3). During the most part of the year phenomena of hypersaturation in O2 take place in the surface (Table III): the highest amounts are reached in May, June, July and August. The highest percentage in O2 is reached during the most part of the year in the surface waters (Fig. 4). This high concentration is simply biologic in origin: it is due to the photosynthetic activity of phytoplankton which has numerically increased in this period. Tables IV, V, VI and VII show the seasonal variations of the dissolved organic matter, of N-NH4, of N-NO3 and P-PO4; they show also that the nutritive substances (mainly N-NO3 and P-PO4 which affect the productivity of the lake of Piediluco, undergo an annual cycle: a phase of consumption on the part of the algal flora living in the surface during spring and summer; a phase of production at the bottom during the summer-autumnal stratification and a winter phase of redistribution of these bottom reserves to all the mass of water owing to the late-autumnal turnover. The qualitative (Table VIII and Table IX) and quantitative (Table 10/A and 10/B) analysis show an absolute prevalence of Diatoms during all the period in which the planktonic hauls have been carried out. On a quantitative point of view they have a very large majority with regard to the other classes: Diatoms by themselves reaches 98%. The Algae which tipically characterize the lake of Piediluco are: Cyclotella kuetzingiana, C. melosiroides, Fragilaria crotonensis, Synedra ulna, S. danica, S. acus, Asterionella formosa; these are followed by: Peridinium inconspicuum, P. cinctum, Dinobryon divergens, Rhizochrysis limnetica, Gloeococcus schroeteri and Closterium aciculare. The prevalence of Diatoms during all the year is due not to one species, but to three species which characterize our lake (Table XI): in January and February there is a facies with Asterionella formosa, in April a facies with Synedra ulna and with Asterionella formosa, Cyclotella kuetzingiana, Synedra danica and Fragilaria crotonensis; in May and June a facies with Cyclotella kuetzingiana with Cyclotella melosiroides and Synedra ulna, while in August we have again a facies with Synedra ulna, and with Synedra acus and Cyclotella kuetzingiana. September and October are characterized by Synedra ulna, even though in a smaller amount. The analysis of two qualitative hauls of surface carried out in the zone with a vegetation mainly constituted by Arundo phragmites (Table XII) put in evidence that the littoral vegetation affects the phytoplanktonic composition because it is on a qualitative point of view more rich of algal species than the eulimnic zone. Before that Nera River was put in the lake, the phytoplankton of this lake was characterized by Peridinium cinctum and Ceratium hirundinella; now these species reduced themselves very much. The Authors, in order to explain this modification in the algological composition, attach importance to the changing of the thermic conditions and perhaps to the increased turbulence of waters caused by the putting in of Nera River (on an average 16–17 mc/sec.). Valuing the degree of trophism of the lake of Piediluco with the Diatom quozient (Nygaard et al.) and the Compound index (Nygaard, 1949), the lake must be considered eutrophic, but if we consider some environmental factors and the presence of some Algae, we conclude that the lake has a character tending to eutrophy. |
