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Abstract
The following work points out the results on the microalgae living in the sediments of three sampling stations located at a different distance from the opening of Chioggia harbour-channel (south basin of Venice lagoon). The three examinated areas are characterized by a different granulometry and are differently interested by the supplies of both sea and continental waters. The samplings were made seasonally (March, June, September, December) by taking samples of sediments, from which sub samples were obtained at several levels (0 cm, −6 cm, −12 cm). Besides the identification and the counting (cells mm−3) of the microalgae, almost all belonged to Diatoms, the chlorophyll a concentration as well as the quantity of organic matter and the biomass were determined. The obtained results demonstrated the role of the grain sizes of the solid matrix on the colonization by Diatoms; density values, ranged from 18 to 700 cells mm−3, were higher in the sediments having a reduced granulometry and characterized by high organic matter values. This is important for those species living in the sediments and able to survive in absence of light, using metabolic adaptations that allow the shift to a heterotrophic nutrition. The most important species were Achnanthes delicatula, Amphora exigua, Cocconeis molesta, C. scutellum, Navicula cryptocephala and N. cincta. The comparison among chlorophyll a concentrations as well as biomass values with density data seem to be very difficult to compare.