CONICET | Buscador de Institutos y Recursos Humanos

Species composition, biomass and diversity of the phytoplankton in the Soto pond (27º 29S; 58º 45W) were studied from April 1995 to March 1996. One or two samples were taken monthly in the limnetic and in the littoral area. Cell counts were done by the Utermöhl method (1958). Phytoplankton biomass varied among 3 and 417 mm3.m-2 with a mean value of 99.4 (± 143.6) mm3.m-2. The population density oscillated among 0.8x106 and 38.3x106 ind.cm-2 (x=9.3x106 ±12.5x106). Cyanophyta was the dominant group all the year contributing among 14 and 99% of the total of biomass. Cylindrospermopsis raciborskii, Aphanizomenon sp., Aphanocapsa elachista, A. pulchra and Microcystis aeruginosa were the dominant species. The Chlorophyta, second group in importance, contributed among 1 and 50% being Botryococcus braunii and Sphaerocystis schroeteri the dominant algae. Bacillariophyceae, Chrysophyceae, Cryptophyceae and Euglenophyceae they did not surpass 37% of total biomass. Dinophyceae+Xanthophyceae they did not surpass 14% of total biomass. Phytoplankton density diversity varied among 0.1 and 3.5 bits.ind-1. Biomass diversity varied among 0.05 and 3.06 bits.mm-3, with a similar variations in the depths of the pond. The lowest density and biomass were always founded in the littoral zone. Strategists S (with a low SA/V ratios) such as Aphanocapsa pulchra, Microcystis aeruginosa and Botryococcus braunii prevailed during the greater part of the year. The Soto pond presents eutrophic characteristic, by the physical and chemistries properties of their waters as well as by the density and composition of the phytoplankton species. The utilization of their beaches for recreation would increase the process of antropic eutrophication. This is manifested by the high algae concentration, particularly of some few species of Cyanophyceae during the summer.

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