CONTRIBUTION OF PHYTOPLANKTON AND PERIPHYTON TO THE PRODUCTION IN A RESERVOIR OF S.W. SPAIN

Resum

The phytoplanktonic and periphytic communities of La Minilla reservoir were studied from September 1987 to August 1988. The aim of the present paper is to assess their contribution to total primary production. The sampling station for phytoplankton was situated in the middle of the reservoir, 500 m from the dam, above the deepest point. The periphyton was studied in a closed bay using slates as experimental substrata. Two kinds of samplers were used with these experimental substrata: potential production samplers (PPS) that always had the same position with respect to the water top, and real production samplers (RPS) that always had the same position with respect to the bottom. The dominant species in both communities were small and unicellular: Cyclotella ocellata, Rhodomonas minuta and Chrysidcilis peritaphrena in phytoplankton, and Achnanthes minutissima, A. linearis and Geitleribactron periphyticum in periphyton. The production of both communities was estimated by the amounts of chlorophyll a. In order to compare the contribution of each community to the total production, the volume of water in the reservoir was estimated and the surface able to be colonized was determined by echosounding. The annual mean of potential production of the periphytic community was the 5% of the total production (maximum 16% in spring) and the annual mean of real production was 2.44% of the total production (maximum 6% in spring). The differences between potential and real periphytic production were due to the water level fluctuations in autumn and winter and because of grazing in spring and summer.