SEASONAL FLUCTUATION OF PLANKTONIC ROTIFERS IN AZIBO RESERVOIR (PORTUGAL)

The rotifers of the recently-commissioned Azibo reservoir were studied during two years. The community was similar to that of other portugue se reservoirs with Keratella cochlearis, Polyarthra spp., Asplanchna p riodonta and Collotheca pelagica dominant, although densities varied s trongly. Maximum rotifer density was 640 ind l-1 in 1987 and 315 ind l -1 in 1988. During the first year two density peaks were observed, whi le in the second year fluctuations were irregular. Phytoplankton avail ability was considered the main reason for the fluctuation of rotifer density, although temperature, dissolved oxygen and cladoceran interfe rence may also have played a role. A high density of Anabaena flos-aqu ae might have facilitated the dominance of K. cochlearis in 1988.