A study on the spatial and temporal variations of the primary productivity by phytoplankton was conducted in Jurumirim Reservoir (Paranapanema River, São Paulo, Brazil) from October, 1995 to July, 1996. Six sampling stations were selected, three in Paranapanema Arm, two in Taquari Arm and one in das Posses Arm. The C14 method was used for measuring the primary production and a study on the correlated physical and chemical variables was made. In November, 1995 and in January-February, 1996 the T2 station (transition region of Taquari Arm) presented the highest value of primary production (23.9 and 20.1 mgC.m-2.h-1, respectively) and in the other two studied periods (April and July, 1996), it was the second most productive station (6.0 and 10.2 mgC.m-2.h-1, respectively). In das Posses station, the primary production presented the highest value in April (19.8 mgC.m-2.h-1) and July, 1996 (15.2 mgC.m-2.h-1), when compared with the other five stations. In Taquari Arm, an increase on the primary production occurred from the T1 (riverine zone) to T2 stations (transition region), in all the four studied periods. In Paranapanema Arm, an increase on the primary production was also observed from the P1 (riverine zone) to P2 (transition zone) and to P3 stations (main water body zone) in October-November, 1995 and July, 1996. In January-February 1996, the increase in the three successive stations was low, while in April, 1996 no significant variation was observed on the primary production. A variance analysis showed that there were significant modifications in the suspended matter amounts, in the water conductivity and in the inorganic phosphate and nitrate concentrations in the different periods of the year. The seasonal variations can be attributed to the water precipitation, to the allochthonous loads of suspended solids, nitrogen and phosphorus introduced by the two tributaries, and to internal recycling of nutrients. The statistical analysis also showed significant differences between sampling stations on the primary productivity by phytoplankton, on the euphotic zones, on the water conductivity and on the silicate concentrations. The spatial differences of the primary productivity are due to the modifications on the amplitudes of the euphotic zones. The euphotic layer presents longitudinal variations, that can be attributed, besides the solar radiation intensities, to the different allochthonous material loads, the tripton sedimentation and the sediment ressuspension caused by the water circulation. The spatial differences in the silicate concentrations and in the water conductivity are due to the geological influences of the watershed of each tributary (Paranapanema, Taquari and das Posses Rivers). The ionsand silicate inputs by the three rivers present a different dilution when introduced in each compartment of the reservoir.
primary production; phytoplankton; limiting factors; tropical reservoir
