Temporal variations in phytoplankton primary production in a tropical reservoir (Barra Bonita, SP – Brazil)

Temporal variations of phytoplankton primary production in Barra Bonita Reservoir (22° 29′ 5 S, 48° 34′ W, São Paulo State, Brazil) were evaluated by monthly in situ observations in the period July 1993 to June 1994 and by frequent (every 2 days for 4 weeks) sampling during the dry and colder (July) and wet and warmer (January/February) periods. Highest primary production was observed in April (654 mgC m−2 h−1), which also coincided with the period of longest theoretical water retention time. In July, the primary production was the lowest (20 mgC m−2 h−1). Nanoplankton production was the highest in October (192 mgC m−2 h−1) corresponding to 81% of the total. June represented the period with the lowest share of nanoplankton production (17%, 9 mgC m−2 h−1). Nanoplankton was predominant during 8 of the 12 months of observation, representing an average of 41% of the total community primary production. During January/February, most organisms were smaller than 20 μm. Microphytoplankton production was higher in the colder and dryer period. The production values found during the periods of intensive measurements were higher in the wet January/February period, with the average value of 135 mgC m−2 h−1, while the lowest production values were found in the dry winter (July) when they represented 90 mgC m−2 h−1. The cause of the high January values was partially bigger loads of nutrients from the watershed during the high flow, but probably also faster nutrient regeneration at higher temperatures. Barra Bonita primary production is currently three times higher than that observed 15 years ago.