Limnetica 30

A spatial and temporal evaluation was made of the contribution of phytoplankton fractions (> 100 µm, 50-100 µm, 20-50 µm, 1.2-20 µm and < 1.2 µm) to total primary production in two shallow tropical reservoirs with different trophy, Ninféias (mesotrophic) and Garças (hypertrophic), located in southeastern Brazil. Material was collected for 14 consecutive months, from November 2000 to December 2001, at different depths. At Ninféias Reservoir, both the total and fractioned production were distinct during rain and dry periods and stratification and mixing processes, which promoted nutrient homogenisation in the system. Light intensity was an inhibitory agent, mainly at the more superficial layers of the reservoir. The phytoplankton fractions varied both spatially and temporally, but algae < 20 µm (nanoplankton) always dominated. At Garças Reservoir, the primary production values were usually greater than those of Ninféias Reservoir, whereas the fractioned production was dominated by algae between 1.2 and 20 µm and was virtually restricted to the reservoir layer between the surface and a depth of 0.5 m. Light was the limiting factor in the latter system as well as the phytoplankton blooms that lasted for the entire study period, affecting light penetration and reducing the extension of the euphotic zone to 0.59 m depth. The primary production temporal variation was influenced by stratification and mixing processes at Ninféias Reservoir. This in fluence was practically null at Garças Reservoir. The permanent stratification of Garças Reservoir restrained total production to its superficial layer. At Ninféias Reservoir, production was greater at the superficial layer, but the contribution of the lower layers was significant due to the extensive euphotic zone and the occurrence of photo inhibition. Nanoplankton was the most photosynthetically efficient and productive fraction in the latter reservoir, independent of the system’s trophy. At Garças Reservoir, light availability was the primary limiting factor, favouring micro and nanoplankton production at the surface. At Ninféias Reservoir, the limiting factors were stratification and mixing processes acting on light and nutrient availability. During the mixing period, however, there was a better distribution of different primary production fractions along the water column.