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Laguna Bufeos is a pristine varzea lake in the upper Madera basin, close to the Andes mountains. Two sets of experiments were carried out during the low-water and high-water season to determine bottom-up and top-down regulation of phytoplankton growth rates. Nutrient addition assays indicated that phytoplankton was primarily limited by N during both the high- and low-water season. Measurements of phytoplankton growth rates at different depths in the lakes indicated photoinhibition close to the water surface and slightly negative growth rates below 1 m depth. Incubations of phytoplankton in the presence and absence of mesozooplankton indicated no sig- nifi cant grazing pressure of mesozooplankton. Microzooplankton grazing rates determined using dilution experi- ments were low (about 0.2 day -1 ). Nevertheless, given the low in situ growth rates of phytoplankton due to nutrient limitation, this grazing impact was not negligible. Nutrient-deletion dilution and grazing experiments indicated that phytoplankton growth rates were supported by internal and external nutrient pools rather than by nutrients recycled by micro- or mesozooplankton.