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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.

Climate change and water extraction may result in increased exposition of the biota to ultraviolet-B radiation (UVB) in high-altitude Andean lakes. Although exposition to lethal doses in these lakes is unlikely, sub-lethal UVB doses may have strong impacts in key compartments such as zooplankton. Here, we aimed at determining the effect of sub-lethal UVB doses on filtration rates of two cladoceran species (Daphnia pulicaria and Ceriodaphnia dubia). We firstly estimated the Incipient Limiting Concentration (ILC) and the Gut Passage Time (GPT) for both species. Thereafter we exposed clones of each species to four increasing UVB doses (treatments): i) DUV-0 (Control), ii) DUV-1 (0.02 MJ m2), iii) DUV-2 (0.03 MJ m2) and iv) DUV-3 (0.15 MJ m2); and estimated their filtration rates using fluorescent micro-spheres. Our results suggest that increasing sub-lethal doses of UVB radiation may strongly disturb the structure and functioning of high-altitude Andean lakes. Filtration rates of D. pulicaria were not affected by the lowest dose applied (DUV-1), but decreased by 50% in treatments DUV-2 and DUV-3. Filtration rates for C. dubia were reduced by more than 80% in treatments DUV-1 and DUV-2 and 100% of mortality occurred at the highest UVB dose applied (DUV-3).

Whether the primary role of bacterioplankton is to act as “remineralizers” of nutrients or as direct nutritional source for higher trophic levels will depend on factors controlling their production and abundance. In tropical lakes, low nutrient concentration is probably the main factor limiting bacterial growth, while grazing by microzooplankton is generally assumed to be the main loss factor for bacteria. Bottom-up and top-down regulation of microbial abundance was studied in six nutrient limitation and dilution gradient-size fractionation in situ experiments. Bacteria, heterotrophic nanoflagellates (HNF), ciliates and rotifers showed relatively low densities. Predation losses of HNF and ciliates accounted for a major part of their daily production, suggesting a top-down regulation of protistan populations by rotifers. Phosphorus was found to be strongly limiting for bacterial growth, whereas no response to enrichment with Nitrogen or DOC was detected. HNF were the major grazers on bacteria (g=0.43 d-1), the grazing coefficient increased when ciliates were added (g= 0.80 d-1) but decreased when rotifers were added (g= 0.23 d-1) probably due to nutrient recycling or top-down control of HNF and ciliates by rotifers. Que el bacterioplancton juegue básicamente un papel de reciclaje de nutrientes, o sea una fuente directa de nutrientes, depende de varios factores que afectan su producción y abundancia. En los lagos tropicales, la baja concentración de nutrientes es posiblemente el principal factor limitante del crecimiento bacteriano, y suele suponerse que la mayor pérdida poblacional de bacterias se debe a depredación por parte del microzooplancton. Estudiamos la regulación ascendente (“de abajo hacia arriba”) y descendente (“de arriba hacia abajo”) de abundancia bacteriana mediante seis experimentos in situ de limitación de nutrientes y de fraccionamiento de la dilución tamañogradiente. Bacterias, nanoflagelados heterotróficos (NHT), ciliados y rotíferos tienen densidades relativamente bajas. Las pérdidas por depredación de ciliados y de NHT explicaron la mayoría de la producción diaria, lo que sugiere que las poblaciones de protistas son reguladas por los rotíferos de forma descendente. El fósforo resultó ser un limitante fuerte del crecimiento bacteriano, pero no se detectó efecto del enriquecimiento con nitrógeno ni con “DOC”. Los NHT fueron los principales depredadores de bacterias (g=0.43 d-1). El coeficiente de ramoneo aumentó al agregar los ciliados (g= 0.80 d-1) pero disminuyó al agregar los rotíferos (g= 0.23 d-1) probablemente debido a reciclaje de nutrientes o a control descendente de los NHT y ciliados por parte de los rotíferos.