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Fish growth is an indicator of populations’ life conditions that could be used to detect stress due to contamination. In this study, age of fishes of four species (Psectrogaster essequibensis, Schizodon fasciatus, Triportheus angulatus and Pygocentrus nattereri) from the Itenez basin (Upper Madera, Amazon) were estimated by otolith readings, and growth parameters were estimated by using the Von Bertalanffy Growth Function. Populations of four sites were compared in order to detect the effect of natural and anthropogenical water chemistry differences. One of these sites was directly impacted by gold mining activities whereas the three others were only influenced by mild human activities and presented white or clear waters. Species growth coefficient (K) varied from 0.28 (P. nattereri), to 0.39 (P. essequibensis), 0.69 (S. fasciatus) and 1.71 (T. angulatus). Due to the limited size samples, these values have to be considered as preliminary regional approximation. For S. fasciatus, P. nattereri and T. angulatus, residuals analysis of the growth function showed differences among the three unimpacted sites. However, each species exhibited different patterns of growth variations among sites, thus preventing to clearly demonstrate a general effect of water chemistry on the fish growth of these populations. On the contrary, for these three species fish growth appeared weaker in the gold-mining site.

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.

We describe the limnological changes between 1989 and 2006 in an urban, shallow lake, Laguna Alalay, located in the Andean valley of Cochabamba (Bolivia). Until 1960, water diversion to the lake was used to lower the inundation risk of Cochabamba city. In the 1980s and 1990s, the high waterfowl diversity and recreational services provided by the lake increased its conservation value. However, the population increase and the discharge of wastewater rich in nutrients increased eutrophication, and the lake became characterized by an annual alternation of submerged macrophytes and phytoplankton. The main aim of the present study is to analyze the response of the lake to manipulations implemented by local authorities: (a) sediment removal and accidental introduction of the exotic fish species Odontesthes bonariensis in 1997 and (b) manual mass removal of floating macrophytes during 2004–2006. The sediment removal and species introduction had several unpredictable consequences for the functioning of the lake, namely the transition to a permanent turbid water state and the persistent dominance of floating macrophytes. A general conclusion of our study is that any lake recovery measures in Bolivia should consider not only ecological, but also socio-economic and political aspects. Taking these into account, restoration of the submerged macrophyte-dominated state may not be that universally desirable as is widely held.

We studied diel vertical migration (DVM) of a variety of zooplanktonic taxa. Our results fit the predictions of the predator avoidance hypothesis, with larger taxa performing normal migrations to avoid fish predation and smaller taxa performing reversed migrations, probably to avoid predation by Chaoborus. Cladocerans and adults of copepods displayed normal DVM, whereas cyclopoid copepodites did not migrate. Five rotifer taxa migrated in a normal pattern whereas four taxa migrated in a reverse pattern. Our results suggest that during the day microcrustaceans moved close to the bottom, while rotifers showed narrower migration amplitudes. Larvae of the invertebrate predator Chaoborus displayed strong normal DVM, and appeared to synchronize its life cycle with the lunar cycle, with mass emergence of adults around New Moon.

Laguna Bufeos is a floodplain lake of the river Ichilo, a tributary of the Amazon basin situated in Bolivia. Nutrient addition assays involving whole water (

Laguna Bufeos is a white-water floodplain lake located in the upper Amazon basin (Ichilo River, Bolivia). To study nutrient limitation of bacteria and to determine the sources support- ing nutrient-limited bacterial growth, 5 nutrient deletion/dilution experiments were carried out in Laguna Bufeos. We carried out 3 experiments during 3 consecutive low-water seasons and 2 experi- ments during 1 high-water season. No evidence of N or P limitation was observed during the 2 exper- iments in the high-water season. Conversely, bacteria were limited by P in the 3 experiments in the low-water season. Limitation by N occurred only in one of the low-water experiments. Nutrient- limited bacterial growth rates equaled 37 to 86% of nutrient-saturated growth rates. Nutrients recy- cled by microzooplankton, mainly heterotrophic nanoflagellates and oligotrich ciliates, were the major nutrient source supporting P- or N-limited growth. Our results suggest a strong control of bac- terial growth rates by P in Amazonian white-water lakes during the low-water season, while this lim- itation is alleviated during the high-water season, probably through supply of new nutrients from the river. The strong nutrient limitation of bacteria and the dependence of bacteria on nutrients supplied by their predators can be expected to slow down the decomposition of organic matter in Amazonian floodplain lakes. This may partly explain why these ecosystems are often rich in organic matter.

Summary 1. Two enclosure experiments were carried out in Laguna Bufeos, a neotropical varzea lake located in the floodplain of River Ichilo (Bolivia). The experiments aimed (i) to assess the relative importance of bottom-up and top-down control on the plankton community, (ii) to assess the relative impact of direct and indirect effects of planktivorous fish on the zooplankton, and (iii) to attempt to identify the mechanisms responsible for these effects. 2. During the first experiment, bottom-up control seemed to dominate the planktonic food web. Compared with fishless enclosures, oxygen concentrations, chlorophyll a levels and the population densities of all cladoceran zooplankton taxa increased in enclosures with fish. Birth rates of Moina minuta, the dominant taxon, were substantially higher in the presence than in the absence of fish, whereas death rates did not differ between treatments. These results are the first to suggest that the positive effects of fish on crustacean zooplankton via effects on nutrient cycling and the enhancement of primary production can compensate for losses because of fish-related mortality. 3. During the second experiment, the direction of control appeared to vary between trophic levels: the phytoplankton appeared to be bottom-up controlled whereas the zooplankton was mainly top-down controlled. Chlorophyll a concentrations were enhanced by both fish and nutrient additions. The majority of the zooplankton taxa were reduced by the presence of fish. Birth rates of most cladoceran taxa did not differ between treatments, whereas death rates were higher in the enclosures with fish than in the fishless enclosures. Bosminopsis deitersi reached higher densities in the presence of fish, probably because of a release from predation by Chaoborus. 4. We convincingly showed strong deviations from trophic cascade-based expectations, supporting the idea that trophic cascades may be weak in tropical lakes.

Components of the pelagic food web in four eutrophic shallow lakes in two wetland reserves in Belgium ('Blankaart' and 'De Maten') were monitored during the course of 1998-1999. In each wetland reserve, a clearwater and a turbid lake were sampled. The two lakes in each wetland reserve had similar nutrient loadings and occurred in close proximity of each other. In accordance with the alternative stable states theory, food web structure differed strongly between the clearwater and turbid lakes. Phytoplankton biomass was higher in the turbid than the clearwater lakes. Whereas chlorophytes dominated the phytoplankton in the turbid lakes, cryptophytes were the most important phytoplankton group in the clearwater lakes. The biomass of microheterotrophs (bacteria, heterotrophic nanoflagellates and ciliates) was higher in the turbid than the clearwater lakes. Biomass and community composition of micro- and macrozooplankton was not clearly related to water clarity. The ratio of macrozooplankton to phytoplankton biomass - an indicator of zooplankton grazing pressure on phytoplankton was higher in the clearwater when compared to the turbid lakes. The factors potentially regulating water clarity, phytoplankton, microheterotrophs and macrozooplankton are discussed. Implications for the management of these lakes are discussed.

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.

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.