Browsing by Autor "Danny Rejas"

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Diel vertical migration of zooplankton in an Amazonian várzea lake (Laguna Bufeos, Bolivia)
(Taylor & Francis, 2006) Danny Rejas; Luc De Meester; Lena Ferrufino; Mabel Maldonado; F. Ollevier
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.
Efectos de los peces sobre el fitoplanton de una Laguna de Várzea (Laguna Bufeos, Bolivia)
(2003) Rosmery Ayala; Danny Rejas; Steven Declerck
Effect of water quality on growth of four fish species in the Iténez basin (Upper Madera, Amazon)
(Springer Science+Business Media, 2012) Esther López Siangas; Marc Pouilly; Adamit Vallejos; Tamara Pérez; Danny Rejas
Effects of the non-native Arapaima gigas on native fish species in Amazonian oxbow lakes (Bolivia)
(Public Library of Science, 2025) Danny Rejas; Monika Winder; Reinaldo Cholima; Thierry Oberdorff
The introduction of non-native fish species into new environments has raised global concerns due to potential ecological impacts on recipient ecosystems. A previous study focusing on the introduced fish species Arapaima gigas in Bolivian Amazon waters showed that its isotopic niche significantly overlapped with most co-occurring native fish species, suggesting potential competition. To evaluate this hypothesis, we extended here the investigation by comparing the trophic position and isotopic niche width of eleven abundant native fish species inhabiting both colonized and non-colonized floodplain lakes. We found lower trophic positions in colonized versus non-colonized lakes only for native piscivores, mostly driven by a shift towards increased dietary proportion of detritivorous fishes. Conversely, results showed that the isotopic niche width of most fish species analyzed (i.e. 10 over 11 species) did not significantly decrease in colonized compared to non-colonized lakes. Our overall results suggest potentially low competitive interactions between A. gigas and native fishes, with the notable exception of piscivorous species. We attribute our findings to the high abundance of available resources in Amazon oxbow lakes.
Effects of UVB radiation on grazing of two cladocerans from high-altitude Andean lakes
(Public Library of Science, 2017) Carla E. Fernández; Danny Rejas
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).
Local adaptation to <scp>UV</scp> radiation in zooplankton: a behavioral and physiological approach
(Wiley, 2020) Carla E. Fernández; Melina Campero; Giuseppe Bianco; Mikael T. Ekvall; Danny Rejas; Cíntia Bertacchi Uvo; Lars‐Anders Hansson
Abstract Ultraviolet radiation ( UVR ) is recognized as a driving force for phenotypic divergence. Here, we aim at assessing the ability of zooplankton to induce UVR tolerance and disentangle the relative importance of local adaptations behind the expression of such tolerance. Two populations of Daphnia pulex, derived from environments strongly differing in UVR conditions, were exposed to UVR for 70 d to induce production of photo‐protective compounds and changes in behavioral responses. We expected greater tolerance to UVR in individuals from the high‐ UVR (H‐U) environment as well as a refuge demand inversely related to the level of pigmentation. However, the complementarity between physiological and behavioral strategies was only observed on animals from the Low‐ UVR environment (L‐U). L‐U animals developed photo‐protective compounds and decreased their refuge demand when re‐exposed to UVR , that is, tolerated more UVR , compared to their control siblings. Conversely, UVR ‐exposed individuals from the H‐U environment even having developed higher levels of photo‐protective compounds increased their refuge demand staying deeper in the water column compared to the control animals, likely expressing an evolutionary memory to seek refuge in deeper waters irrespective of the UVR level. Stronger changes were observed in the H‐U population compared to the L‐U population; thus, our results suggest that although changes in tolerance after UVR exposure were evident for both populations, the strength of the inductions was more related to local adaptation independently of the rearing environment, showing that UVR tolerance is dependent on the evolutionary history of each population.
Management of Laguna Alalay: a case study of lake restoration in Andean valleys in Bolivia
(Springer Science+Business Media, 2007) Rosmery Ayala; Francisca Acosta; Wolf M. Mooij; Danny Rejas; Paul A. Van Damme
Mercury bioaccumulation patterns in fish from the Iténez river basin, Bolivian Amazon
(Elsevier BV, 2012) Marc Pouilly; Tamara Pérez; Danny Rejas; F. Monroy Guzman; Giovanni Crespo; Jean‐Louis Duprey; Jean-Remy D. Guimarães
Nutrient limitation of bacteria and sources of nutrients supporting nutrient-limited bacterial growth in an Amazonian floodplain lake
(Inter-Research Science Center, 2005) Danny Rejas; Koenraad Muylaert; Luc De Meester
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 supporting 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 experiments during 1 high-water season. No evidence of N or P limitation was observed during the 2 experiments 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. Nutrientlimited bacterial growth rates equaled 37 to 86% of nutrient-saturated growth rates. Nutrients recycled 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 bacterial growth rates by P in Amazonian white-water lakes during the low-water season, while this limitation 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.
Phytoplankton–bacterioplankton interactions in a neotropical floodplain lake (Laguna Bufeos, Bolivia)
(Springer Science+Business Media, 2005) Danny Rejas; Koenraad Muylaert; Luc De Meester
Plankton dynamics in a tropical floodplain lake: fish, nutrients, and the relative importance of bottom‐up and top‐down control
(Wiley, 2004) Danny Rejas; Steven Declerck; Johan Auwerkerken; Paulus Tak; Luc De Meester
Summary 1. Two enclosure experiments were carried out in Laguna Bufeos, a neotropical várzea 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.
The introduced <i>Arapaima gigas</i> in the Bolivian Amazon: Trophic position and isotopic niche overlap with native species
(Wiley, 2023) Danny Rejas; Thierry Oberdorff; Steven Declerck; Monika Winder
Abstract Non‐native fish species may generate major ecological impacts on native assemblages. This study aims to assess the potential impact of the introduced Arapaima gigas on native fish assemblages in two oxbow lakes of the Bolivian Amazon. Stable isotope data were used to determine trophic position (TP) and isotopic niche overlap, to evaluate potential predation and competition interactions, respectively. Results suggest that A. gigas is more an omnivore than a top predator, as often claimed. Arapaima gigas occupied an intermediate TP between detritivore/herbivore and piscivore fish species and showed broader isotopic niche compared to most native species analysed. The isotopic niche of A. gigas significantly overlapped with most native fish species in one lake (i.e. Lake Mentiroso), while there was low niche overlap in the second (i.e. Lake Miraflores). Given its omnivorous tendencies, the predation impact of A. gigas on other fish species is likely less than currently claimed and likely varies with the food web structure of the ecosystem. More precise data on resource availability and use are necessary to infer whether niche overlap will have negative impacts on native fish species through potential competitive interactions. Increasing our understanding on the processes generating impact of these introductions on resident communities through food web ecology will pave the way for better resource management and conservation efforts.
Trophic interactions within the microbial food web in a tropical floodplain lake (Laguna Bufeos, Bolivia).
(National Institutes of Health, 2007) Danny Rejas; Koenraad Muylaert; Luc De Meester
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.
Trophic Structure and Mercury Biomagnification in Tropical Fish Assemblages, Iténez River, Bolivia
(Public Library of Science, 2013) Marc Pouilly; Danny Rejas; Tamara Pérez; Jean‐Louis Duprey; Carlos I. Molina; Cédric Hubas; Jean-Remy D. Guimarães
We examined mercury concentrations in three fish assemblages to estimate biomagnification rates in the Iténez main river, affected by anthropogenic activities, and two unperturbed rivers from the Iténez basin, Bolivian Amazon. Rivers presented low to moderate water mercury concentrations (from 1.25 ng L(-1) to 2.96 ng L(-1)) and natural differences in terms of sediment load. Mercury biomagnification rates were confronted to trophic structure depicted by carbon and nitrogen stable isotopes composition (δ(15)N; δ(13)C) of primary trophic sources, invertebrates and fishes. Results showed a slight fish contamination in the Iténez River compared to the unperturbed rivers, with higher mercury concentrations in piscivore species (0.15 µg g(-1) vs. 0.11 µg g(-1) in the unperturbed rivers) and a higher biomagnification rate. Trophic structure analysis showed that the higher biomagnification rate in the Iténez River could not be attributed to a longer food chain. Nevertheless, it revealed for the Iténez River a higher contribution of periphyton to the diet of the primary consumers fish species; and more negative δ(13)C values for primary trophic sources, invertebrates and fishes that could indicate a higher contribution of methanotrophic bacteria. These two factors may enhance methylation and methyl mercury transfer in the food web and thus, alternatively or complementarily to the impact of the anthropogenic activities, may explain mercury differences observed in fishes from the Iténez River in comparison to the two other rivers.
Variación de la amplitud del nicho isotópico de tres especies de peces en un gradiente de disponibilidad de recursos
(Centre National de la Recherche Scientifique, 2020) Aldo Echeverría; Marc Pouilly; Danny Rejas
International audience
Variaciones espacio-temporales de la composición de la comunidad de peces de una laguna de la várzea del río Ichilo (Cochabamba, Bolivia)
(2000) Danny Rejas; Mabel Maldonado
SPATIAL-TEMPORAL VARIATIONS IN THE COMPOSITION OF THE FISH COMMUNITY OF A SHALLOW LAKE IN THE FLOODPLALN OF THE ICHILO RIVER COCHABAMBA, BOLIVIA) El presente trabajo se realizo con el objetivo de describir las variaciones estructurales de la ictiocenosis en dos habitat diferentes y en dos epocas hidrologicas extremas. El estudio se llevo a cabo en la laguna Bufeos ubicada en la planicie de inundacion del rio Ichilo a 16.4SOS y 64. 700.Se capturaron 35 especies, distribuidas en 14 familias y cuatro ordenes. La familia Characidae fue la representada por un mayor numero de especies y la familia Curimatidae fue la mas abundante en numero de individuos y biomasa.La estructura de la comunidad presento variaciones espacio-temporales, tanto en diversidad como en rendimiento. Se encontro un mayor rendimiento de pesca en epoca de aguas bajas, sin embargo la riqueza especifica y la diversidad se incrementaron despues de la inundacion.En conjunto la estructura biocenotica muestra cambios de poca magnitud en relacion a otros ambientes similares donde el pulso de inundacion es mas prolongado y predecible que en el rio Ichilo. Palabras clave: Ictiocenosis, laguna, varzea, variaciones espacio-temporales. Bolivia. ABSTRACT The objective of this study was to describe the variations in the ichthyocoenosis structure in two different habitats and the two extreme of the hydrologic cycle. The field work was done in the Bufeos Laguna, a shallow lake in the floodplain of the Ichilo River (16. 43 oS y 64.70°W).During the study, 35 species, distributed in 14 families and tour orders, were obtained. The Characidae family was most abundant with respect to species numbet; while, the Curimatidae family was the most abundant, both in number of individuals and biomass. The community structure showed spatial-temporal variations, both in diversity and in yield. The catch increased during the low-water season. However species richness and diversity increased after flooding.The overall biocenotic structure showed only slight variations in comparison with similar environments where the flooding is longer and more predictable than in the Ichilo River. Key Words: Ichthyocoenosis, shal!ow lake, floodplain, spatial-temporal variations, Bolivia.