Browsing by Autor "Luc De Meester"

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Asexuality and polyploidy in Daphnia from the tropical Andes
(Wiley, 2007) Ximena Aguilera; Joachim Mergeay; An Wollebrants; Steven Declerck; Luc De Meester
We assessed genetic variation at microsatellite loci within and among populations of the planktonic crustacean Daphnia pulex in 12 Bolivian Andean lakes, located above 4,000 m above sea level. Genetic analyses show that all populations consisted of obligately asexual lineages, a fact that was confirmed by observations from laboratory cultures. Moreover, microsatellite phenotypes indicate that these tropical lineages are polyploid. Levels of genetic diversity were comparable to those found in polyploid Daphnia from arctic regions, indicating a local origin rather than an accidental colonization from arctic regions. This is the first record of polyploid cladocerans in a tropical region. We suggest that their origination and abundance have probably been facilitated by the extreme environmental conditions in Andean lakes. Our analysis of multilocus genotype frequencies in relation to variation in environmental conditions indicates lineage sorting along a food availability and fish predation gradient.
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.
Diversity and Zoogeography of Rotifera (Monogononta) in a Flood Plain Lake of the Ichilo River, Bolivia, with Notes on Little‐Known Species
(Wiley, 1998) Hendrik Segers; Niniver L. Ferrufino; Luc De Meester
Abstract Results are presented on a study of the momentary species diversity of Rotifera in “Laguna Bufeos”, a flood plain lake of the Ichilo River in Bolivia. A total of 104 morphospecies of monogonont Rotifera were identified from three samples collected on June 1, 1997. The species record includes 11% Neotropical endemics, 5% pantropical and 13% tropicopolitan taxa. Four taxa, viz. Aspelta lestes Harring and Myers, 1928; Colurella denticauda Carlin, 1939; Ptygura intermedia (Davis, 1867) and Trichocerca kostei Segers, 1993 are newly recorded from South America, 67 morphospecies are new to the Bolivian fauna. Notes are added on some selected taxa. Brachionus amazonica Koste and Robertson, 1983 (stat. nov.) is elevated to morphospecies rank.
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 genetic legacy of polyploid Bolivian Daphnia: the tropical Andes as a source for the North and South American D. pulicaria complex
(Wiley, 2008) Joachim Mergeay; Ximena Aguilera; Steven Declerck; Adam Petrusek; Tine Huyse; Luc De Meester
We investigated genetic variation in asexual polyploid members of the water flea Daphnia pulex complex from a set of 12 Bolivian high-altitude lakes. We used nuclear microsatellite markers to study genetic relationships among all encountered multilocus genotypes, and combined this with a phylogenetic approach using DNA sequence data of three mitochondrial genes. Analyses of mitochondrial gene sequence divergence showed the presence of three very distinct clades that likely represent cryptic undescribed species. Our phylogenetic results suggest that the Daphnia pulicaria group, a complex of predominantly North American species that has diversified rapidly since the Pleistocene, has its origin in South America, as specific tests of topology indicated that all three South American lineages are ancestral to the North American members of this species group. A comparison between variation of nuclear and mitochondrial markers revealed that closely related polyploid nuclear genotypes sometimes belonged to very divergent mitochondrial lineages, while distantly related nuclear genotypes often belonged to the same mitochondrial lineage. This discrepancy suggests that these South American water fleas originated through reciprocal hybridization between different endemic, sexually reproducing parental lineages. It is also likely that polyploidy of the investigated lineages resulted from this hybridization. Nevertheless, no putative diploid parental lineages were found in the studied region.
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.