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Apeldoorn, Netherlands

Van De Vijver B.,National Botanic Garden of Belgium | Verweij G.L.,Koeman en Bijkerk bv | Van Der Wal J.,AQUON Boxtel | Mertens A.,Grontmij BV
Phytotaxa | Year: 2012

A new cymbelloid diatom species, Encyonopsis neerlandica sp. nov., is described from several moorland pools from The Netherlands. The new species belongs to the complex of species around E. microcephala. Based on light and detailed scanning electron microscopy, the morphology of Encyonopsis neerlandica is discussed and compared to similar species from this complex. Encyonopsis neerlandica is characterized by its rather large valve dimensions, the narrowly to clearly lanceolate valve outline with convex to weakly convex margins and rostrate to subcapitate apices. The distal raphe fissures are ventrally deflected. There is a clear difference between the rounded areolae near the valve margin and the transapically elongated areolae near the axial area. Notes on the ecology of the species are included. © 2012 Magnolia Press.

McCartney K.,University of Maine at Presque Isle | Witkowski J.,University Of Szczecin | Jordan R.W.,Yamagata University | Daugbjerg N.,Universitetsparken 4 | And 11 more authors.
Marine Micropaleontology | Year: 2014

Silicoflagellate double skeletons are commonly considered to be pre-division stages, even though their life cycle is only partially resolved, especially with respect to reproduction. Double skeletons of the modern silicoflagellate genera Dictyocha Ehrenberg, Distephanus Stöhr, and Octactis Schiller are for the first time examined in detail by scanning electron microscopy in order to improve our understanding of how skeletal morphology relates to paired skeletons. A number of genus-specific mechanisms enable sibling skeletons to be held together at their abbasal surfaces, including a zig-zag design of the basal ring achieved via apical structure, strut attachment and pike rotation (in Distephanus and Dictyocha), and the presence of organic material binding the generally planar basal rings (in Octactis). Contrary to what is generally understood, the siblings are not mirror images of one another. Instead, the triple junctions formed by the skeletal elements of each apical structure are transposed across the middle of the dividing cell to produce a copy with the same rotation. Thus, two dome-shaped skeletons represent halves of a more spherical design, which suggests that the role of the silicoflagellate basal ring is to enable double skeleton formation, but the full implications of this have yet to be explored. Although the purpose of double skeleton formation in silicoflagellates remains unclear, observations from the fossil record indicate that differences in the relative alignment of doublet members can have a high significance for phylogeny. Differences in the doublet structure of living silicoflagellates call for a combined biological and geological perspective of the utility of maintaining Dictyocha, Distephanus and Octactis as separate genera. © 2014 Elsevier B.V.

Goudswaard P.C.,Leiden University | Goudswaard P.C.,Institute for Marine Resources and Ecosystem Studies | Katunzi E.F.B.,Tanzania Fisheries Research Institute | Wanink J.H.,Leiden University | And 3 more authors.
African Journal of Aquatic Science | Year: 2011

Although Nile perch Lates niloticus is assumed to be sensitive to low oxygen concentrations, it was found in deep water in Lake Victoria, where oxygen depletion is common during the rainy season. Since factors determining Nile perch distribution are not well understood its spatial distribution in the Mwanza Gulf of Lake Victoria was analysed in relation to depth, temperature and dissolved oxygen (DO) concentrations. Of these factors, DO concentration and the interaction of depth and DO concentration explained the distribution of Nile perch, whilst temperature had no significant effect. In periods of normoxia, Nile perch preferred to stay near the bottom at depths of 12-35 m, where densities of shrimps, their main prey, were high. However, Nile perch were apparently driven away from these areas by hypoxic conditions in the rainy season. They apparently escaped the seasonally hypoxic layers in deep water by horizontal inshore migration and by vertical movement in upward direction. Sudden upwelling of these deep layers is a threat to the Nile perch stock and the Nile perch fishery. © NISC (Pty) Ltd.

Witte F.,Leiden University | Witte F.,Netherlands Center for Biodiversity | Silsbe G.M.,Netherlands Institute of Ecology | Hecky R.E.,University of Minnesota | And 7 more authors.
Hydrobiologia | Year: 2012

Possible causes of the increased algal blooms in Lake Victoria in the 1980s have been disputed by several authors; some suggested a top-down effect by the introduced Nile perch, whereas others suggested a bottom-up effect due to eutrophication. In this article the potential impact is established of grazing by fish on phytoplankton densities, before the Nile perch upsurge and the concomitant algal blooms in the Mwanza Gulf. The biomass and trophic composition of fish in the sublittoral area of the Mwanza Gulf were calculated based on catch data from bottom trawls, and from gill nets covering the whole water column. Estimates of phytoplankton production in the same area were made from Secchi values and chlorophyll concentrations. The total phytoplankton intake by fish was estimated at 230 mg DW m-2 day-1. The daily gross production ranged from 6,200 to 7,100 mg DW m-2 day-1 and the net production from 1,900 to 2,200 mg DW m-2 day-1. Thus, losses of phytoplankton through grazing by fish were about 3-4% of daily gross and 10-12% of daily net phytoplankton production. As a consequence it is unlikely that the phytoplankton blooms in the second half of the 1980s were due to a top-down effect caused by a strong decline in phytoplankton grazing by fish. © 2011 The Author(s).

Kishe-Machumu M.A.,Tanzania Fisheries Research Institute | Kishe-Machumu M.A.,Leiden University | Voogd T.,Leiden University | Wanink J.H.,Leiden University | And 2 more authors.
Environmental Biology of Fishes | Year: 2015

The introduced Nile perch, Lates niloticus Linnaeus 1758 in Lake Victoria is considered to be a major contributor to the decline of haplochromine species. In the 1990s, the abundance of Nile perch declined and the recovery of some haplochromine species, mainly detritivores and zooplanktivores was observed. However, the resurgence of detritivores group was much slower than that of zooplanktivores. Differential Nile perch predation is thought to be the main cause for the differential recovery of these two groups. To test this, the frequency of occurrence (FOO) of the detritivores and zooplanktivores in the stomachs of the Nile perch and their FOO in the lake were compared. On their vulnerability to predation, body shape of the two groups was measured and compared with Nile perch mouth gape. The FOO for haplochromines in Nile perch stomach contents were 13.7 % detritivores and 86.3 % zooplanktivores, whereas in trawl catches they were 24.3 and 75.7 % respectively. The FOO of detritivores in the Nile perch diet was significantly lower than in the environment. The same holds for the FOO of juvenile haplochromines (13 % in the diet and 45 % in the environment). Body depth of detritivores (0.8–1.9 cm) in the diet of Nile perches was significantly larger than that of the zooplanktivores (0.6–1.7 cm). However, based on Nile perch mouth gape (2.0–5.3 cm), only Nile perches <13 cm TL (<1.9 m mouth gape) would not be able to swallow the largest detritivores. Thus, selective predation by Nile perch cannot explain the relatively slow resurgence of detritivores. © Springer Science+Business Media Dordrecht 2014.

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