Browsing by Author "Olafsson, Emil"

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    Effects of Intensive Seaweed Farming on the Meiobenthos in a Tropical Lagoon
    (Elsevier, 1995) Olafsson, Emil; Johnstone, Ron W.; Ndaro, Simon G. M.
    Open water aquaculture of the seaweed Eucheuma spinosum, imported to Zanzibar from the Philippines in 1989, is presently a large scale operation on the island, with algal farms covering around a 1000 ha of the intertidal area. To assess the effects of the farming on benthic communities both field and laboratory studies were conducted. Two field studies conducted at different times showed that all major meiofaunal taxa were found in significantly lower numbers within the farm area compared with control areas. Multidimensional scaling ordination of nematode species, genera and family abundance data separated samples from farmed and control areas. There was a significant difference in the nematode assemblage structure among areas in pairwise comparisons using ANOSIM. The trophic structure of the nematode assemblage was characterised by a high number of epistrate feeders in all areas ranging from 73 to 96% of total numbers in the samples. To test the hypothesis that toxic substances excreted by the seaweed were responsible for lower abundance inside the farm area, a laboratory experiment was conducted. Eucheuma plants were added to several microcosms and allowed to grow there for 40 days. The results indicated no effects of the seaweed on the density of the major infauna taxa as no significant difference was found among the treatments. It is concluded that other factors such as increased predation by benthic feeding fish and the mechanical disturbance of the sediments may better explain the observed differences in infauna abundance inside and outside the algal culture farms.
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    The Impact of the Mangrove Crabs Uca Annulipes and Dotilla Fenestrata on Meiobenthos
    (1997) Olafsson, Emil; Ndaro, Simon G. M.
    To assess the effects of 2 mangrove crabs on benthic meiofauna a laboratory experiment was performed in Zanzibar, eastern Africa. The 2 ocypodidae crab species Uca annulipes and Dotilla fenestrata are commonly found at mid to high water levels among Avicennia marina trees. Both genera are borrowers in soft sediments and feed upon drained surface deposits by forming pseudofaecal pellets. They are efficient bioturbers of the uppermost few mm of the sediment and some Uca species may prey directly on meiobenthos. The 2 species were added to microcosms containing sediment with natural meiofauna populations. After 10 d of enclosure, numbers of harpacticoid copepods in the top 1 cm layer were significantly lower in microcosms containing U. annulipes than in control microcosms. Two nematode assemblages were found in the microcosms, one in the surface layer and one deeper down. The crabs did not alter the structure of these assemblages. It appears that the nematodes are quite resilient to the reworking of the sediment surface. We conclude that the ocypodid crabs do not regulate resident nematode assemblages, but may inhibit settlement of colonisers that have not adapted to the intense surface disturbance created by these crabs. Such biological control is most likely to be in those areas where tidal water contains relatively high numbers of migrating meiofauna.
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    Meiobenthos of Hypersaline Tropical Mangrove Sediment in Relation to Spring Tide Inundation
    (2000) Olafsson, Emil; Carlström, Susanna; Ndaro, Simon G. M.
    Tropical intertidal sediments often contain porewater of relatively high salinity, especially in areas exposed to longer periods without seawater inundation and high evaporation. Such an area exists on the west coast of Zanzibar: a high intertidal mangrove plateau, flooded only during spring high tides, with sediment porewater salinities commonly exceeding 100 ppt. A field survey was conducted in this area to examine variations in population density of major meiofaunal taxa and the assemblage structure of free-living marine nematodes during spring-neap tidal cycles. Samples were taken on seven occasions for two months, starting from the end of the rainy season. Porewater salinity remained high throughout the sampling period, ranging from 89 to 160 ppt. Neither spring tide inundation nor heavy rains lowered the salinity markedly. The meiofauna consisted only of four taxa, present on all sampling occasions: nematodes, harpacticoid copepods, plathyhelminthes and chironomids. Densities in surface sediments (0–5 cm) were low compared to other mangrove areas, ranging from 271 to 656 animals 10 cm−2 with nematodes dominant on all sampling occasions (58–87%). Density fluctuations could not be explained by the effects of spring tide inundation, but the meiofauna showed significant correlations with grain size and organic material. Despite the wide range of salinity, only the numbers of chironomids were negatively correlated with increased salinity. Nematode species diversity was low in all samples, although altogether 28 species were recorded in the samples. Four species occurred in more than 50% of the samples (Microlaimus sp. (100%), Metalinhomoeus sp. (76%), Daptonema sp.l (56%), Chromadorina sp. (56%)) while 12 species were found only in one or two samples. Multidimensional scaling ordination (MDS) of the nematode species abundance data indicated little effects of spring tide inundation on the assemblage structure, but rather a successive change from wet to dry season with a reduction in species diversity and increased numbers of the dominant nematode species Microlaimus sp.
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    Small-Scale Variation in Major Meiofaunal Taxa and Sediment Chemistry in Tropical Sediments
    (1995) Ndaro, Simon G. M.; Sjoling, S.; Olafsson, Emil
    Variations in sediment biochemistry and abundance of meiofauna were investigated in three tropical habitats: mangrove forest, intertidal lagoon, and subtidal reef area in eastern Africa. Both the biochemical components of the pore water and the meiofauna varied substantially over small distances in all three habitats. In the mangrove area, the color of the pore water was found to be positively correlated with the major meiofaunal taxa, especially the nematodes. In the coastal lagoon, polychaetes showed negative correlation with particulate organic matter while other groups showed no correlation with any of the chemical components analyzed. In the subtidal reef area no chemical components were found to correlate with the meiofauna. When data from the three habitats are taken together two clear associations emerge. First, grain size shows a highly significant relationship with both total fauna and nematode numbers. Second, there is clear association between the amount of particulate organic carbon and particulate organic nitrogen in the pore water.
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    Soft-Bottom Fauna with Emphasis on Nematode Assemblage Structure in a Tropical Lagoon in Zanzibar, Eastern Africa: I. Spatial Variability
    (Kluwer Academic, 1999) Ndaro, Simon G. M.; Olafsson, Emil
    Twenty stations were sampled in a reef-protected lagoon on the east coast of Zanzibar from a variety of habitats: seagrass beds, fine sand, bare medium sand and coarse sand. Total meiofauna densities ranged from 219 to 3422 ind./10 cm2. Stations in the upper lagoon (< 400 m from shore) had relatively higher densities of meiofauna than those of other habitats, apart from seagrass beds. Of the 12 major taxa recorded, Nematoda normally was the most abundant taxon, contributing between 13% and 97% of total fauna, followed by Harpacticoida, Polychaeta and Turbellaria. Nematodes were dominated by epistrate feeders in all habitat-types and altogether 100 nematode genera were found within the lagoon. Clear nematode assemblage structures were depicted by non-metric multidimensional scaling ordination in seagrass bed, fine sand and coarse sand habitats. On a larger scale, sediment characteristics seem to determine the gross assemblage structure. Samples from the same station were more similar in terms of generic occurrence and population structure than samples from different stations in similar habitats. This indicates that there are localised factors influencing the nematode populations on a small scale in each habitat-type.