|Vegetatie-ecologische studie van de zeegrasgemeenschap (incl. wiercomponent) in Gazi Bay, Kenia = Vegetation ecological study of the seagrass community (incl. algae component) in Gazi Bay, Kenya|
De Wit, M. (1988). Vegetatie-ecologische studie van de zeegrasgemeenschap (incl. wiercomponent) in Gazi Bay, Kenia = Vegetation ecological study of the seagrass community (incl. algae component) in Gazi Bay, Kenya. Ir Thesis. Rijksuniversiteit Gent - Faculteit van de Landbouwwetenschappen: Ghent. 189, 93 appendix pp.
Algae; Sea grass; Avrainvillea obscura (C.Agardh) J.Agardh, 1887 [WoRMS]; Boodleopsis pusilla (F.S.Collins) W.R.Taylor, A.B.Joly & Bernatowicz, 1953 [WoRMS]; Cymodocea rotundata Ascherson & Schweinfurth, 1870 [WoRMS]; Cymodocea serrulata (R.Brown) Ascherson & Magnus [WoRMS]; Enhalus acoroides (Linnaeus f.) Royle, 1839 [WoRMS]; Gracilaria corticata (J.Agardh) J.Agardh, 1852 [WoRMS]; Gracilaria salicornia (C.Agardh) E.Y.Dawson, 1954 [WoRMS]; Halimeda macroloba Decaisne, 1841 [WoRMS]; Halimeda opuntia (Linnaeus) J.V.Lamouroux, 1816 [WoRMS]; Halodule uninervis (Forsskål) Ascherson, 1882 [WoRMS]; Halodule wrightii Ascherson, 1868 [WoRMS]; Halophila ovalis (R.Brown) J.D.Hooker, 1858 [WoRMS]; Halophila stipulacea (Forsskål) Ascherson, 1867 [WoRMS]; Syringodium isoetifolium (Ascherson) Dandy, 1939 [WoRMS]; Thalassia hemprichii (Ehrenberg) Ascherson, 1871 [WoRMS]; Thalassodendron ciliatum (Forsskål) den Hartog, 1970 [WoRMS]; ISW, Kenya, Gazi Bay [Marine Regions]; Marine
The field work for this study was done in July and August 1987, at the western part of Gazi Bay. In order to cover the most representative seagrass associations, seven linear transects were traced in the seagrass community of Gazi Bay. At low tide the floristic structure of the seagrasses, benthic seaweeds, and epiphytic algae on Thalassodendron ciliatum were observed. On the medio- and infra-littoral transects 11 species of seagrasses, 31 species of benthic seaweeds, and 19 species of epiphytic algae were observed. Abiotic factors such as salinity and water temperature were investigated as well. Substrate was sampled on the 1 m2 sample locations of floristic observations along the transects. These samples were analyzed for particle size distribution, concentration of Ca, K, Na, Mg, P, as well as organic material. All data on benthic plant species and abiotic factors were submitted to multivariate computer analysis. STATGRAPHICS calculated simple correlations between benthic plant species, and also between benthic plant species and abiotic factors. DECORANA arranged benthic plant species and the 1 m2 sample locations in a particular ordination. TWINSPAN clustered benthic plant species and the 1 m2 sample locations in a particular classification. PCA with the present set of data did not give a satisfactory result. Globalizing the results of the different methods induced to a hypothesis concerning zonation and succession as follows: rough substrate fractions are characteristic for original naked san planes because the finer fractions are easily disturbed by waves and winds in case of submersion resp. emersion. The correlations of 18.104.22.168 suggest a succession in which colonizing seagrass species grow first on these rough substrates. They fix the finer sediments with their stems and leaves while altering the substrate with their roots, thus preparing the substrate to suit to further plant communities. In the infra-littoral zone Halophila balfourii is the colonizer able of fixing the sand quickly with the roots of far reaching stolons. It is not quite evident if Cymodocea ciliata is an intermediate species in the infra-littoral succession. Thalassodendron ciliatum might be the climax species of the never exposed infra-littoral., whereas Syringodium isoetifolium and Enhalus acoroides might be the climax species of the infra-littoral belt that is emerged at extremely low tide. The infra-littoral species may occur in deep stagnating water of the medio-littoral as well, the so-called ‘up lifting’. Where this is the case, they proceed in the same succession i.e. colonizer, intermediate species, climax species. The infra-littoral climax species, Thalassodendron, Syringodium and Enhalus, by the way the biggest seagrass species occurring in Gazi Bay, are morphologically well suited to fix fine sediment particles. They are thus able of lifting the substrate towards belonging to the medio-littoral zone forcing the plant community to change. In the medio-littoral zone Halodule wrightii and Halophila minor, the latter with far reaching stolons, may be the colonizing species. In the low and middle medio-littoral they may be followed by a seagrass community with Halophila ovalis as an intermediate species, on almost fine substrate, and with Cymodocea rotundata and Halodule uninervis on fine substrate, and Thalassia hemprichii on almost every sedimentary substrate, as climax species. In the high medio-littoral, where the emersion period is getting extremely long, they probably can be followed by the mangrove community, possibly with a Boodleopsis pusilla – community as an intermediate vegetation on very fine mud. A different succession occurs on a hard substrate from low and middle medio-littoral: Laurencia papillosa is growing on naked dead coral substrate, fixing sand. Sand fixing continues until the substrate is well suited for colonizing seagrasses. The data on the epiphytic algae-layer of Thalassodendron was also submitted to multivariate computer analysis. Variance analysis with STATGRAPHICS proved that the average coverage with epiphytes on older parts of the leaves is higher than on the younger parts. With DECORANA and TWINSPAN a clear distinction was made between samples of photophilic and of the sciaphylic community. A further distinction could not be made, because sampling was done only in one season, and because no age marks were put on the leaves.
- Seagrasses and associated macroalgae at West Gazi bay during July and August 1987 and abiotic data of the corresponding sampling locations, more