The subject of this study is the vegetation of the Gazi Bay tidal channel on the south coast of Kenya, more specifically the seagrasses and the benthic macro-algae of the sublittoral zone. For this purpose thirteen transects were made perpendicularly on the direction of the tidal flow, from the field laboratory down to the lagoon. We also investigated a side channel, which showed a great variety of species, as well as an upstream section of the main channel. During our field work the seagrasses were easy to recognize, but some of the macro-algae had to be collected for later identification. The seagrasses were quantified by estimating the degree of coverage (in %) whereas for the algae only presence or absence were noted. In each case the type of substratum has been taken into account (hard or loose). To be able to specify the current velocity, sediment samples were taken all along the transects, in order to determine their particle size. The field work took place in July and August 1989 and was carried out scuba-diving. The analysis and determination of some algae did not always result in the identification of the species: the genus Dictyota could only be classified into morphological entities, whereas the lack of different growth phases of the genus Sargassum prevented us from determining the precise species. However we were able to compile a list of eleven-species of seagrasses and one of seventy different algae: the epiphytes were only briefly touched. The sublittoral zone of the shallow Gazi Bay, containing the tidal channel as well as the lagoon, is a homogeneous biotope: due to the tidal movements salinity, turbidity and pH are fairly conform. Varying parameters here are: substratum: hard: - immobile: dead tree trunks, branches, coral boulders; - moving along with the current: shells, coral fragments; . loose: particle size distribution: >2 mm: shells 1-2 mm: shell fragments; 2 mm: very coarse sand; 0,5-1 mm: coarse sand; 0,2-05 mm: medium coarse sand; 0,1-0,2 mm: medium fine sand; 0,05-0,1 mm: fine sand; 2-50 µm: loam; <2 µm: clay; orientation within a transect: channel rim, channel slope, channel bottom; (3) distance up to the most, upstream observation. All the data on the vegetation and on the environmental parameters were analysed with mathematical techniques in order to get a clear insight into the ecology and zonation pattern of the seagrasses and the macro-algae. The classification programme TWINSPAN did classify the vegetation according to the substratum. Three different groups were distinguished: the species found in sandy substratum, among them the seagrasses, the rheophilic species growing on hard, moving substrata and the rheophilic species growing on hard, fixed substrata. Canonical Correspondence Analysis (C.C.A.) allowed us to determine in a direct way the relationship between vegetation and environmental parameters. Distance up to the most upstream observation point appeared to be the most relevant factor, with Thalassodendron ciliatum, Enhalus acoroides and Syringodium isoetifolium as associated species. These are typical lagoon seagrasses (sheltered sublittoral biotope). They are only to be found here because they are unable to attach themselves in the main channel, where the current is too strong. Typical algae for this area are Amphiroa rigida and Jania rubens, which can form sandy bumps under Thalassodendron. The second most important factor is the channel rim, which represents the sublittoral fringe. Halodule wrightii and Halophila minor are associated with it. These are the pioneers of the midlittoral, but they can reach the sublittoral zone. When the channel is flooded and the water flows over into the midlittoral, the erosion is particularly strong along the rim; the same phenomenon takes place when the tide goes out. In this way a stabilized pioneer association comes into existence. Particle size distribution did vary with current velocity; however no specific relation between particle size fraction and seagrass species could be detected. The ‘at random’ distribution aspect of seagrasses on the channel slopes is the result of the successive stages of the vegetation: due to relocation of the tidal channel, sand banks are built up as well as sand slopes along the leeside of the meander. With strong tidal movements the sand banks remain bare; if on the other hand they are protected from strong currents, these bare areas are colonised by the fast growing Halophila stipulacea. When the rheophilic character remains too strong, this stage rests as it is. If the area is more sheltered Halodule uninervis, Cvmodocea serrulata and C . rotundata take the place of Halophila stipulacea. In some cases finally, the vegetation can develop into a climax vegetation, consisting of Thalassia hemprichii, such as to be found in the midlittoral zone. If however the current should build up (by relocation of a meander), these specific seagrasses tend to disappear again, leaving the sand bare. In this way the whole cycle can start all over. This does explain the patchy growth of the seagrasses, yet we were unable to find a suitable explanation for the patchiness of certain algae in this biotope (Halimeda macroloba, Avrainvillea obscura, Udotea orientalis). The channel bottom, subject to strong currents, is covered with an important number of shell fragments. Typical for this area are the algae Gracilaria verrucosa, Hypnea cornuta, Solieria robusta and Cladophora sp. - although the coverage is limited. Upstream the landscape levels down, the channel gets wider and the water flow is dammed up by sandbanks; this results in the reduction of the current velocity and consequently of the particle size. Gradually more and more species of seagrasses and algae disappear, until at last only the pioneer Halophila stipulacea remains. The side channel is characterized by a rich variety in species. This may be the result of its sheltered situation - the current however being fairly strong - and of the very varied substrata. The importance of the seagrasses is not restricted to their own biotope: large amounts of seagrass leaves (mainly of Thalassodendron ciliatum) and of drifting Sargassum (brown algae) are brought into the mangrove ecosystem at spring tide. As their biomass seems to exceed largely that of the mangrove leaves, it might be interesting to make a precise quantification, in order to understand the nutrient flow of the mangrove. |
