In this study the meiobenthic composition and diversity near five seagrasses Halodule wrightii Asch., Halophila ovalis (Braun) Hooker, Thalassia hemprichii (Ehrenb.) Asch., Syringodium isoetifolium (Asch.) Dandy and Halophila stipulacea (Forssk.) Asch. was investigated. Two quadrants of each seagrass species, situated in an intertidal seagrass bed at Gazi Bay (Kenya) were sampled. Sediment samples were taken to a depth of ten centimetre using a plastic hand core. They were divided into six depth layers (0-1, 1-2, 2-3, 3-4, 4-5, 5-10 cm). Relevant abiotic factors such as sediment characteristics and organic matter, pigment and nutrient content were also quantified. Four communities were distinguished in the seagrass bed, based on meiofaunal composition and prevailing abiotic factors. This spatial distribution was similar to the one found by COPPEJANS et al. (1992), based on the presence and cover of seagrasses as well as macroalgae. This matches the successional pattern in a seagrass bed, proposed by DEN Hartog (1967, 1977). The first community, consisting of meiobenthos near H. wrightii and H. ovalis, which are pioneers in the eulittoral zone, is characterised by a high total meiofauna density, a high density of nematodes, low densities of copepods and nauplii and a low diversity at the higher taxon level. A small median grain size and low organic matter, pigment and nutrient concentrations are the most distinctive abiotic variables. The other three communities, which are found in the sublittoral zone, have higher abundances of copepods and nauplii and a lower total meiobenthic density. Median grain size is larger, as are pigment, nutrient and organic matter contents. The community associated with the climax-vegetation T. hemprichii is the most diverse, followed by the community of S. isoetifolium, which is a climax species restricted to deeper regions. H. stipulacea is a coloniser of the sublittoral, which has a less diverse meiobenthic composition than the climax species. Moreover, the pigment, nutrient and organic matter contents are also lower. Highest abundances of nematodes were found in fine sediments, whereas the density of copepods increases in sediments with higher median grain size. Next to oxygen availability and water content, meiobenthic distribution is also related to food availability (such as detritus) in the sediment (GlERE, 1993), which in turn is linked to the degree of habitat complexity of the seagrass plant. An increase in leaf surface area ultimately leads to a higher sediment detritus content once the plant dies. Detritus and associated micro-flora form a potential food source for meiofaunal organisms (Meyer-Reil & Faubel, 1980). The vertical distribution pattern of meiobenthos shows a decrease in abundances with increasing depth. Nematodes are the most abundant taxon and their contribution to total meiofaunal density increases in the lower centimeters. Copepods and nauplii are abundant mostly in the uppermost centimeter(s). The number of taxa diminishes with increasing depth. The meiofauna found in the upper two centimeters is the most diverse and is clearly distinct from that of the underlying depths. Relatively high densities of copepods and nauplii are restricted to the top centimeter in the community near H. wrightii/H. ovalis. A considerable amount of kinorhynchs is also present at that depth. The communities of T. hemprichii and H. stipulacea are characterised by the presence of copepods and nauplii at all depths. They each make up more than 3% in every depth layer. The fourth community associated with S. isoetifolium has high densities of copepods and nauplii in the upper centimetre and between 4 and 5 centimeter. The benthic harpacticoids associated with both quadrants of H. ovalis and H. stipulacea show a larger variation between seagrasses than between quadrants. H. stipulacea shows a higher density and diversity (on family level) than H. ovalis. A comparison between benthic and epiphytic communities of these quadrants (De Troch , unpubl. data) revealed a strong mutual similarity in each quadrant. The increased densities of Thalestridae and Laophontidae, two mainly epiphytic families, in the two quadrants of H. stipulacea indicate a rather epiphytic community which appears mainly in complex and detritus rich habitats (HICKS, 1980). The H. ovalis community is characterised by Ectinosomatidae and Cletodidae, two families which are rather benthic (HICKS & COULL, 1983). The spatial variation of meiofaunal taxa between pioneer and climax species was also found on a smaller scale for harpacticoid families between two morphologically similar seagrasses. Epiphytic and benthic communities of H. stipulacea (larger leaf surface area, higher detritus input) were characterised by a higher diversity and higher densities. The more stable and enriched environment where H. stipulacea prevails probably contributes to these findings. |
