Meiofauna plays an important role in the seagrass ecosystem. The epiphytic meiofauna of Thalassodendron ciliatum has been studied at two different sit.es along the Kenyan Coast : Nyali Beach and Maftaha (Gazi) Bay. In Gazi, two other species of seagrasses were also sampled : Enhalus acoroides and Cymodocea rotundata . In Nyali, two samples were studied; in Gazi we took six samples. The first treatment was done in the laboratories of the Kenyan Marine and Fisheries Research Institute at Mombasa, Kenya. The seagrasses studied here, are also discussed in a short summary. The most important character of their ecology is that they aII appear in the sublittoral zone and have an enormous primary production. Seagrasses are the only Angiosperms that can live and reproduce in salt water, because of different adaptations e.g. hydrophyllous pollination; rhizomes. A seagrass ecosystem plays an important ecological and economical role. We found in literature for example that they give shelter to a lot of animals including juveniles of commercial pelagic fishes; they keep the substrate together. and protect it against erosion and currents; they strip the nutrients out of the water; they serve as food for some animals (e.g. Serenia). Some economic advantages are the sometimes eatable leafs and fruits and sometimes they can be used in the paper-industry as well. The seagrasses have of coarse also an immediate influence on their epifauna, This is the reason why we also measured some parameters of those seagrasses : Iength, volume, dry weight (DW), ash-free dry weight and the Kjeldahl Nitrogen amount. DW is the factor we used to standardize our density-results in Numbers/gram of DW. The Kjeldahl Nitrogen amount is an alternative to measure the biomass of available food. The DW of the sampled grasses lies between 0.950 and 21. 180. Kjeldahl N. varies from 1699.500 to &81,4. 510. The sediment composition consists of medium sand in all samples; two samples have a small amount of silt as well (1%). This 1% silt has a big influence on the meiofauna population : the density increases with the amount of silt. We know from literature that seagrasses appear always in sandy sediments. The analysis is given together with the cumulative frequency distribution curves. Following abiotic parameters are correlated with the meiofauna : water currents and quantity of epiphytes on the seagrasses are of very great importance (literature) but we didn’t measure these factors. The influence of water currents is obvious in Nyali where the lengths of the seagrass-stems differ with distance to the tide channel and to MLWL (mean low water level). The higher the stress, because of current and waves, the shorter the stems. The depths of the sample sites ranges from 0.2m till about 3.5m. pH varied from 6.71- to 8.1-5 in Gazi Bay. During the sampling period, the temperature of the seawater diurnally changed from 25-27.5 ?C. The average salinity for Gazi Bay is 33 promille. Light is of course towards Angiosperms of primarily interest in function of their photosynthetic activity and primary production. Often it plays a limiting role. Eighteen taxonomical levels are distinguished within the meiofauna; Nematoda and Copepoda are the dominant taxa (resp. 39.34-64.22% and 16.66-44.29%). Only in samples 3 and 10 the Copepoda were the most abundant taxon resp. 40 and 38%. We can decide out of a range of studies of literature that what counts for different species algae, also counts for the different seagrass species : although we studied meiofauna from seagrasses of two different places and of three different seagrass species, the different taxa do not differ at the different sites; only in absolute meiofauna densities a clear difference between Nyali and Gazi has been shown, Gazi has a much bigger quantity of epiphytic meiofauna (Gazi: 1351-6796 N/g DW; Nyali : 331-623 N/g DW). The nematodes have been studied at genus level. In total, 1487 nematodes have been determined, belonging to 25 different families. The dominant family is Chromadoridae, which has also the most ( 18 ) different genera. The dominant genera are : Anticoma, Chromodora, Euchromadora, Prochromadorella, Spilophorella, Viscosia. Juveniles made always 40-60% of total density, while males are far less abundant than females. There is however a clear difference between the stem fauna and fauna of the leaves. The leaves have more adults and most of them belong to the larger nematodes; Symplocostoma is one of them. This genus migrates during summer from the stem to the leaves when an increase in habitat complexity occurs because of an increase in epiphytic algae. Juveniles then disappear or are reduced in numbers. In general, the bigger nematodes occur on the leaves in summer as adults; and on the stems and leaves as juveniles during the whole year. Examples are Euchromadora, Viscosia, Graphonema and Halalaimus. The Chromadoridae occur more on the leaves and so do the Enchelidiidae and the Oncholaimidae. The Chromadoridae, Oxystominidae, Epsilonematidae, Draconematidae, Microlaimidae and Desmoscolecidae are the dominant families of the stems. In sample 8 (with most silt) the Linhomoeidae, Leptolaimidae, Diplolaimidae and Oncholaimidae increase in density. In Gazi, the most abundant families are the Anticomidae, Oncholaimidae, Enchelidiidae, Cyatholaimidae, Chromadoridae, Desmodoridae, Monhysteridae. The Chromadoridae are in Nyali more dominant than they are in Gazi. The Linhomoeidae are limited to Gazi. The dominant genera are almost different in all samples, but they almost belong always to the Chromadoridae (with some exceptions). The most dominant genera are Anticoma, Chromadora, Euchromadora, Prochromadorella, Spilophorella, Viscosia, Graphonema, Araeolaimus, Eleutherolainrus, Leptolajmus, Paradesmodora. In sample 8A Eleutherolaimus, Leptolaimus and Prochromadorella are most abundant. The most important leaves genera are : Viscosia, Symplocostoma, Euchromadora, Chromadorina, Araeolaimus, Camacolaimus, Draconema, Nemanema, ProchromadoreIla. Those of the stems are Euchromadora, Chromadora, Halalaimus, Epsilonema Spilc,phorella. Anticoma. Aponema. Graphonema. In Nyali, Chromadora, Chromadorina and Euchromadora are clearly more dominant. The determining factors for the distribution of meiofauna are water currents , sedimentation ( = amount of silt ) and the quantity of epiphytic algae. The algae cause an increase in habitat complexity; increasing currents cause decreasing sedimentation. The higher the habitat complexity, the higher the nematode-density and Iarger nematodes appear. Higher water currents cause a decrease in meiofauna. In Gazi, were more epiphytes, while in Nyali there was a stronger water current (that changed with the tides), which explains the higher meiofauna densities in Gazi. Nematodes are divided into four feeding-types : 1A : selective deposit feeders, 1B : non-selective deposit feeders, 2A : epistratum feeders and 2B : predators. They are characterized by the following characteristics : 1A: No teeth present, no or a small mouth. 1B: No teeth present, a small cylindrical mouth. 2A: with teeth to scrape food from the substratum. 2B: with large teeth to catch preys. The dominant feeding-type in the seagrass community is 2A. Feeding-type 1B is positively correlated with the amount of silt; its abundance increases with increasing percentage of silt. The abundance is about 10% in sample B that has the most silt (1%). Feeding-type 1A has a constant abundance of 20-25% and is more present on the stems of seagrasses, while the predators (2B) occur more on the leaves by adults because its correlation with the large nematodes. The selective deposit feeders (1A) have always a higher abundance than the predators ( 2B). Literature shows that on algae and seagrasses the dominant feeding-type is always 2A and this mainly because of the Chromadoridae (2A), Monhysteridae and Linhomoeidae (1B), different families of 1A, Oncholaimidae, Ironidae (Svrineolaimus ) and Enchelidiidae (Symplocostoma and Eurystomina). Different diversity-indices have been measured, but the exponent of the Shannon-Weaver index (N1 = exp H’) is the most standardized one in literature. The family-diversity N1 varies from 2.67 - 13.81 and for genera from 9.01-22.07. All samples in Nyali have a significant lower diversity than in Gazi. On the leaves of seagrasses, the diversity is also lower than on the stems. With the results of abundances of taxa, families and genera, we determined the similarity between the samples. Following analyses were performed : TWINSPAN (Two-Way Indicator Species Analysis) KLUS Bray- Curtis - Similarity DCA (Detrended Correspondance Analysis). 1) without downweighting the rare groups 2) with downweighting as a comparison with the results of TWINSPAN and Bray-Curtis. 3 ) with downweighting and transformation vv (to minimalize the variance of the input-data). 4 ) we connected to the last one our abiotic parameters to find out which parameters are the base for similar clusters. The results of these analyses are that at each different level (taxon, family or genus ), the groups of samples with similar animal composition are distinguished. These groups are: * Sample 8 always stands apart because of its higher percentage of silt. Silt causes also a significant increase in Gastropoda and the nematode families Linhomoeidae, Diplolaimidae and Leptolaimidae. * The stem samples are clusterd because of a correlation with ash-free dry weight and the percentage of sediment bigger than 1 mm. On the stems the Microlaimidae, Oxystominidae, Desmoscolecidae, Epsilonematidae and Phanodermatidae are also correlated with these abiotic factors. * The leave samples are clustered because of their correlation with the dry weight * The Gazi samples 7, 8, 9 are related to MdØ (median of the sand fraction), volume of the seagrasses and the Kjeldahl Nitrogen. These factors determine the occurrence of Selachinematidae, Xyalidae, Anticomidae, Desmodoridae and Cyatholaimidae. |
