An Ecological Assessment of Littoral Seagrass Communities In Diani and Galu Coastal Beaches, Kenya

Abstract

Most studies along the Kenyan coast have concentrated on coral reefs and mangrove swamps. Limited studies have been carried out on seagrasses though they provide an important link between the mangroves and the coral reefs. Seagrass beds are located in intertidal lagoons which lie directly in front of coastal hotels, hence they are thought to be affected by hotel sewage through underground seepage particularly where the sewage disposal systems are located close to the shore. The aim of this study was to ascertain the effects of this on the marine life sustained by the seagrass community. The flora, fauna and physico-chemical parameters of the littoral seagrass community in Diani Beach, a tour i st resort located in the south coast of Kenya, was compared to the one in Galu Beach, a relatively unexploited area in terms of beach hotel development. The mean biomass of seagrass was significantly higher in Diani Beach (H = 6.96, p ~ 0.05). Though the algal biomass was higher in Diani Beach, a comparison between the two areas revealed that the difference was not statistically significant. The epiphytic load on the stems of the seagrass Thalassodendron ciliatum (Forssk.) den Hartog was significantly different (H = 18.13, p ~ 0.05). However, the abundance of the epiphytes was governed by the availability of T. ciliatum stems in both Diani and Galu Beaches. The mean number of epibenthic fauna and infauna were higher in Diani Beach though it was not statistically significant when the two areas were compared. The fauna, unlike the flora, experienced significant seasonal fluctuations attributed to the seasonal environmental changes particularly in the sediment characteristics. Detectable nutrient levels in the water column in both Diani and Galu Beaches indicated the presence of nutrient seepage into the seagrass ecosystem. The overall levels of nitrates were significantly different (H = 4.57, p ~ 0.05) when the two areas were compared with higher levels recorded in Diani Beach. However the levels of ammonium and phosphate were similar in the two areas. Nutrient levels in the two areas appeared to be related to the maximum spring tidal heights and the tidal pumping of groundwaters as well as rainfall levels. The levels of biochemical oxygen demand were less than the critical level of 10 mg °2/1 in.both Diani and-Galu Beaches. In spite of the high concentration of beach hotels in Diani, tidal out-flushing and strong circulation patterns prevent the eutrophic accumulation of nutrients in this lagoon.