Groundwater effects on diversity and abundance of lagoonal seagrasses in Kenya and on Zanzibar Island (East Africa)

The second objective of the GROFLO project is to elucidate differences in nearshore community structures and ecosystem functions in relation to groundwater outflow. In March 1997 and February 1998 two seagrass surveys were carried out. In the framework of the GROFLO project, models of groundwater outflow along the coasts of Kenya and Zanzibar Island were constructed (see contribution of VUB in this report). This made it possible to select study sites with contrasting groundwater-outflow rates. The aim of our surveys was to relate the rate of coastal groundwater outwelling to productivity and vitality of lagoonal seagrasses. The objective of the present study is to relate the rate of coastal groundwater outwelling to the abundance and species diversity of lagoonal seagrasses in East Africa. For the dominant species Thalassodendron ciliatum, supplemental data on nitrogen content and natural abundance of nitrogen isotopes in leaves were also collected. In addition, leaf-production rates, shoot demography, and flowering frequency of T. ciliatum were determined.