Zonation and distribution of creek and fringe mangroves in the semi-arid Kenyan coast

Abstract

Patterns of zonation and distribution of mangroves in relation to climate, hydrology and hydrographic factors are described. There are eight species of mangroves found along the semi-arid Kenyan coast where they may occur as creek or fringe mangroves. The climate along the Kenyan coastal belt is under the influence of two seasonal monsoonal winds, namely the South East Monsoon (SEM), which begins from April to October, and the North East Monsoon (NEM) from November to March. The rains are bimodal, the heaviest rainfall occurring in SEM (range 55 to 272mm) and the second peak may appear in NEM (range 8 to 84 mm). The ranges of temperatures in SEM and NEM are 20-31° C and 23-32° C, respectively. There are only two permanent rivers and a few seasonal rivers. However, there is considerable discharge of underground water to the sea floor which creates the brackish water conditions required by mangroves. Geographically the bulk of the mangrove forest cover is found far away from the permanent rivers and in some cases at places without any surface flow. The latter areas, however, correspond to the areas receiving heavy discharges of underground water. The range at the Kenyan coast is wide (4m), and it is therefore common to find zonation in creek mangrove. Based on the principal species, the mangrove zonation observed in an upward shore direction was as follows: Sonneratia alba J.Sm., Rhizophora mucronata Lam., Ceriops tagal (perr) C.B. Robinson, Avicennia marina (Forks.) Vierche and Lumnitzera racemosa Willd. However, Avicennia marina has the widest vertical distribution and almost exclusively occupies the salty flats.