Sierra Leone - State of the Marine Environment 2015

2. Major marine ecosystems

2.1. Oceanic Habitat in the TerritorialWater

Major relief features include the coastal valleys of the Futa- Jallon highlands composed mainly of Paleozoic sands. The coastal valleys are covered with weathered and erosion products of the Futa-Jallon highlands. High temperature and moisture enhance intensive chemical weathering. The weathered material finds its way into rivers and is carried to the coast, where it is transported alongshore. The relief of the rivers catchments enables the movement of large quantities of terrigenous material (mainly quartz) into the ocean with waters of the surface flow. Other sediment sources including biogenic sediment sources are of secondary importance to the region. The chemical composition of the sedimentary material has a wide range and various types can be identified. The middle shelf zone lies at depths between 20–30 meter and 60–70 meter and is usually the widest part of the shelf with a comparatively smooth surface. The bottom slopes at an angle of some few minutes and at some locations it is less than a minute. The outer shelf lies below 60–70 meter depth and is smaller in width with greater angles of inclination of the bottom. In some parts bed rock is common. This part of the shelf is commonly incised by the heads of canyons.

The Exclusive Economic Zone (EEZ) of Sierra Leone covers about 160,000 km 2 (Fig. 2.1, Tab. 2.1). The continental shelf of the coast of Sierra Leone is about 100 km wide in the north and tapers to about 13 km in the south towards Liberia. The total continental shelf area covers about 30,000 km 2 and it is perennially enriched by nutrients from the river networks, rendering the coastal environment a unique ecosystem, which serves not only as an important habitat for assemblages of marine organisms but also as a feeding and breeding ground for most economically targeted species. The Sierra Leone continental shelf can be divided into four zones: the inner shelf, the middle shelf, the outer shelf and the shelf edge. The shelf is characterized by relatively plain surfaces inclined at angles of a few minutes and with an average width of about 62 km. The outer shelf limit lies at an average depth of 160 m. Each shelf zone is characterized by different angles of inclination of the bottom and they lie parallel to the coast in extensive strips. The inner shelf zone could be traced up to depths of about 20–30 meter, and is the zone of active wave activity. The geomorphology of this zone is closely related to that of the adjacent coast.

Figure 2.1: Maritime boundaries of Sierra Leone (GRID-Arendal)

13

Made with