Sierra Leone - State of the Marine Environment 2015

Analysis of limited hydrographic data suggests that upwelling off the Sierra Leone coast is characterised by a shallower thermocline and nutrient enrichment below the surface at 20m depth (Johnson & Johnson 1996). The upwelling phenomenon has not been studied in detail with regards to causes, timing, persistence and impacts climate and productivity of the Sierra Leonean coastal zone. Productivity and upwelling are inextricably interwoven. 6.5. Tides At the northern mouth of the Sierra Leone River estuary, a sand bar of considerable magnitude obstructs the flow of water from and into the estuary and consequently the velocity of the tidal stream is very great. In the dry season, during spring tides, the flood stream runs from 1 to 1,5 knots lasting for about 5 hours, and the stream on the ebb tide varies from 2 to 2,5 knots for about 7 hours. In the wet season, however, when the tidal prism is augmented by the flood discharge from the river, the ebb stream attains velocities in the region of 5 knots. The tide brings in fair quantities of sediments from the upper reaches of the estuary. The highest astronomical tide of maximum tidal range goes up to 3.38 metres above the lowest astronomical tide or the admiralty chart datum. Mean high water neaps are 3.0 meter above Chart Datum whereas mean low water neaps are 1.0 meter above Chart Datum. 6.6. Waves and Swells Almost a perennial swell of moderate height, coming in from the Atlantic disturb ships at their moorings within the estuary. This phenomenon is particularly evident during the wet season when both frequency and amplitude are greatest. The maximum amplitude is in the region of 1.0 meter. Winds waves, usually not higher than 1.0 meter, only occur for brief spells usually just before the change of seasons about May and October. 6.7. Sediment Transport Processes Longshore drift current is the main mechanism by which sediments are transported along the Sierra Leone coast. The sediment transport takes place mainly within 1–10m water depth. Three main longshore drift current directions can be recognized along the Sierra Leone coastline. These currents flow in a north eastern direction causing erosion of the northern coastline around Yelliboya Island and Konakridee. Similar south easterly flowing currents in the south carry sediments from the Freetown Peninsula beaches and along the entire southern coastline of Sierra Leone. Tidal currents also influence the sediment transport dynamics particularly those of very fine sand and mud mainly at the entrances of bays and estuaries.

Hemisphere when the Southeast Trades noticeably weaken, the Northeast trades are full developed. During this period the Canary current intensifies bringing cool water to the coast of Sierra Leone. This current generally flows in a south- easterly direction at the surface in the near-shelf regions. The Canary current is mainly southward from August to April. When this current approaches the equator, it turns westward as the North equatorial current. The monsoon period generally lasts from July to August; during the Northern Hemisphere summer. During this period, the equatorial counter current is strongly developed and is the source of much water joining the Guinea current. In the winter months (December to February) however, the equatorial counter current ceases to be of importance and the canary current is the main source of water joining the Guinea current. In the autumn the southeast trades strengthen reaching maximum strength in August. During the May – July period, the canary current moves northward carrying low salinity Liberian surface waters to the north as far as Senegal (Berrit, 1969). 6.4. Seasonal Upwelling and Productivity The Sierra Leone coastal zone lies at the southernmost extension of the upwelling system which occurs along the entire northwest African coast Dakar – Freetown region. In Sierra Leone, the continental shelf is isolated from the seasonal coastal upwelling areas of North-West Africa and central Gulf of Guinea by a complex of shoals (submerged sand banks) that comprises the Bissagos Archipelago, off Guinea-Bissau in the North and the shoals of Saint Ann to the South. The area of the continental shelf up to a depth of 200 m is about 30,000 km 2 . The western tip of Sherbro Island delimits two contrasting coastal waters. The narrow southern shelf has limited fish resources and is influenced by the eastward flowing Guinea current.The northern Sierra Leonean coast on the other hand constitutes the productive shelf of Sierra Leone. Therefore, most of the artisanal fishing activities in Sierra Leone occur in the North. Here, there are three major estuaries: the Scarcies River, the Sierra Leone River and the Sherbro River, as well as the Yawri Bay. The continental shelf has good yield potential for demersal and pelagic fish as well as shrimps. According to Coutin (1989), a strong upwelling occurs each year along the coast of North-West Africa between December and April and especially between October and February for the Dakar – Freetown region during the Harmattan. These Harmattan winds cause aerobic upwelling along the entire northwest African coast. Longhurst (1983) indicated that the continental shelf waters of Sierra Leone are influenced by the Cape Verde divergence to the north and the convergence at the northern margin of the Equatorial Counter Current (ECC) further south of the equator. Upwelling is characteristic of the former whilst down welling is a feature of the latter.

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