Sierra Leone - State of the Marine Environment 2015

5.18. Plankton

reaches (Leigh 1973). Seawater temperature reaches a second peak in April and May during which period plankton production decreases. In general high plankton production is between the end of the rainy season to the middle of the dry season (October- February). There is a decline from March to June, which extends into the rainy season. Themajor phytoplankton species are: Thalassiosira, Nitzchia, Pleurosigma, Coscinodiscus, Thalassoinema, Skeletonema, Amphora, Ceratium, Peridinum, and Oscillatoria . Some blue green algae may occur in the rainy season. Dominant phytoplankton species in the dry season appears to be Coscinodiscus and Thalassiothrix . In the rainy season the dominant species are Thalassiothrix , Coscinodiscus and Thalassiosira . Algae species are not known to be exploited, no major changes in composition have been observed and are therefore in good condition. 5.19. Summary (species assessment) A total of 29 parameters (species and species groups) were assessed at the workshop. The average score for species indicates a good condition for most places (Fig. 5.3). The condition of species in the best 10% places is assessed as being good, ranging from good to very good. The condition of species in the worst 10% places is assessed as being poor, ranging from poor to good.

The plankton of the Sierra Leone River estuary and parts of its creeks and bays have been studied by many researchers (Watts 1958, Aleem 1979, Leigh 1973, Findlay 1978, Conteh 2001). Horizontal distribution and seasonal fluctuations in plankton production are intricately linked with the changes in climate during the rainy and dry season. During the rainy season (May–September), there is a reduction in solar insolation, increased discharge of freshwater, increase in sediment input on the shelf and the lowering of the temperature. During this period there is instability. Stability returns in November after a complete mixing of the estuarine water in October by strong winds. In the dry season (November–April) there is a reduction in river discharges, reduction in stream velocity, increase in wave and tidal effects, and increase in solar radiation and temperature leading to stratification. Phytoplankton production increases in December followed by an increase in zooplankton density in January and February. There is rapid reduction in nutrients due to corresponding uptake by phytoplankton and loss from surface waters due to dying plankton sinking to the bottom. There is a gradual increase in salinity due to intrusion of oceanic waters and a high evaporation rate reaching a maximum in May or June. Cold oceanic water intrusion during the prevailing Harmattan depresses temperature at optimum salinities, thus favouring high standing stocks of plankton during the middle of the dry season at the middle

Figure 5.3: Summary of the species assessment. Average, high and low for the condition of species in the best 10%, most and worst 10% of places

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