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5.18. Plankton
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
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.