Abstract:
Surface water concentrations of inorganic nutrients and suspended particulate matter
(SPM) components were examined and compared between Mtwapa and Shirazi creeks in
Kenya. This was aimed at assesing the ecological situation of the two creeks and
determine the influence of sewage discharge. The results obtained were further compared
with those from Ramisi, an estuarine system. Mtwapa recorded higher nutrient,
chlorophyll a and phytoplankton carbon concentrations than Shirazi. The two creeks also
recorded different phytoplankton stocks and groups. Dinoflagellates dominated Mtwapa
in the stations within the vicinity of sewage discharge points whereas Shirazi was
dominated by pennate and centric diatoms, though at lower concentrations. Shirazi
recorded the highest particulate organic carbon (POC) / phytoplankton carbon ratio. The
Ramisi estuarine stations were characterised by high concentrations of dry weight (DW),
centric diatoms, phytoplankton carbon, detritus and POC. Cluster analysis revealed three
main clusters; the first cluster of pure estuarine stations, a second cluster comprised of
stations from Mtwapa and Shirazi and a third cluster of two Mtwapa stations which were
located within the vicinity of sewage discharge points. A PCA sites scatter plot produced
similar clusters. A PCA species-sites biplot showed that stations in the first cluster were
characterised by high concentrations of phytoplankton carbon, centric diatoms, DW, POC
and detritus, 'species' which were highly correlated with axis 2; stations in the second
cluster were characterised by high concentrations of POC / phytoplankton carbon ratio
whereas stations in the third cluster were characterised by high concentrations of
dinoflagellates, a 'species' which was highly correlated with axis 1. The three systems
however had detritus as the main POC component contributing above 60% of the total
POC.
