Seasonal variability of phytoplankton diversity and biomass in the coastal waters of Unguja Island, Zanzibar

The primary focus of this study was to describe patterns in phytoplankton distribution, composition, abundance, diversity and chlorophyll-a (Chl-a) concentration in Unguja Island coastal waters. The field study was conducted from May, 2012 to May, 2013 in Bawe and Chwaka Bay of Unguja Island. It involved species identification, abundance, diversity and chlorophyll-a determination in relation to physicochemical parameters with regard to spatial and temporal variations. A total of 260 species of phytoplankton were identified, belonging to 94 genera and six classes: Bacillariophyceae (171 species), Dinophyceae (71 species), Cyanophyceae (15 species), and one species each for Chlorophyceae, Crysophyceae and Dictyochophyceae. Species diversity at Bawe was not significantly different between seasons (p > 0.05), while at Chwaka Bay diversity was higher during the Southeast monsoon (SEM) than the Northeast monsoon (NEM) (p < 0.05). Species diversity index average was above 3.0, indicating that Unguja Island coastal waters have moderate diversity. Chl-a concentration was high near the shore of Stone Town and decreased further towards the open waters (p < 0.05). At Chwaka Bay, the spatial variability in Chl-a concentration was not significantly different among stations (p > 0.05). Temporal variation in Chl-a for insitu data revealed higher concentration of Chl-a during SEM than NEM (p < 0.05). The insitu measurements corresponded with satellite data which also showed high Chl-a concentration during the SEM that coincided with low sea surface temperature (SST). Time series satellite data for ten years further showed an inverse relationship between the interannual variation in Chl-a concentration and SST