Abstract

Phytoplankton are important in aquatic ecosystems since they are the primary producers. Chlorophyll-a (Chl-a) concentration is used as a proxy in estimation of phytoplankton biomass. The present study was conducted to determine the spatial and temporal phytoplankton distribution and abundance in relation to the physico-chemical variables using remote sensing Chl-a and in situ collected Chl-a measurements along Zanzibar and Pemba channels. In situ Chl-a was based on fluorometric method while the satellite Chl-a was acquired from MODIS sensor. There was a significant difference in Chl-a (H=66.23, 11 d.f, p<0.05) and SST (H =244.27, 11 d.f, p<0.05) variation between the months, with higher Chl-a concentration in April-May and July-August. High Chl-a concentration were observed on the nearshore waters at both Zanzibar and Pemba channels. The rainy season was having significantly higher Chl-a concentration than the dry season (U=19, 1d.f., p=9.15×10-21) with the mean values above 0.31mg/m3 and below 0.06mg/m3 respectively. Spatially, the Zanzibar channel had significantly higher Chl-a concentration than the Pemba channel (F3, 116=16.96, p=3.36×10-9). The Chl-a was positively correlated with nitrate, phosphate, DO and pH and negatively correlated with SST and salinity during the dry and rainy seasons. The satellite Chl-a was positively and significantly correlated with in situ Chl-a for the rainy (r = 0.86, p < 0.05) and the dry (r = 0.74, p < 0.05) season. However, satellite Chl-a was significantly higher than the in situ values. The present study revealed that phytoplankton distribution and abundance at the sampled sites was influenced by nitrate and phosphate in the dry season and nitrate and salinity during the rainy season.