Browsing by Author "Semba, M."

Now showing 1 - 3 of 3
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    Coastal upwelling and seasonal variation in phytoplankton biomass in the Pemba Channel
    (WIOMSA, 2020) Kyewalyanga, Margareth S.; Peter, Nyamisi; Semba, M.; Mahongo, S.B.
    This study was conducted in the Pemba Channel off Tanga Region in northern Tanzania to investigate physical and chemical factors that drive changes in phytoplankton biomass. Three transects off Mwaboza, Vyeru and Sahare were selected. For each transect, ten stations were sampled. Phytoplankton biomass was determined as chlorophyll-a (Chl-a) concentration. Similarly, physico-chemical variables (temperature, salinity, dissolved oxygen, pH and nutri ents) were determined. It was observed that the Chl-a concentration was significantly higher during the northeast monsoon (median 1.44 mg m-3) as compared to the southeast monsoon (median 1.19 mg m-3; W = 2216, p = 0.029). The higher productivity during the northeast monsoon is attributed to the presence of high-nutrient water caused by coastal upwelling. It is concluded that indication of upwelling, observed through relatively low temperatures during the northeast monsoon season, could be responsible for bringing nutrient-rich waters to the surface, which in turn stimulated the increase in Chl-a concentration.
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    The decline in phytoplankton biomass and prawn catches in the Rufiji-Mafia Channel, Tanzania
    (WIOMSA, 2016) Semba, M.; Kimirei, I.; Kyewalyanga, M.; Peter, N.; Brendonck, L.; Somers, B.
    The world’s oceans have seen significant declines in phytoplankton-the primary food source in the marine environ ment. This decline in primary producers is likely to impact the food chain and functions of most coastal and marine ecosystems. Despite being one of the most productive marine fishing grounds in the Western Indian Ocean (WIO) region, the information about phytoplankton biomass in the Rufiji-Mafia Channel is poor. This study aimed to nar row this information gap by assessing phytoplankton biomass in the Channel and its association with the decline of the prawn fishery. We combined in-situ measurement and ocean colour satellite data to determine and assess trends in phytoplankton biomass and sea surface temperature between 2002 and 2014. These trends were related to a declining prawn fishery in the Channel. While phytoplankton displayed a significant declining trend during the southwest monsoon, sea surface temperature showed an insignificant increasing trend. Phytoplankton declined at the rate of 1.2 percent per year (tau = 1.2, z = 3.52, p = 0.004) between 2002 and 2014. This declining trend in Chl-a matches well with the decreasing trend in the prawn fishery (tau = 0.57, z = 3.39, p = 0.0006) and the insignificant increasing trend in sea surface temperature (tau = 0.02, z = 0.43, p = 0.66). This study provides quantitative evidence of trends in chlorophyll and SST and the link with trends in the prawn fishery, which increases our understanding of the changes in marine primary productivity in the coastal waters of Tanzania.
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    The influence of physical-chemical variables on the spatial and seasonal variation of Chlorophyll-a in coastal waters of Unguja, Zanzibar, Tanzania
    (WIOMSA, 2018) Peter, N.; Semba, M.; Lugomela, C.; Kyewalyanga, M.S.
    Chlorophyll-a (Chl-a) concentrations were measured at four sites around Unguja, Zanzibar during the northeast (NE) and southeast (SE) monsoon seasons. Data for Chl-a, nitrate, phosphate, ammonia, dissolved oxygen, sea sur face temperature, pH and salinity were collected once a month from March 2008 to March 2009. The SE monsoon had insignificantly higher Chl-a compared to the NE monsoon season when Chl-a for Bawe, Chumbe, Pongwe and Mnemba were combined (W = 234, p = 0.93). The drivers of high Chl-a during the SE monsoon were ammonia and nitrate. Results from individual sites showed that Pongwe and Mnemba had higher median Chl-a during the SE than the NE monsoon season. Temperature, dissolved oxygen and ammonia were the major factors that influenced high Chl-a at these sites. In contrast, Chumbe and Bawe had higher median Chl-a during the NE- than the SE mon soon season. The major factors influencing high Chl-a in the NE at Chumbe and Bawe were high levels of nutrients, mainly from sewage effluent and various human activities around the coast in Zanzibar town. The interaction of Chl-a between monsoon seasons (NE and SE) and sites (Bawe, Chumbe, Pongwe and Mnemba) was insignificant (F(1,3) = 1.3144, p = 0.2949). The principal component analysis revealed that different physical and chemical environmental variables affect Chl-a concentration over time and location.