Coastal upwelling and seasonal variation in phytoplankton biomass in the Pemba Channel

Margareth S Kyewalyanga,Shigalla B Mahongo,Nyamisi Peter,Masumbuko Semba

doi:10.4314/wiojms.si2020.1.3

Abstract

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 nutrients) 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.

Highlights

Phytoplankton are important in the ocean food webs as primary producers which transfer energy from the sun to higher trophic levels through photosynthesis (Vargas, 2006) and contribute about half of the earth’s oceanic and terrestrial primary production (Barlow et al, 2008; Field et al, 1998; Gröniger et al, 2000)
The Wilcoxon test showed that the difference in the median of Chl-a concentration between the southeast monsoon (SE) and the northeast monsoon (NE) monsoon seasons was significant (W = 2216, p = 0.029)
Several studies reported that the abundance and distribution of Chl-a in the coastal waters of Tanzania depend on monsoon seasons, with the NE period experiencing relatively low values in Chl-a compared to the SE season (Peter, 2013; Ezekiel, 2014; Semba et al, 2016; Moto and Kyewalyanga, 2017)

Summary

Introduction

Phytoplankton are important in the ocean food webs as primary producers which transfer energy from the sun to higher trophic levels through photosynthesis (Vargas, 2006) and contribute about half of the earth’s oceanic and terrestrial primary production (Barlow et al, 2008; Field et al, 1998; Gröniger et al, 2000). Phytoplankton distribution and production vary both in space and time, caused by variation in chemical and physical factors Some of these factors include carbon dioxide, sunlight, nutrients, temperature, water depth, bottom topography, upwelling, presence of grazers, salinity, dissolved oxygen and water pH (Bouman et al, 2003; Gallienne and Smythe-Wright, 2005; Lamont and Barlow, 2015; Lee et al, 2012; Meyer et al, 2002; Sá et al, 2013). In Tanzanian coastal waters, several studies reported on spatial and temporal variation in Chl-a and phytoplankton species composition (Kyewalyanga, 2002, 2015; Barlow et al, 2011; Peter, 2013; Ezekiel, 2014; Limbu and Kyewalyanga, 2015; Semba et al, 2016; Moto and Kyewalyanga, 2017).

Methods

Results

Discussion

Conclusion