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

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.

Influence of water physico‐chemical parameters on the spatio‐temporal activity variations of west African waterbirds (fam. Ardeidae)

“…Water transparency defines the level of sunlight penetration into the water where aquatic plants (phytoplankton etc.) can photosynthesise (Helbling & Villafane, 2009;Kyewalyanga, 2016;Nielsen et al, 2002). This phytoplankton is the first link in the aquatic food chain (D'Alelio et al, 2016;Davies et al, 2009;Lampman & Makarewicz, 1999).…”

High-frequency dynamics of pH, dissolved oxygen, and temperature in the coastal ecosystems of the Tanga-Pemba Seascape: implications for upwelling-enhanced ocean acidification and deoxygenation

“…Similar to the loggers utilized in coastal ecosystems, pH and DO loggers were securely affixed to the buoy, positioned at a depth of 10 meters below the surface to maintain continuous monitoring of surface water pH, DO, and temperature. The loggers recorded pH, DO, and temperature data throughout the northeast monsoon season, spanning from October 2022 to March 2023, when upwelling is thought to occur in the T-PS (Halo et al, 2020;Kyewalyanga et al, 2020).…”

“…Deoxygenation, the decline in seawater oxygen levels, is caused by ocean warming, which reduces the ocean's capacity to hold DO (Keeling et al, 2010;Limburg et al, 2020). Furthermore, high nutrients in seawater, mostly from agriculture, sewage, and upwelling, lead to excessive growth of phytoplankton that results to algal blooms (Kyewalyanga et al, 2020). When algal blooms die and disintegrate, they absorb oxygen, resulting in deoxygenation in coastal and shelf areas, as well as semi-enclosed seas (Limburg et al, 2020).…”

“…These conditions place resident marine organisms and coastal ecosystems at a high risk of future ocean acidification and deoxygenation (Cornwall et al, 2023;Kroeker et al, 2023). Although the extensive impacts of upwelling events on pH, DO, and temperature dynamics are well-documented globally (Rixen et al, 2012;Schulz et al, 2019;Kroeker et al, 2023), this information is largely unknown in coastal upwelling areas of the Western Indian Ocean (WIO), particularly in the Tanga-Pemba Seascape (T-PS), known to experience upwelling during the northeast monsoon season (Halo et al, 2020;Kyewalyanga et al, 2020). This information is crucial for informing the future of global change biology in the WIO.…”

“…The objective of this study was twofold. Firstly, to understand high-frequency dynamics in pH, DO, and temperature in the open ocean of the T-PS during the northeast monsoon, when upwelling events are thought to occur (Halo et al, 2020;Kyewalyanga et al, 2020), as well as the 24-hour high-frequency dynamics of these variables in mangrove, seagrass, and coral reef ecosystems within the seascape. Secondly, to understand the susceptibility of mangrove, seagrass, and coral reef ecosystems to lower pH and DO conditions induced by upwelling events and whether some of these ecosystems offer any degree of mitigation under such events.…”