Spatial and temporal variations in environmental variables in relation to phytoplankton composition and biomass in coral reef areas around Unguja, Zanzibar, Tanzania

Abstract

Phytoplankton can indirectly indicate health status of coral reefs due to their sensitivity to changes in water quality parameters. This study explored the spatial and temporal variability in water quality and nutrients in relation to phytoplankton community composition and chlorophyll a concentration at Bawe, Mnemba, Chumbe and Pongwe coral reef sites in Unguja Island. In situ measurements of dissolved oxygen, temperature, salinity and pH were done every month for 1 year. Surface water samples were collected for determination of phytoplankton composition, nutrients and chlorophyll a concentration. Dissolved oxygen, temperature, salinity and pH did not differ significantly among the four sites (p > 0.05) but showed significant temporal variations among months (p < 0.05). Bawe had significantly higher phosphate concentration (1.45 ± 0.57 µg L−1) than Chumbe (0.74 ± 0.53 µg L−1), Mnemba (0.42 ± 0.30 µg L−1) and Pongwe (0.28 ± 0.10 µg L−1; p < 0.05). Similarly, Bawe had significantly higher nitrate concentration (0.81 ± 0.43 µg L−1) than Mnemba (0.33 ± 0.14 µg L−1) and Pongwe (0.24 ± 0.13 µg L−1; p < 0.05) but similar to Chumbe (0.90 ± 0.35 µg L−1; p > 0.05). However, values obtained at all the studied sites were less than 3 and 14 mg L−1 for phosphate and nitrate, respectively, for eutrophic oceans. Phytoplankton species were dominated by Bacillariophyceae (70.83 %) and some species identified such as Ceratium sp., Dinophysis sp., Protoperidinium sp., Prorocentrum sp., Oscillatoria sp. and Dictyocha fibula are known to produce toxins that affect fish species. Bawe had significantly higher chlorophyll a concentration (0.47 ± 0.07 mg L−1) than Mnemba (0.33 ± 0.04 mg L−1) and Chumbe (0.33 ± 0.04 mg L−1; p < 0.05). Chlorophyll a concentration was spatially inversely related to distance from Unguja town (p < 0.05) while it was temporally significantly positively correlated with dissolved oxygen, nitrate and phosphate (p < 0.05). The study revealed that, the coral reef sites have low nutrient levels and are in good health. The existence of toxic phytoplankton species suggests careful consumption of fisheries resources at the four coral reef sites and frequent monitoring for Harmful Algal Blooms (HABs) is required. The higher nutrients and chlorophyll a concentrations at Bawe Island compared to other sites calls for mechanisms to limit the release of domestic sewage from households and hotels to safeguard the coral reefs.

Environmental Factors Influencing Chlorophyll-a Concentration in Tri An Reservoir, Vietnam

“…Previous researches also reported that the relationship between Chl-a concentration and NO3was described. The research in coral reef areas around Unguja, Zanzibar, Tanzania, Chl-a concentration was temporally significantly correlated with NO3but it was positive [22]. Besides, a negative relationship between NO3and Chl-a was corroborated in the study in some lakes of the Upper Paraná floodplain [23].…”

“…The studies of relationship between Chl-a and temperature have been widely studied. The temperature had negative significant correlation with Chl-a concentration in coral reef areas around Unguja, Zanzibar, Tanzania [22] while the study in the Bangpakong river reported that the Chl-a content had positive correlation to water temperature [16,30]. Besides that, the study in Srinadharin reservoir, Kanchanaburi province, Thailand, Chl-a concentration determined to have a positive correlation to water temperature and dissolved oxygen [22].…”

“…The study of relationship between DO and Chl-a concentration of Bang Pa Kong Estuary was positive that decreased with depth [26]. It was temporally significantly positively correlated with DO and PO4 3in coral reef areas around Unguja, Zanzibar, Tanzania [22]. DO is one of the most important environmental factors for phytoplankton health because of its effect in a number of biogeochemical processes such as respiration and metabolism that influence their life which related to phytoplankton biomass as determined by Chl-a concentration [27].…”

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

“…However, the relationship between primary production and Chl-a concentration is not always linear (Kyewalyanga, 2002). Due to the difficulty in computing primary production, scientists often use Chl-a concentration as a proxy for primary productivity (Ezekiel, 2014;Limbu and Kyewalyanga, 2015;Moto and Kyewalyanga, 2017;Peter, 2013;Semba et al, 2016). However, Barlow et al (2011) estimated primary productivity in the coastal waters of Unguja and Pemba Islands directly from photosynthesis-irradiance combined with measured Chl-a.…”

Variación Espacial Y Temporal Del Fitoplancton Con Énfasis en Las Floraciones Algales Frente a La Playa De Pescadores Artesanales De Chorrillos / Lima / Perú

“…La composición y abundancia del fitoplancton son consideradas como bio-indicadores naturales de las variaciones en la calidad del agua debido a su sensibilidad y rápidas respuestas a los cambios de las condiciones ambientales como pH, luz, temperatura, salinidad, turbiedad y nutrientes (Ekwu & Sikoki 2006). Así, la composición de especies, abundancia relativa, distribución espacial y temporal de estas biotas acuáticas son una expresión de la salud ambiental o integridad biológica de un cuerpo de agua particular (Limbu & Kyewalyanga, 2015).…”