Limbu S M, Kyewalyanga M S
Department of Aquatic Sciences and Fisheries, University of Dar es Salaam, P.O. Box 35064, Dar es Salaam, Tanzania ; Department of Biology, School of Life Sciences, Laboratory of Aquaculture Nutrition and Environmental Health, East China Normal University, 500 Dong Chuan Road, Shanghai, 200241 China.
Institute of Marine Sciences, University of Dar es Salaam, Mizingani Road, P.O. Box 668, Zanzibar, Tanzania.
Springerplus. 2015 Oct 28;4:646. doi: 10.1186/s40064-015-1439-z. eCollection 2015.
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.
浮游植物对水质参数变化敏感，因此可间接指示珊瑚礁的健康状况。本研究探讨了奔巴岛的巴韦、姆恩巴、楚姆贝和蓬韦珊瑚礁站点水质和养分的时空变化，及其与浮游植物群落组成和叶绿素a浓度的关系。连续1年每月进行溶解氧、温度、盐度和pH值的现场测量。采集地表水样本以测定浮游植物组成、养分和叶绿素a浓度。四个站点的溶解氧、温度、盐度和pH值无显著差异（p>0.05），但在各月份间存在显著的时间变化（p<0.05）。巴韦的磷酸盐浓度（1.45±0.57µgL⁻¹）显著高于楚姆贝（0.74±0.53µgL⁻¹）、姆恩巴（0.42±0.30µgL⁻¹）和蓬韦（0.28±0.10µgL⁻¹；p<0.05）。同样，巴韦的硝酸盐浓度（0.81±0.43µgL⁻¹）显著高于姆恩巴（0.33±0.14µgL⁻¹）和蓬韦（0.24±0.13µgL⁻¹；p<0.05），但与楚姆贝（0.90±0.35µgL⁻¹；p>0.05）相似。然而，所有研究站点的磷酸盐和硝酸盐含量分别低于富营养化海洋的3和14mgL⁻¹。浮游植物种类以硅藻门为主（70.83%），已鉴定出的一些物种，如角藻属、鳍藻属、原甲藻属、原多甲藻属、颤藻属和双尾藻属，已知会产生影响鱼类的毒素。巴韦的叶绿素a浓度（0.47±0.07mgL⁻¹）显著高于姆恩巴（0.33±0.04mgL⁻¹）和楚姆贝（0.33±0.04mgL⁻¹；p<0.05）。叶绿素a浓度在空间上与距奔巴镇的距离呈负相关（p<0.05），而在时间上与溶解氧、硝酸盐和磷酸盐呈显著正相关（p<0.05）。研究表明，珊瑚礁站点的养分水平较低，健康状况良好。有毒浮游植物物种的存在表明，在这四个珊瑚礁站点捕捞渔业资源时需谨慎，并需要对有害藻华进行频繁监测。与其他站点相比，巴韦岛较高的养分和叶绿素a浓度要求采取措施限制家庭和酒店生活污水的排放，以保护珊瑚礁。
