Emmanuel Mgimwa

Introduction The influence of physical-chemical variables on phytoplankton and lesser flamingo (Phoeniconaias minor) abundances have been studied in the East African Rift Valley Soda lakes such as Bogoria (Ballot et al., 2004, Schagerl & Oduor 2008, Tebbs et al., 2013, & Robinson, 2015), Nakuru (Ballot et al., 2004, Schagerl & Oduor 2008, Robinson 2015, & Krienitz et al., 2016), Elementaita (Ballot et al., 2004, Schagerl & Oduor 2008, Robinson, 2015), Oloidien (Robinson, 2015) and Manyara (Kihwele et al., 2014). However, there are no studies on the influence of physical-chemical variables on phytoplankton and lesser flamingo abundances at lake Natron despite being the only important and regular breeding area for the East African lesser flamingo populations (Baker & Baker 2002, & Wildlife Division, 2010). Although migration and erratic movements of lesser flamingo has been documented (Baker 1997, & Baker & Baker, 2002), a sizeable number of flamingos has been found to stay at lake Natron throughout the year. The migrating lesser flamingos spend time at lake Natron before and after the breeding season (October - February but can breed throughout the year if conditions are favourable) making it important to understand the productivity levels of the phytoplankton and the underlying physical-chemical variables. This study investigated the relationship of the lesser flamingo and phytoplankton abundances and how the physical-chemical variables (dissolved oxygen, water turbidity, pH, nitrate (NO3-), ammonium (‎NH4+) and phosphate (PO43-)) affect Chlorophyll-a abundance at lake Natron. The study used Chlorophyll-a as a measure of phytoplankton and selected physical-chemical variables with the greatest influence on the phytoplankton's primary productivity. Conclusion Findings from this study suggest that annual fluctuation in the lesser flamingos numbers in lake Natron is triggered by phytoplankton abundance whose primary productivity is influenced by physical-chemical variables in the lake waters. In turn, these changes are mainly due to both anthropogenic activities and weather changes within the lake basin and its surroundings. The observed trend in physical-chemical variables at the three study lagoons is a result of the spatial distribution of freshwater sources from Pinyinyi, Engaresero and Ewaso Ng’iro rivers and the dilution effect from rainfall and evaporation. The importance of the hydrological balance for the productivity of lake Natron is of paramount for the survival of the lesser flamingos. Being a closed (no river flowing from this lake) and transboundary ecosystem, any hydrological changes will impact the limnology and, in turn affect phytoplankton productivity.引言 针对东非大裂谷的苏打湖，包括博戈里亚湖（Bogoria）、纳库鲁湖（Nakuru）、埃门泰塔湖（Elementaita）、奥洛迪恩湖（Oloidien）与曼雅拉湖（Manyara），学界已开展多项研究，探讨理化变量对浮游植物（phytoplankton）以及小红鹳（lesser flamingo，*Phoeniconaias minor*）种群丰度的影响（相关文献涵盖Ballot等，2004；Schagerl & Oduor，2008；Tebbs等，2013；Robinson，2015；Krienitz等，2016；Kihwele等，2014）。然而，尽管纳特龙湖（Lake Natron）是东非小红鹳种群唯一重要且固定的繁殖栖息地（Baker & Baker，2002；Wildlife Division，2010），目前尚无研究探讨该湖内理化变量对浮游植物与小红鹳种群丰度的影响。尽管小红鹳的迁徙与偶发移动已有文献记录（Baker，1997；Baker & Baker，2002），但观测发现全年均有相当规模的小红鹳栖息于纳特龙湖。迁徙中的小红鹳会在繁殖季前后（10月至次年2月，若环境条件适宜则可全年繁殖）停留于该湖，因此探明该湖浮游植物的生产力水平及其背后的理化驱动机制，具有重要的研究价值。本研究旨在探究纳特龙湖内小红鹳与浮游植物种群丰度的关联，以及各类理化变量——包括溶解氧、水体浊度、pH值、硝酸盐（NO₃⁻）、铵盐（NH₄⁺）与磷酸盐（PO₄³⁻）——对叶绿素a（Chlorophyll-a）丰度的影响。本研究以叶绿素a作为浮游植物生物量的衡量指标，并选取对浮游植物初级生产力影响最为显著的理化变量开展分析。结论 本研究结果显示，纳特龙湖小红鹳种群数量的年度波动由浮游植物丰度驱动，而浮游植物的初级生产力受湖内水体理化变量的调控。此类变化主要源于湖盆及其周边区域的人类活动与气象波动。在三个研究泻湖观测到的理化变量变化趋势，源自皮尼因伊河（Pinyinyi）、恩加雷塞罗河（Engaresero）与埃瓦索恩吉罗河（Ewaso Ng’iro）的淡水水源空间分布，以及降雨与蒸发产生的稀释效应。水文平衡对于纳特龙湖生产力的重要性，对小红鹳的生存而言至关重要。作为一处封闭型（无河流流出该湖）的跨界生态系统，任何水文变化都将影响其湖沼学特征，进而作用于浮游植物的生产力。