Emmanuel Mgimwa
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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.
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Sampling design and data collection methods This study was conducted from March to December 2019. Each site had one sampling location (1 km2) where birds were counted and water samples for physical-chemical variables and phytoplankton abundance collected once a month. The area selected for sampling was that being used by a large number of lesser flamingos at the time of the start of this study showing their importance in supporting food nutrients for each site. Lesser flamingo abundance Lesser flamingo abundance was determined using the ground survey method following Bibby et al. (2000) and Kihwele et al. (2014). To avoid disturbance and to get accurate data, lesser flamingo abundance was estimated prior to taking water samples and recording GPS coordinates. The 1 km2 plot was divided into two halves with birds in each subplot estimated by one person. The two counters distributed themselves in counting birds in each subplot synchronously but independently using Vistron binoculars (10 x 42). Large groups of birds were estimated in subgroups of 100 or 1,000. Lesser flamingo abundances for a particular month and site was determined as the average value from the two independent counters. Because the lesser and greater flamingo intermingles during feeding and that large group of birds were counted in groups of 100 or 1,000 birds, chances of counting greater flamingo together with lesser flamingo were high. To avoid this, counters scanned each subplot using binoculars and telescopes before commencing the counting process. Lesser flamingo abundance was estimated by counting all flamingos in the subplot and deducting the percentage estimate of the greater flamingos (if any) after each count. Chlorophyll-a abundance and physical-chemical variables Plastic bottles of half a litre (500 ml) were used to collect water samples for Chlorophyll-a abundance at the selected sampling sites in the three lagoons. Water samples were collected in the mornings between 7 am to 10 am at the centre of the plot. As water level was changing, different plots were made every month. The plastic bottles were lowered to the lake’s bottom (at the depth of 20-50 cm) to capture phytoplankton in the water column as per Kihwele et al., (2014). The collected water samples were transported to GongaliModel Water Quality Laboratory in Arusha, Tanzania within 48 hours and refrigerated at 6oC before laboratory analysis. In the laboratory, Chlorophyll-a abundance was determined using Aquafluor Handheld Fluorometer as In Vivo Chlorophyll-a; water turbidity was measured using Hach 2100Q Aquameter. Dissolved oxygen (DO) and pH were determined using YSI Pro1020 meter. Nitrate (NO3-), Phosphate (PO43-) and Ammonium (NH4+) were measured from water samples using DR3900 Multiparameter produced by Hach.