Model for trace metal exposure in filter-feeding flamingos at Alkaline Rift Valley Lake, Kenya

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Environmental Toxicology and Chemistry, ISSN: 0730-7268, Vol: 17, Issue: 11, Page: 2302-2309

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Nelson, Yarrow M.; Thampy, Ramesh J.; Motelin, Gideon K.; Raini, Jackson A.; DiSante, Cynthia J.; Lion, Leonard W.
Civil and Environmental Engineering
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Abstract—Toxic trace metals have been implicated as a potential cause of recent flamingo kills at Lake Nakuru, Kenya. Chromium (Cr), copper (Cu), lead (Pb), and zinc (Zn) have accumulated in the lake sediments as a result of unregulated discharges and because this alkaline lake has no natural outlet. Lesser flamingos (Phoeniconaias minor) at Lake Nakuru feed predominantly on the cyanobacterium Spirulina platensis, and because of their filter-feeding mechanism, they are susceptible to exposure to particle-bound metals. Trace metal adsorption isotherms to lake sediments and S. platensis were obtained under simulated lake conditions, and a mathematical model was developed to predict metal exposure via filter feeding based on predicted trace metal phase distributions. Metal adsorption to suspended solids followed the trend Pb >> Zn > Cr > Cu, and isotherms were linear up to 60 pg/L. Adsorption to S. platensis cells followed the trend Pb >> Zn > Cu > Cr and fit Langmuir isotherms for Cr, Cu and Zn and a linear isotherm for Pb. Predicted phase distributions indicated that Cr and Pb in Lake Nakuru are predominantly associated with suspended solids, whereas Cu and Zn are distributed more evenly between the dissolved phase and particulate phases of both S. platensis and suspended solids. Based on established flamingo feeding rates and particle size selection, predicted Cr and Pb exposure occurs predominantly through ingestion of suspended solids, whereas Cu and Zn exposure occurs through ingestion of both suspended solids and S. platensis. For the lake conditions at the time of sampling (1.2 g/L suspended solids, 0.23 g/L S. platensis), predicted ingestion rates based on measured metal concentrations in lake suspended solids were 0.71, 6.2, 0.81, and 13 mg/kg-d for Cr, Cu, Pb, and Zn, respectively. Higher exposure doses are predicted when metal concentrations are determined from sediment concentrations rather than suspended solids concentrations. Also, decreases in the S. platensis population would increase the clearing rate of the flamingos and increase predicted metal exposure via ingestion of suspended solids. For example, with metal concentrations calculated based on average metal concentrations in lake sediments and S. platensis concentration of 0.06 g/L, exposure rates would be 13, 10, 4.4, and 38 mg/kg-d for Cr, Cu, Pb, and Zn, respectively. These ingestion rates, except for Cu, are significantly higher than the no observable adverse effects levels.