MODEL FOR TRACE-METAL EXPOSURE IN FILTER-FEEDING FLAMINGOS AT ALKALINE RIFT-VALLEY LAKE, KENYA

Toxic trace metals have been implicated as a potential cause of recent flamingo kills at Lake Nakuru, Kenya. Chromium (Cr), copper (Cu), lea d (Pb), and zinc (Zn) have accumulated in the lake sediments as a resu lt of unregulated discharges and because this alkaline lake has no nat ural outlet. Lesser flamingos (Phoeniconaias minor) at Lake Nakuru fee d predominantly on the cyanobacterium Spirulina platensis, and because of their filter-feeding mechanism, they are susceptible to exposure t o particle bound metals. Trace metal adsorption isotherms to lake sedi ments and S. platensis were obtained tinder simulated lake conditions, and a mathematical model was developed to predict metal exposure via filter feeding based on predicted trace metal phase distributions. Met al adsorption to suspended solids followed the trend Pb >> Zn > Cr > C u, and isotherms were linear up to 60 mu g/L. Adsorption to S. platens is cells followed the trend Pb >> Zn > Cu > Cr and fit Langmuir isothe rms for Cr, Cu and Zn and a linear isotherm for Pb. Predicted phase di stributions indicated that Cr and Pb in Lake Nakuru are predominantly associated with suspended solids, whereas Cu and Zn are distributed mo re evenly between the dissolved phase and particulate phases of both S . platensis and suspended solids. Based on established flamingo feedin g rates and particle size selection, predicted Cr and Pb exposure occu rs 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 su spended solids, 0.23 g/L S. platensis), predicted ingestion rates base d 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. Highe r exposure doses are predicted when metal concentrations are determine d from sediment concentrations rather than suspended solids concentrat ions. Also, decreases in the S. platensis population would increase th e clearing rate of the flamingos and increase predicted metal exposure via ingestion of suspended solids. For example, with metal concentrat ions calculated based on average metal concentrations in lake sediment s and S. platensis concentration of 0.06 gn, exposure rates would be 1 3, 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.