Bismuth-210: A tracer of dynamic scavenging in aquatic systems.

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Chemical cycling in any aquatic system is strongly affected by the transport of particles through that system, and a better understanding of particle dynamics is particularly needed in lakes and shallow marine systems, which play a disproportionately large, yet poorly quantified role in global biogeochemical cycling. Our current understanding of particle flux and attenuation down through the water column has largely relied on the use of sediment traps and naturally occurring particle-reactive radionuclides. However, particle dynamics are chronically understudied in shallow aquatic systems largely because of methodological limitations. The downward transport of particulate matter in shallow waters cannot be reliably quantified with a bottom tethered sediment trap and the choice of radionuclide tracers in high flux and freshwater systems is severely limited. The primary goal of this proposed work is to demonstrate that combined measurements of the natural radionuclide lead-210 (210Pb) and its daughter bismuth-210 (210Bi) can be used to determine particulate flux in a shallow and dynamic aquatic system. A thorough demonstration and testing of the 210Bi/210Pb tracer is imperative before any general application of the tracer is made to study process rates. The project will support two undergraduate students as field and laboratory assistants during summer months and one MS graduate student for the duration of this project. Sample collection and field work will take place in Lake Michigan and will be incorporated into a graduate field course.The goals of this proposed work are to: (i) demonstrate the practicality of a simple technique for measuring the 210Pb-210Bi-210Po radionuclide trio in aquatic systems; (ii) show that 210Bi/210Pb derived particle fluxes are concordant with 234Th/238U derived particle fluxes in a dynamic and “calibrated” nearshore system; and (iii) show that the in situ Kd of 210Bi is greater than that of 210Pb under varying conditions of particle concentration and organic matter content, across a gradient of dissolved oxygen concentrations. Nearshore scavenging processes are chronically understudied due to methodological limitations. Developing a relatively easy to measure and widely occurring particle tracer based on 210Bi/210Pb disequilibria will allow for future process studies of dynamic particle cycling that are critical to understanding system-wide chemistry as well as practical problems associated with the management of trophic status, hypoxia, harmful algal blooms, and contaminant fate and transport.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.