Date of Award

August 2024

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences

First Advisor

James T Waples

Committee Members

Kanchan Maiti, Erin E Black, Ryan J Newton

Abstract

210Bi is a short-lived (t1/2: 5.01 days) naturally occurring radionuclide and a potential tracer of particle dynamics in high flux aquatic environments such as lakes, coastal oceans, and continental shelves. While its parent, 210Pb, and daughter, 210Po, have been used to estimate particle removal rates from the ocean, few studies have investigated the behavior of 210Bi in aquatic systems or its use as a tracer of particle export fluxes. In this study, a time series experiment was conducted where 210Pb, 210Bi, and 210Po activities were measured in rainwater and in the water column on the slope of southwestern Lake Michigan. Particulate matter was filtered from the lake water samples to separate radionuclide activities into operationally defined particle-bound and dissolved (<0.45 μm) activity fractions. The geometric mean of particle distribution coefficients (Kd) for 210Pb, 210Bi and 210Po were 1.48 × 106, 1.85 × 106, and 3.56 × 106 L kg-1, respectively. The ratio of the distribution coefficient means – or the fractionation factor (F) – are 1.3, 1.9, and 1.4 for F-Bi/Pb, FPo/Pb, and FPo/Bi, respectively. 210Po was never in equilibrium with 210Pb, and substantial disequilibrium between 210Bi and its parent 210Pb was observed on 18 September. Non-steady-state (NSS) fluxes of radionuclide export from the water column at 75 m were estimated for the period spanning 13 to 22 September 2023. One-dimensional (1-D) conditions were assumed, and vertical convection accompanied by bottom scavenging were assumed to be the dominant mechanism of removal. Using 210Bi/210Pb and 210Po/210Pb disequilibria, radionuclide removal flux estimates were 308 dpm 210Bi m-2 d-1 and 115 dpm 210Po m-2 d-1. Particulate organic carbon (POC) export fluxes were estimated as the product of radionuclide fluxes and the ratio of POC to daughter nuclide total activity (POC/A_D^total). 210Bi- and 210Po-derived POC export flux estimates from 13 to 22 September were 404 ± 92 and 423 ± 43 mg C m-2 d-1, respectively. Three key results from this work support the use of 210Bi as a tracer of high and variable particle fluxes in lakes and shallow marine environments: 1) 210Bi displayed high particle affinity as was evidenced through high Kd values and a FBi/Pb value >1. 2) 210Bi was observed to be in disequilibrium with its parent 210Pb. 3) 210Bi-derived POC export flux estimates were in agreement with concurrent 210Po-derived flux estimates.

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