Date of Award

August 2024

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences

First Advisor

James T Waples

Committee Members

Gillian M Stewart, Ryan J Newton, Kanchan Maiti

Abstract

In this study, I examine the potential of using the naturally occurring radionuclide Polonium-210 (half-life = 138.376 days) to estimate the in situ consumption of particulate organic carbon (POC) by dreissenid mussels in Lake Michigan. To estimate POC uptake, I measured the ratio of POC to particle-bound 210Po in the water column above the lakebed, as well as the inventory of 210Po being stored in mussel tissue on the lakebed at three sites with depths of 40, 50 and 80 meters. Additionally, I conducted lab incubation experiments to determine the biological half-life of 210Po in mussel tissue. These measurements were combined to estimate the amount of POC that mussels are taking up in situ. Different assimilation efficiencies were used to estimate the amount of POC being consumed by mussels. The average POC:210Po ratio from water samples taken from 5 meters above the lakebed in Lake Michigan was 12 ± 5 mg/dpm at 40m, 15 ± 6 mg/dpm at 50m, and 10 ± 2 mg/dpm at 80m. The average dreissenid density at these sites was 442 ± 327 mussels/m2 at 40m, 2,126 ± 1,689 mussels/m2 at 50m, and 6,079 ± 1,856 mussels/m2 at 80m and the average dreissenid biomass was 9 ± 6 g DW m-2 at 40 meters, 18 ± 14 g DW m-2 at 50 meters, and 62 ± 16 g DW m-2 at 80 meters. To calculate the inventory of 210Po being stored in mussel tissue per meter squared of the lakebed, the activity of 210Po in tissue was measured for four different size classes of mussels. The four size classes were 3-7mm, 8-17mm, 18-25mm, and 25+mm. The average – combined, 3-station – concentration of 210Po in tissue for each size class was 0.010 ± 0.003 dpm/mg, 0.012 ± 0.003 dpm/mg, 0.011 ±0.002 dpm/mg, and 0.012 ± 0.004 dpm/mg, respectively. These values were applied to biomass measurements to estimate the amount of 210Po being stored in tissue on the lakebed, which was 125 ± 89 dpm/m2 at 40m, 246 ± 212 dpm/m2 at 50m, and 557 ± 150 dpm/m2 at 80m. Three incubation experiments were conducted to determine the biological half-life of 210Po in mussel tissue. A temperature incubation was conducted to observe the change in 210Po in mussel tissue over time, at both 20˚C and 4˚C. The results from this incubation showed that temperature does not have a significant effect on the biological half-life of 210Po in dreissenid tissue. A feeding incubation was conducted to determine if mussels would lose 210Po from their tissue if being fed an algae culture that was free of 210Po. Results from this incubation determined that the biological decay constant of 210Po in mussel tissue was 0.0459 ± 0.001 d-1, with a corresponding half-life of 15.1 days. Multiplying the biological half-life by the amount of 210Po being stored in mussel tissue per meter squared if the lakebed gave a minimum amount of 210Po that mussels must be consuming each day, assuming 100% assimilation efficiency. For 40m depth the average amount of particulate 210Po that would need to be delivered to maintain 210Po inventories in mussel tissues was 6 ± 4 dpm m-2d-1. For 50m this minimum value was 11 ± 10 dpm m-2 d-1 and for 80m depth this value was 26 ± 7 dpm m-2d-1. Multiplying these values by the ratio of POC:210Po in the water column gives a minimum amount of POC that mussels must be consuming, which was 72 ± 57, 165 ± 164, and 260 ± 87 mg C m-2 d-1 at the 40m, 50, and 80m stations, respectively. Assuming an organic carbon assimilation efficiency of 81%, I estimate POC by dreissenid mussels to be 89 ± 70 mg C m-2 d-1 at 40m, 204 ± 202 mg C m-2 d-1 at 50m, and 321 ± 108 mg C m-2 d-1 at 80m.

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