https://scholars.wlu.ca/etd/2338

Henderson, Rachel (2020). Investigating diel, seasonal, and interannual variability of dissolved and particulate stable carbon isotope values in a eutrophic boreal lake https://scholars.wlu.ca/etd/2338

Lakes are globally significant sources, sinks, and conduits of carbon. Cultural eutrophication of freshwater promotes the growth of phytoplankton blooms, which can transform, respire, and sequester large amounts of carbon. Analysis of stable carbon isotopes is a common tool to study the movement of carbon through the aquatic carbon cycle. The causes of temporal variability in δ13C values at different timescales are not well understood, and differ between lakes. In this study, I investigated diel, seasonal, and interannual variability in δ13C values of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) in Lake 227, an artificially eutrophic lake at the IISD Experimental Lakes Area in northwestern Ontario, Canada. Weekly phosphorus additions to Lake 227 promote the growth of annual phytoplankton blooms. I interpreted hourly overnight change in δ13C-DIC values to determine the relative contributions of atmospheric gas exchange and ecosystem respiration (ER) to the DIC pool on a diel timescale. I also interpreted the drivers of temporal variability in δ13C-DIC and δ13C-POC values on weekly to seasonal timescales from regular sampling events during the ice-free season since 2010.

Collecting δ13C-DIC samples overnight is labour-intensive and this study is one of few that reports overnight δ13C-DIC values for a lake. I collected hourly samples between sunset and sunrise on three occasions: during an epilimnion phytoplankton bloom dominated by cyanobacteria, a bloom composed of cyanobacteria and chlorophytes, and a period of biomass decline between the two blooms. 1 m samples represented conditions in the well-mixed epilimnion, and 3 m samples represented a depth of highly-concentrated biomass in the metalimnion. Overnight change in δ13C-DIC values varied between a 3 ‰ increase and 18 ‰ decrease. The values of δ13C-DIC at 1 m were closer to the value of δ13C-DIC produced by ER during the blooms, but closer to the δ13C-DIC value in equilibrium with the atmosphere during the lower-biomass period. Gas exchange and lake metabolism calculations confirmed that ER was the dominant source of DIC to the epilimnion during the blooms. The rate of ER controls the overnight increase in [DIC] and decline in pH. Daytime pH tended to reach 9.5-9.7 during the blooms, which allowed for a low rate of chemically enhanced diffusion (CED). Accounting for CED, the rate of gas exchange was lower or equal to the rate of ER during the ice-free season. The 3 m sampling depth acted as a closed system as lake metabolism drove changes in DIC concentration. Understanding diel variability in DIC concentration and δ13C-DIC is important for modeling values of photosynthetic fractionation, and interpreting seasonal δ13C-DIC and δ13C-POC values.

Changes in weekly mid-morning δ13C-DIC and δ13C-POC values were driven by changes in the concentration of phytoplankton biomass. Phytoplankton blooms in Lake 227 lower the DIC concentration below equilibrium with the atmosphere. Lower DIC concentrations occurred in conjunction with high δ13C-POC values and low δ13C-DIC values. Since sampling began in 2010, when the epilimnion concentration of chlorophyll a was greater than 30 μg L-1 and the DIC concentration was less than 50 μg L-1, the range in δ13C‑POC values was narrow: between -26 ‰ and -23 ‰. Outside of these thresholds, observed values of δ13C-POC were as low as -36 ‰. Similar relationships did not exist at 3 m, as the phytoplankton peak shifted vertically in the water column over each ice-free season. The lowest daytime δ13C‑DIC values at 1 m corresponded with the lowest DIC concentrations, and the overnight δ13C-DIC values were also lowest when the DIC concentration was low during the phytoplankton blooms. DIC fixation caused pH to increase during the phytoplankton blooms, reducing the proportion of DIC that was dissolved CO2, and decreasing the amount of photosynthetic fractionation. These trends repeated across years, and should lead to a high, consistent δ13C value of the lake sediments.

Lake 227 is an ideal setting to investigate the drivers of temporal variability in δ13C-DIC and δ13C-POC in a eutrophic lake. The concentration of biomass and phytoplankton species composition tends to repeat each year, and there is a long history of comprehensive physical, geochemical, and biological monitoring. The data I present in this thesis provide new insight into how changes in phytoplankton concentration and carbonate geochemistry drive temporal variability in δ13C values on diel, seasonal, and interannual timescales.

Masters, Wilfrid Laurier University, FORMBLOOM

https://scholars.wlu.ca/etd/2338