Air-water CO2 Fluxes In Seasonal Hypoxia-influenced Green Bay, Lake Michigan

Increasing anthropogenic nutrient enrichment has led to seasonal hypoxia in Green Bay, the largest freshwater estuary in the Laurentian Great Lakes, but change in carbon dynamics associated with the development of hypoxia remains poorly understood. Variations in alkalinity, abundance of carbon species, and air-water CO₂ fluxes were quantified under contrasting hypoxic conditions during summer 2014. Green Bay was characterized with high pH (average 8.62 ± 0.16 in August), high DIC concentrations (2113 - 3213 µmol/kg) and high pCO₂ in the water column. pCO₂ was mostly >700 µatm in June, resulting in a net CO₂ source to the air, while pCO₂ was mostly <650 µatm in August when hypoxic conditions occurred in Green Bay. In central Green Bay, pCO₂ was the highest during both sampling months, accompanying by low dissolved oxygen (DO) and lower pH in the water column. In August, pCO₂ was inversely correlated with DOC concentration and increased with DOC/DOP ratio, suggesting a control by organic matter on air-water CO₂ dynamics and consumption of DO in Green Bay. Positive CO₂ fluxes to the atmosphere during August were only observed in northern bay but a CO₂ sink was found in southern Green Bay (~40% of study area) with high biological production and terrestrial DOM. Daily CO₂ flux ranged from 10.9 to 48.5 mmol-C m^-2 d^-1 in June with an average of 18.29 ± 7.44 mmol-C m^-2 d^-1, whereas it varied from 1.82 ± 1.18 mmol m^-2 d^-1 in the north to -2.05 ± 1.89 mmol m^-2 d^-1 in the south of Green Bay in August. Even though strong biological production reduced the CO₂ emission, daily CO₂ fluxes from Green Bay to the air were as high as 7.4 × 10⁷ mole-C in June and 4.6 × 10⁶ mole-C in August, suggesting a significant role of high-DIC lakes in global CO₂ budget and cycling.