The Metabolic Balance of Lake Superior's Mixed Layer

Understanding carbon (C) dynamics in large lake ecosystems poses challenges due to their size, causing uncertainty in their contribution to the global C cycle. Here, we used autonomous underwater vehicles to calculate upper mixed-column (UMC) metabolism in Lake Superior. We paired 7 years of high-resolution dissolved oxygen and temperature data from 17 glider missions with in situ meteorological data to calculate daily gross primary production, ecosystem respiration, and net ecosystem metabolism. We found the UMC of Lake Superior to be net heterotrophic (primary production < ecosystem respiration) most of the time, suggesting its role as a net source of CO₂ to the atmosphere. Heterotrophy was pronounced during cold and transitional days (<14°C) and in nearshore regions (<10 km distance from the shore). When scaled, our results indicate that Lake Superior released 84–112 Tg CO₂ y⁻¹, which is twice the rate of C it fixes from the UMC itself.