The metabolic balance of Lake Superior as measured from autonomous underwater vehicles

The metabolic balance of aquatic ecosystems determines whether these environments are a net source or sink of CO2 relative to the atmosphere and is an important component of the global carbon cycle. However, the metabolism of many large lake ecosystems remains unsubstantiated given the challenges of estimating gross primary production and ecosystem respiration over vast areas. Autonomous underwater vehicles (AUV; a.k.a. gliders) commonly used in marine environments may provide an opportunity to leverage advancements in technology to better understand the ecology of large lake ecosystems. Through a multi-institutional collaboration, the Consortium of Great Lakes Gliders has directed both research and development, and observational AUV missions in the Great Lakes for over a decade. Here, we took advantage of archived high resolution limnological data collected over large spatial (i.e., horizontal, and vertical) and temporal scales to refine estimates of ecosystem metabolism in Lake Superior, the world's largest freshwater lake by surface area. Flight paths differed in location and time of year but were biased to the southwestern portion of the lake and the summer season. Yet, several of the areas surveyed were expected to differ in ecosystem metabolism due to differences in nearshore proximity, depth, and prevailing nutrient concentration. Continuous limnological data (i.e., dissolved oxygen and temperature) was used to estimate daily rates of epilimnetic gross primary production and respiration from each glider mission. Spatial and temporal patterns in rates of metabolism and other limnological variables (i.e., chlorophyll a) were then summarized to describe the contemporary metabolic balance of Lake Superior and associated drivers of heterogeneity. Lake Superior is, for the most part, net autotrophic, but does exhibit variability particularly in the more productive and shallow areas of the lake.