Remote sensing derived trends (2003-2018) in carbon fixation for the world’s eleven largest lakes

Abstract:

Freshwater lakes are an important component of the global carbon cycle but they have not always received the attention they deserve, largely due to the small fraction of the earth’s surface area covered, the large and diverse number of freshwater lakes, and the complex carbon cycle of individual lakes. Recent work suggests that the carbon cycle of individual lakes can vary on significant temporal and spatial scales depending on thermal stratification, allochthonous loading, trophic state, and degree of anthropogenic influence. One of the principal inputs in any lake’s carbon budget is the rate of carbon fixation, yet we do not know this rate for most lakes of the world. The traditional methods for estimating carbon fixation, C-14 uptake or oxygen evolution, require time consuming bottle incubations limiting observations to lakes in more accessible and developed regions. Recent studies have used remote sensing to determine chlorophyll concentrations in the world’s lakes and provide detailed estimates of lake-wide carbon fixation. The eleven largest lakes on the planet contain over 50% of the world’s surface freshwater by volume and area. As part of a NASA Carbon Monitoring System (CMS) funded project, we used satellite imagery to estimate annual carbon fixation for these lakes from 2003-2018 allowing us to examine trends among a subset of important lakes. Several lakes exhibited significant changes in carbon fixation due to changing climate. For example, North America’s Great Slave and Great Bear lakes both experienced increases in carbon fixation as a result of increasing water temperature and solar irradiance, while fixation in Lake Tanganyika has decreased due to oligotrophication as a result of increased wind speed. And while the other large lakes showed no significant changes in carbon fixation, all experienced unique climate change forcing that resulted in offsetting effects in phytoplankton production. These new time-series products can be used to provide a better understanding of how anthropogenic forcing and climate change affect carbon fixation of the freshwater lakes in the various ecological regions throughout the globe.