Carbon fixation by the phytoplankton community across Lake Winnipeg

Abstract

Lakes are important sites for carbon fixation and carbon dioxide (CO2) exchange with the atmosphere. Carbon fixation rates have not previously been published for Lake Winnipeg but are important for quantifying the lake’s role in the regional greenhouse gas budget and the lake’s trophic structure and fish habitat. This study measured net ecosystem production (NEP), gross primary production (GPP), and gross respiration (GR) across the lake using a custom-built automated incubator connected to a ship’s water intake during a research cruise between July 31 and August 17, 2018 on Lake Winnipeg. The incubator estimated NEP, GR, and GPP every 60 min while moving along the ship’s track and at anchor, providing high-resolution data that are not obtainable through conventional incubations. The mean NEP for Lake Winnipeg during our survey was −8.4 ± 5.6 g C m−2 d−1, suggesting that the lake was net heterotrophic and thus a net CO2 source to the atmosphere during the 2018 summer cruise. The high-resolution data revealed significant spatiotemporal variability, including short-lived, highly net productive events that preceded remotely sensed chlorophyll a blooms by several days. Conversely, in regions with high chlorophyll a concentrations, we observed strong net heterotrophy and low nutrients, suggesting respiration was fueled by the degradation of mature, nutrient-limited phytoplankton blooms. The incubator system used in this study demonstrated its utility for monitoring rapid changes in NEP over short spatial scales in a lake which shows heightened regional variability in its physical, biogeochemical, and biological make-up.