Modeling CO2 emissions from boreal lakes in present and future

Abstract

In this work, carbon dioxide (CO2) dynamics in boreal lakes were studied using vertical process-based coupled physical-biogeochemical lake models. Many lakes in the boreal zone are at present supersaturated with the greenhouse gas CO2 and emit it to the atmosphere. Transport of terrestrially produced organic carbon to boreal lakes and its in-lake degradation is predicted to increase under the ongoing climate change, and lakes may be even greater CO2 sources to the atmosphere in the future. Two process-based models of different complexity were developed for the simulation of carbon cycling in lakes. The models are based on a one-dimensional vertical lake model called MyLake, which simulates lake thermal conditions and phosphorus-phytoplankton dynamics. The applications of each model to humic boreal lakes were calibrated and validated with comprehensive measurements of water column CO2 concentration. The performance of the simpler model was found to be inadequate, partially because of the unusual behavior of the phytoplankton community in the study lake, but the output of the more advanced model agreed well with measurements. Further, the applicability of different parameterizations for the gas exchange velocity for CO2 incorporated into the more advanced model was assessed. None of the applied parameterizations resulted in an improved lake model performance regarding water column CO2 concentration or air-water CO2 flux. The use of more sophisticated and possibly more correct gas exchange models, on the contrary, led to difficulties in obtaining sufficient gain of CO2 in the water column. Impacts of warmer air temperature and climate-induced changes in stream inflow on CO2 dynamics in a boreal lake between the periods 1980–2009 and 2070–2099 were assessed using projections for air temperature from recent-generation global climate models with different radiative forcing scenarios and literature estimates for changes in seasonal stream inflow volume. Also, the effects of additional increases in the inflow concentrations of CO2 and dissolved organic carbon (DOC) were compared. The results imply that CO2 concentrations in boreal lakes will be elevated in future climate and, in consequence, the release of CO2 to the atmosphere will further increase.

Keywords: lakes, gas exchange, carbon dioxide, modelling, biogeochemistry, carbon cycling, climate change