Modelling the burial of organic carbon in lakes within the ISIMIP framework

Burial of organic carbon in lake sediments is a key process in the global carbon cycle, acting as a permanent sink for both for aquatic and terrestrial carbon. Warming can enhance carbon burial in lakes by promoting prolonged stratification and anoxic conditions. Human activities, particularly land-use changes, have also been shown to significantly influence organic carbon burial rates. Here, we developed a process-based model of particulate organic carbon (POC) burial in lake sediments under the framework of The Inter-Sectoral Impact Model Intercomparison Project (ISIMIP, https://www.isimip.org/), which allows global-scale hind- and forecasting. The model was validated using the time series data of Lake Erken (Sweden). The model includes lake temperature, stratification and mixing dynamics derived from a one-dimensional hydrodynamic lake model. Additionally, it includes oxygen dynamics, accounting for oxygen consumption in both the water-column and sediment. Both aquatic and terrestrial contributions of POC were considered, with inputs from primary production and loading from the catchment. These inputs have three possible fates: export via hydrologic pathways, permanent burial in lake sediments and mineralization to inorganic carbon. Estimates of internal production are from a two-layer process-based model, where phytoplankton growth in the epilimnion was primarily limited by light and nutrients, with temperature influencing these factors. Loss mechanisms include respiration, sinking and loss from the outflow, as well as entrainment from the hypolimnion. Loading from the catchment of sediments, POC and nutrients was formulated as a function of river discharge, land cover and land use. We also developed a new scaling approach to calculate lake-specific river inflows from the ISIMIP gridded hydrological data. The model components (hydrodynamics, oxygen, phytoplankton, river loading) performed well when validated against data from Lake Erken, giving confidence in the modelled carbon burial rate and opening the door to spatially and temporally resolved estimation of carbon burial in global lakes.