Submerged macrophyte-dominated systems fed by karst groundwater produce a significant autochthonous carbon sink in sediment: A mesocosm experiment study

The inland water portion of the carbon cycle is an essential component of the global carbon cycle and is a promising direction to seek missing carbon sinks. Inland waters fix inorganic carbon to form autochthonous organic carbon (Auto-OC) and accept laterally transferred terrestrial OC. Calculating the carbon sink flux of inland water requires a quantitative estimation of the proportion of the aforementioned processes. In the current study, n-alkanes are used as biomarkers and the dual carbon isotope method (Bayesian mixing model) is applied to estimate the proportions of Auto-OC in the sediment of a simulation site comprising five shallow submerged macrophyte-dominated subsystems. The study results showed that a high proportion of Auto-OC was present in all sediment, regardless of the season or subsystem. However, the proportions were higher in the warm-humid season than in the cold-dry season. Results from a correlation analysis showed that temperature-controlled seasonal variations in the photosynthetic strength of aquatic organisms are the most likely cause of this difference. The average deposition rates of total organic carbon and Auto-OC were high (66.7 and 58.2 g C/m2/yr, respectively). Throughout the year, the weighted average percentage of Auto-OC ranged from 76% to 90%, with a mean value of 86% in the five aquatic subsystems. Establishing and maintaining submerged macrophyte-dominated systems have a potential of decreasing carbon dioxide (CO2) evasion and sequestrating more carbon (C) in headwaters. Working for clear submerged macrophyte-dominated lakes is beneficial for increasing carbon sinks.