Extreme13 C enrichments in a shallow hypereutrophic lake: Implications for carbon cycling

Abstract

An analysis of stable carbon isotope (δ13C) ratios in Lake Apopka, Florida, reveals extreme 13C enrichments of dissolved inorganic carbon (DIC) pools in the water column and sediment pore water of this shallow polymictic and hypereutrophic lake. The sediment pore water had high averageδ13C of DIC (26.4‰), DIC (8.37 mmol L−1), and methane (CH4) concentrations (1.23 mmol L−1). The extreme 13C enrichment in the sediment pore‐water DIC pool is attributed to methanogenesis, which produces 13C‐rich carbon dioxide (CO2) and 13C‐poor CH4 during the bacterial fermentation of organic matter. The δ13C in the water‐column DIC pool ranged from 5‰ to 13‰ with an average of 9.0‰. The flux‐weighted δ13C from the DIC due to external loading and sediment respiration was estimated as −12‰, whereas the δ13C from particulate organic carbon (POC) due to water‐column production was −13‰. The 13C enrichment in the water column is attributed directly to the diffusion and advection of isotopically heavy DIC from the sediment and to the isotopic fractionation by phytoplankton photosynthesis and is attributed indirectly to the removal of isotopically light CH4 by ebullition and organic matter by sedimentation and outflow. Atmospheric invasion and sedimentation were the most important source and sink, respectively, in the carbon mass balance. CH4 oxidation, atmospheric invasion, anaerobic respiration, and sedimentation are the important flux terms affecting the isotopic mass balance. A combination of shallow water depth, frequent wind mixing, anoxic sediments with high rates of methanogenesis, high phytoplankton productivity, and lack of external loading dominated by terrestrial carbon led to the 13C enrichment of the water‐column DIC pool in Lake Apopka.