Extreme ¹³C enrichments in a shallow hypereutrophic lake: Implications for carbon cycling
Gu, Binhe, Claire L. Schelske, and David A. Hodell
Limnol. Oceanogr., 49(4), 2004, 1152–1159

An analysis of stable carbon isotope (¹³C) 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 average ¹³C 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 ¹³C in the water-column DIC pool ranged from 5‰ to 13‰ with an average of 9.0‰. The flux-weighted ¹³C from the DIC due to external loading and sediment respiration was estimated as -12‰, whereas the ¹³C 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.