Carbon budget of a mid-latitude, groundwater-controlled lake: Isotopic evidence for the importance of dissolved inorganic carbon recycling

Abstract

The carbon cycle of the groundwater-controlled Lake Gościaż, Poland, was investigated. The study covered a period of intensive primary production from April to November 1993. Physiochemical and isotope characteristics of the lake system (depth profiles of temperature, pH, alkalinity, concentrations of Ca 2+, dissolved inorganic carbon (DIC), and δ 13C of DIC) monitored on a monthly basis. Carbon isotope compositions of calcite samples collected in the sediment traps were determined. The carbon cycle of the lake was simulated using a numerical model which accounts for all basic processes controlling concentration and carbon isotope composition of DIC. The model allowed researchers to quantify the relative importance of sources and sinks of carbon influencing the δ 13C DIC in the epilimnion and, consequently, of the authigenic calcite deposited in the sediments. Inorganic carbon dissolved in groundwaters feeding the lake appeared to be insufficient to maintain the concentration and δ 13C of DIC in the epilimnion in the observed range. DIC originating from decomposition of organic matter is a crucial element of the carbon budget of the lake. For the entire studied period, it accounted for about two-thirds of the total DIC supplied to the epilimnion; its average δ 13C was −25.5%c. Decomposition of organic matter within the lake appeared to be at least one-third higher than its production. Of the external sources of carbon, groundwater inflow contributes 50%, terrestial biomass, 30%, and atmospheric CO 2, 20% of the total carbon input to the lake. Evasion of CO 2 and CH 4 drives the average δ 13C DIC in the epilimnion to the values which are considerably higher than δ 13C of the external sources of carbon.