A stable carbon isotope study of dissolved inorganic carbon cycling in a softwater lake

The dissolved inorganic carbon (DIC) cycle in a softwater lake was studied using natural variations of the stable isotopes of carbon,¹²C and¹³C. During summer stratification there was a progressive decrease in epilimnion DIC concentration with a concomitant increase in δ¹³C_DIC), due to preferential uptake of¹²C by phytoplankton and a change in the dominant CO₂ source from inflow and in situ oxidation to invasion from the atmosphere. There was an increase in hypolimnion DIC concentration throughout summer with a concomitant general decrease in δ¹³C_DIC from oxidation of the isotopically light particulate organic carbon that sank down through the thermocline from the epilimnion. Mass balance calculations of DI¹²C and DI¹³C in the epilimnion for the summer (June 23-September 25) yield a mean rate of net conversion of DIC to organic carbon (C_org) of 430 ± 150 moles d^-1 (6.5 ± 1.8 m moles m^-2 d^-1. Net CO₂ invasion from the atmosphere was 420 ± 120 moles d^-1 (6.2 ± 1.8 m moles m^-2 d^-1) with an exchange coefficient of 0.6 ± 0.3m d^-1. These results imply that at least for the summer months the phytoplankton obtained about 90% of their carbon from atmosphere CO₂. About 50% of CO₂ invasion and conversion to C_org for the summer occurred during a two week interval in mid-summer. DIC concentration increased in the hypolimnion at a rate of 350 ± 70 moles DIC d^-1 during summer stratification. The amount of DIC added to the hypolimnion was equivalent to 75 ± 20% of net conversion of DIC to C_org in the euphotic zone over spring and summer implying rapid degradation of POC in the hypolimnion. The δ¹³C of DIC added to the deep water (-22‰.) was too heavy to have been derived from oxidation of particulate organic carbon alone. About 20% of the added DIC must have diffused from hypolimnetic sediments where relatively heavy CO₂ (-7‰) was produced by a combination of POC oxidation and as a by-product of methanogenesis.