The carbon cycle for Lake Washington—A stable isotope study1

Abstract

We investigate the carbon cycle in Lake Washington for the year 1980 using monthly measurements of the dissolved inorganic carbon (DIC) and its ¹³C : ¹²C isotopic composition. Mass balances of DIC and ¹³C : ¹²C yield estimates of CO₂ gas exchange rates and net organic carbon production rates. Between 24 June and 13 August, the calculated CO₂ gas invasion rate of 0.80 × 10⁶ mol C d⁻¹ is nearly equal to the river DIC inflow rate. The calculated epilimnetic net organic carbon production rate is 0.68 × 10⁶ mol C d⁻¹, about 20–30% of primary productivity estimated from ¹⁴C‐fixation experiments and ETS‐derived respiration rates. Metalimnetic and hypolimnetic DIC increase rates and porewater DIC gradients in hypolimnetic sediments indicate that remineralization of particulate organic carbon (POC) previously deposited in the sediments is a major (0.5 × 10⁶ mol C d⁻¹) DIC source to the lake during summer.For the whole year, summertime CO₂ gas invasion balances wintertime CO₂ gas evasion and DIC and POC outflow balance DIC and POC inflow rates, implying no net carbon burial in the sediments during 1980. This contrasts with the measured long term sedimentation‐rate‐derived carbon burial rate of 0.8 × 10⁶ mol C d⁻¹. Year‐to‐year variability in summertime primary production rates largely determines net gains or losses of carbon via CO₂ gas exchange and sedimentation.