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.