Gas Exchange, Photosynthetic Uptake, and Carbon Budget for a Radiocarbon Addition to a Small Enclosure in a Stratified Lake

Abstract

9250 kBq (250 μCi) of 14C as NaHCO3 were added to the mixed-layer waters inside a long, cylindrical plastic enclosure anchored in an oligotrophic lake of the Canadian Shield. Loss of 14C from the epilimnion was predominantly in the form of irreversible gas-exchange across the liquid–air interface. This loss was measured by 14C inventory of the epilimnion and thermocline waters. Using the Lewis and Whitman boundary layer model, values for the mass transfer coefficient of 126, 58, and 100 cm/d were determined for three distinct phases in the deepening of the epilimnion during autumn cooling. The relationship between these mass transfer coefficients and the average wind speeds over the same three time periods were consistent with the results of previous wind-tunnel, gas-exchange experiments.Two significant features of the carbon budget during the course of the experiment were the large net outflux of CO2 from the corral (with [Formula: see text] in the epilimnion 3–7 times atmospheric levels) and the doubling of the total dissolved inorganic carbon (DIC) content of the epilimnion. The major source of carbon for these two processes was the entrainment of dissolved inorganic carbon as the epilimnion deepened during the cool days of late summer. Particulate organic carbon was also entrained and its oxidation contributed to the net DIC increase and CO2 loss. Simultaneous determinations of daily integral primary productivity by an incubator technique and by direct measurement of 14C uptake inside the enclosure were consistent. Dark respiration was 45–53% of daily integral primary productivity, but total respiration was nearly two times that for dark plus light respiration. Net primary productivity was thus substantially negative.Key words: Gas exchange, photosynthetic uptake, carbon budget