Anomalous carbon isotope fractionation between atmospheric CO2 and dissolved inorganic carbon induced by intense photosynthesis

A stable isotope mass-balance of dissolved inorganic carbon during a blue-green algae bloom in a softwater lake demonstrates that at low partial pressure of carbon dioxide there must be a large net negative carbon isotope fractionation between atmospheric CO1 and the CO2 absorbed by lake water at pH = 9.5. The net fractionation of CO*(g) with respect to HCO; was about -13%~ compared with about +8%~ for water at equilibrium with atmospheric CO* at pH o 7. Chemical enhancement of CO2 invasion at high pH by the reaction CO2 + OH+ HCOS at large apparent film thicknesses may result in carbon isotope fractionation approaching that for a hydroxide solution. This phenomenon, coupled with a decrease in the photosynthetic fractionation, forced the surface water of a softwater lake to achieve increasingly negative hL3C values during an algal bloom, which is in the opposite sense to the trend that results from photosynthesis under less extreme conditions. This and other similar systems must operate under non-equilibrium (kinetic) conditions, causing a large kinetic fractionation during CO2 invasion at pH > 8 and relatively large film thicknesses (i.e., low wind stress). INTRODUCTION CARBON ISOTOPE FRACTIONATION INTENSE BLUE-GREEN algae blooms in freshwater lakes are commonly accompanied by very low partial pressures of carbon dioxide (PCo,) and consequent high pH (>9) (SCHINDLER and FEE, 1973; SCHINDLER et al., 1973; EMERSON, 1975a; TALLING, 1976). Fractionation of carbon isotopes between blue-green algae and dissolved carbon dioxide (CO*(aq)) at low PCol has been shown to be as low as O%O (DEUSER et al., 1968; CALDER and PARKER, 1973). The fractionation is about 13%0 for conditions where algae are not CO2 limited (CRAIG, 1953; DEGENS et al., 1968). This paper focuses on anomalous trends in the measured carbon isotope composition of dissolved inorganic carbon (DIC) during a blue-green algal bloom in a freshwater lake and the implications of these observed phenomena to other natural systems. Equilibrium fractionation The isotopic equilibrium reaction involving transfer of CO1 across the air-water interface and dissociation of dissolved COz to HCO; may be written as a composite of two processes (i.e., gas exchange and dissociation of carbonic acid): 13COz(g) + H’*CO;(aq) = ‘*C02(g) + H13CO;(aq)