Photochemical production of carbon dioxide during a coastal phytoplankton bloom

Photochemical oxidation of coloured dissolved organic matter (CDOM) produces dissolved inorganic carbon (DIC: CO2, HCO3−, CO3−2). The efficiency of this reaction is affected by the source of CDOM and by irradiation history. Here we demonstrate the effect of local, hydrological factors on photochemical efficiency during a phytoplankton bloom in Saanich Inlet, B.C. The rate of photochemical production of DIC decreased during the bloom. We hypothesized that the decrease was caused by: (1) lower reaction efficiency of algally derived CDOM; (2) nitrate-limitation of photochemical oxidation during the bloom; or (3) a change in water mass and source of CDOM. Subsequent laboratory irradiations of a representative algal culture and of CDOM collected from local rivers indicated that the algally derived CDOM was a more efficient photochemical substrate than was the terrigenous material. In addition, while nitrate was drawn down during the bloom, its concentration was not statistically related to the variation in efficiency. Consequently, and in light of hydrographic measurements made during the field experiment, the most likely explanation for the decreased efficiency during the bloom was a change in water mass. During the bloom, the CDOM in surface water was probably dominated by allochthonous material, because the bloom was initiated by the inflow of a nutrient-rich water mass. During this experiment, DIC was produced photochemically at usually <3% (0.3–10%) of the rate of its uptake by total primary production.