Impact of a coccolithophorid bloom on dissolved carbon dioxide in sea water enclosures in a Norwegian fjord

Abstract

During an experiment conducted using four, 11 m³ mesocosms situated in a small Norwegian fjord, the partial pressure of carbon dioxide (PCO₂) and dissolved oxygen were determined in each enclosure over a 4 week period at dawn and dusk between 25 April and 21 May 1992. The control enclosure, without added inorganic nutrients, showed little evidence of phytoplankton growth over the period of the experiment, and PCO₂ gradually increased from an initial 250 µatm to atmospheric equilibrium ca 350 µatm. In a high inorganic phosphate enriched enclosure (N : P = 16 : 5) a mainly non-calcifying population of phytoplankton developed and resulted in a reduction in PCO₂ and increase in oxygen saturation. In a Redfield ratio (balanced) nutrient enriched enclosure (N : P = 16 : 1), a mixed population developed of both Emiliania huxleyi (Lohmann) Hay et Mohler and Phaeocystis sp. resulting in a similarly rapid rate of depletion of PCO₂ and increase in dissolved oxygen. In a low phosphate enriched enclosure (N : P = 16: 0.2), however, a transient net increase in PCO₂ was detected during a 4 day rapid growth phase of an almost pure E. huxleyi population. A comparison of PCO₂ and total carbon dioxide (TC02) relative removal rates in each enclosure from derived estimates of the Revelle or buffer factor, confirmed that calcification by coccolithophorids can substantially reduce the potential air-sea gradient in PCO₂. Possible explanations for the net daily increase in PCO₂ detected during the development of the bloom of E. huxleyi in the low phosphate enclosure are considered.