The effect of seawater CO2 concentration on growth of a natural phytoplankton assemblage in a controlled mesocosm experiment

We examine the effects of seawater pCO2 concentration of 25, 41, and 76 kPa (250, 400, and 750 matm) on the growth rate of a natural assemblage of mixed phytoplankton obtained from a carefully controlled, 14-d mesocosm experiment. Throughout the experiment period, in all enclosures, two phytoplankton taxa (microflagellates and cryptomonads) and two diatom species (Skeletonema costatum and Nitzschia spp.) account for approximately 90% of the phytoplankton community. During the nutrient-replete period from day 9 to day 14 populations of Skeletonema costatum and Nitzschia spp. increased substantially; however, only Skeletonema costatum showed an increase in growth rate with increasing seawater pCO2. Not all diatom species in Korean coastal waters are sensitive to seawater pCO2 under nutrient-replete conditions. Since the industrial revolution, human activities have released ever-increasing quantities of CO2 into the atmosphere. However, only about half of this CO2 has remained in the atmosphere; the rest has been absorbed into the ocean and land biosphere. The global oceanic sink of fossilfuel CO2 has been estimated to be 118 6 19 petagrams of carbon, accounting for 30% of the total emission during the period from 1800 to 1994 (Sabine et al. 2004). The fossilfuel CO2 absorbed by the ocean is not evenly distributed throughout the depths because it is taken up by the ocean through air–sea gas exchange. The highest concentrations of fossil-fuel CO2 are found in the upper oceans. Away from the wind-driven upper oceans or deep water formation regions, fossil-fuel CO2 can only penetrate to deeper depths by slow diffusive processes, and, as a consequence, the concentration of fossil-fuel CO2 is below the current detection limits in much of the deep ocean (Sabine et al. 2004). The absorption of fossil-fuel CO2 has profoundly changed the upper ocean carbonate chemistry (Chung et al. 2004; Feely et al. 2004); for example it has led to an increase in gaseous CO2 concentration and a decrease in pH and carbonate ion (CO 22 3 ) concentration, which has in turn served to lower the saturation state of seawater with respect to biogenic calcite and aragonite. Over the typical range of surface water pH (7.8–8.2), bicarbonate ions (HCO 3 ) are a major contributor (,90%) to the total dissolved inorganic carbon concentration ([CO2] + [HCO 3 ] + [CO 22 3 ]), followed by CO 22 3 (,9%). Less than 1% of the total dissolved inorganic carbon is in the form of gaseous CO2. Riebesell et al. (1993) first suggested that some large phytoplankton species may rely on CO2 diffusion through the cell membrane for growth and, therefore, that the low concentration of CO2 (5–15 mmol kg21) in the present-day surface ocean would likely hinder phytoplankton growth. When in a low CO2 environment some phytoplankton 1 Corresponding author ([email protected]).