Increased growth rates of marine phytoplankton S. costatum in controlled mesocosm experiments in response to increased atmospheric CO2

Abstract

Rising carbon dioxide (CO2) concentrations in the atmosphere over the past two centuries have resulted in greater CO2 uptakes by the oceans. Such natural acidification process has profoundly changed the upper ocean carbonate chemistry, including the decrease in carbonate ion concentration and the increase in gaseous CO2 concentration. Several lines of evidence suggest that the accumulated oceanic burden of fossil fuel CO2 reduces CaCO3 production ofmarine organisms. However, experiments addressing the effect of rising oceanic CO2 to primary production are shortfall. Here we present results from a carefully-controlled mesocosm experiment, in which the same mixed phytoplankton assemblage in each set of triplicate enclosures was exposed to three different levels of CO2 concentrations (280, 380, and 700 ppm). In the final phase of the controlled experiment, a deliberate addition of nitrate and phosphate into all nine enclosures quickly stimulated primary production and underwent the exponential growth phase. Measured growth rates of several dominant species showed that only S. Costatum in the phytoplankton assemblage sensitively responded to changes in seawater CO2 concentrations. In the exponential growth phase of this experiment, the average growth rates of S. Costatum in 700 ppm-pCO2 treatments were approximately 30% higher than those in 280 ppm- or 380 ppm-pCO2 treatments.