Direct Effect of CO2 Concentration on Phytoplankton Bloom.

Engel, A., Zondervan, I., Aerts, K., et al. (2005) Testing the direct effect of CO2 concentration on a bloom of the coccolithophorid Emiliania huxleyi in mesocosm experiments. Limnol. Oceanogr., 50(2): 493–507.

An extensive paper which investigates the direct effect of CO2 and the associated alterations in carbonate chemistry in sea water on marine planktonic organisms. The authors give a synopsis on the biogeochemistry and development of a bloom of E. huxleyi.

During the last 250 years, a rapid increase in CO2 concentration has been observed with estimates proposing a further increase up until 2100. It is unclear how the steady increase of CO2 concentration affects the fundamental cycling and surface ocean chemical equilibrium that maintains the marine environment. Environmental parameters are suggested to affect marine organism physiology by means of species composition, competition and the cycling of key biogeochemical elements (C, N and P.).

Changes in carbonate chemistry are suggested to effect calcification rates of marine phytoplankton through a change in the ratio of particulate organic carbon (POC) to particulate inorganic carbon (PIC).The quantity of biological calcification and rate of organic assimilation has the potential to manipulate the efficiency of global biological carbon pumps.

An experiment was designed enabling the manipulation of seawater CO2 concentration and maintaining the most natural conditions possible. Nine enclosures were created. The atmospheric and seawater pCO2 were manipulated to create three different CO2 concentrations in triplicate.
1. High pCO2 – Similar to CO2 concentrations expected in 2100.
2. Present pCO2 - Similar to CO2 concentrations to present day.
3. Low pCO2 - Similar to CO2 concentrations of glacial atmosphere.

Nutrient induced blooms of E. huxleyi were monitored over a 19 day period. An overview of all 28 variables that were determined during the study is described, as well as details of their individual methods. Phytoplankton counts were performed and measurements of size and weight were recorded.

Bloom of E. Huxleyi occurred concurrently in all nine enclosures showing exponential growth until total assimilation of nitrate and phosphate was complete. The characteristics of the comparative bloom associated with CO2 related effects were hidden by a variable concentration of particulates observed in the replicate mesocosms. E. huxleyi was affected by CO2 related treatments in the formation of calcite. This decreased as CO2 concentration increased.

Nonetheless, results obtained should be treated with vigilance when determining differences between the enclosures. It must be noted that, any environmental parameters acting upon on the E. huxleyi bloom were kept as constant as possible. This is not representative of the development of the oceanographic and chemical condition of the ocean between glacial times and the present day.

The changes identified in the calcification of cells, net specific growth rates, and alterations in the processes that maintain the essential chemical balance of the ocean allows the authors to advocate that E. huxleyi is susceptible to changes in CO2 concentration. They conclude that CO2 concentration could directly affect biogeochemical cycling and the carbon chemistry of the ocean, thus leading to the potential manipulation of food web dynamics and carbon export.

Nevertheless, the authors concede that further investigation is needed to determine how the rate and timing of sinking particles, as well as the fundamental composition of exported material affect CO2 exportation rates. They also suggest further examination of heterotrophic processes that could be effected by CO2 concentration.