Effect of increased CO₂ and iron levels on the marine plankton food web during a mesocosm experiment

Abstract

A mesocosm experiment was carried out in the Raunefjord (Norway) during 25 days, to investigate the interactive effects of increased CO₂ and iron availability on the plankton community. The seawater carbonate system in the mesocosms was manipulated to achieve two different CO₂ levels, corresponding to the present (390 ppmv, LC) and to levels predicted for year 2100 (900 ppmv, HC), in combination with ambient and increased dissolved Fe (dFe) concentrations in a full factorial design. We observed a shift in the plankton community structure, initially dominated by picoeukaryotes and small nanoeukaryotes, changing to an <em>Emiliania huxleyi</em> dominated bloom. <em>E. huxleyi</em> and <em>Synechococcus</em> were the most sensitive organisms to changes in CO2 and Fe levels, being negatively affected by increased CO₂ and favoured by high dFe levels. Picoeukaryotes, large nanoplankton, viruses and ciliates abundances were not affected by changes in CO₂ or dFe levels. Bacterial abundance showed a significant positive response to high CO₂ but it was unaffected by dFe. Total mesozooplankton abundances did not change significantly. The relevance of these results within the global change scenario will be discussed.