Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 5 4 2008 10.5194/bg-5-1007-2008 http://www.biogeosciences.net/5/1007/2008/ http://www.biogeosciences.net/5/1007/2008/bg-5-1007-2008.html http://www.biogeosciences.net/5/1007/2008/bg-5-1007-2008.pdf 1007 1022 2008-07-21 Coupling of heterotrophic bacteria to phytoplankton bloom development at different <i>p</i>CO₂ levels: a mesocosm study M. Allgaier U. Riebesell M. Vogt R. Thyrhaug H.-P. Grossart hgrossart@igb-berlin.de Leibniz-Institute of Freshwater Ecology and Inland Fisheries; Department Limnology of Stratified Lakes; Alte Fischerhuette 2; D-16775 Stechlin-Neuglobsow, Germany Leibniz- Institute for Marine Sciences, University of Kiel, Duesternbrooker Weg 20, D-24105 Kiel, Germany Laboratory for Global Marine and Atmospheric Chemistry, School of Environmental Sciences, University of East Anglia, Norwich NR 7TJ, UK Department of Biology, Jahnebakken 5, University of Bergen, Norway currant address: DOE Joint genome Inst., Microbial Ecology Program, 2800 Mitchell Drive, Walnut Creek, CA 94598, USA The predicted rise in anthropogenic CO₂ emissions will increase CO₂ concentrations and decrease seawater pH in the upper ocean. Recent studies have revealed effects of <i>p</i>CO₂ induced changes in seawater chemistry on a variety of marine life forms, in particular calcifying organisms. To test whether the predicted increase in <i>p</i>CO₂ will directly or indirectly (via changes in phytoplankton dynamics) affect abundance, activities, and community composition of heterotrophic bacteria during phytoplankton bloom development, we have aerated mesocosms with CO₂ to obtain triplicates with three different partial pressures of CO₂ (<i>p</i>CO₂): 350 μatm (1&times;CO₂), 700 μatm (2&times;CO₂) and 1050 μatm (3&times;CO₂). The development of a phytoplankton bloom was initiated by the addition of nitrate and phosphate. In accordance to an elevated carbon to nitrogen drawdown at increasing <i>p</i>CO₂, bacterial production (BPP) of free-living and attached bacteria as well as cell-specific BPP (csBPP) of attached bacteria were related to the C:N ratio of suspended matter. These relationships significantly differed among treatments. However, bacterial abundance and activities were not statistically different among treatments. 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