Abstract
Abstract. The potential impact of rising carbon dioxide (CO2) on carbon transfer from phytoplankton to bacteria was investigated during the 2005 PeECE III mesocosm study in Bergen, Norway. Sets of mesocosms, in which a phytoplankton bloom was induced by nutrient addition, were incubated under 1× (~350 μatm), 2× (~700 μatm), and 3× present day CO2 (~1050 μatm) initial seawater and sustained atmospheric CO2 levels for 3 weeks. 13C labelled bicarbonate was added to all mesocosms to follow the transfer of carbon from dissolved inorganic carbon (DIC) into phytoplankton and subsequently heterotrophic bacteria, and settling particles. Isotope ratios of polar-lipid-derived fatty acids (PLFA) were used to infer the biomass and production of phytoplankton and bacteria. Phytoplankton PLFA were enriched within one day after label addition, whilst it took another 3 days before bacteria showed substantial enrichment. Group-specific primary production measurements revealed that coccolithophores showed higher primary production than green algae and diatoms. Elevated CO2 had a significant positive effect on post-bloom biomass of green algae, diatoms, and bacteria. A simple model based on measured isotope ratios of phytoplankton and bacteria revealed that CO2 had no significant effect on the carbon transfer efficiency from phytoplankton to bacteria during the bloom. There was no indication of CO2 effects on enhanced settling based on isotope mixing models during the phytoplankton bloom, but this could not be determined in the post-bloom phase. Our results suggest that CO2 effects are most pronounced in the post-bloom phase, under nutrient limitation.
Highlights
The ocean is one of the largest reservoirs of CO2 on earth and one of the largest sinks for anthropogenic CO2 emissions (Sabine et al, 2004)
In which a phytoplankton bloom was induced by nutrient addition, were incubated under 1× (∼350 μatm), 2× (∼700 μatm), and 3× present day CO2 (∼1050 μatm) initial seawater and sustained atmospheric CO2 levels for 3 weeks. 13C labelled bicarbonate was added to all mesocosms to follow the transfer of carbon from dissolved inorganic carbon (DIC) into phytoplankton and subsequently heterotrophic bacteria, and settling particles
We applied the combined technique to determine the uptake of dissolved inorganic carbon by phytoplankton and subsequent transfer within the plankton community under different CO2 levels
Summary
The ocean is one of the largest reservoirs of CO2 on earth and one of the largest sinks for anthropogenic CO2 emissions (Sabine et al, 2004). The strength of the biological pump is largely controlled by three processes: primary production, community respiration and the export rate of particulate organic matter (POM) into the deep ocean. Community respiration in the euphotic zone, dominated by heterotrophic bacteria, converts organic carbon back into CO2 and decreases the oceans’ CO2 uptake capacity (Rivkin and Legendre, 2001). The coupling between phytoplankton and heterotrophic bacteria is mainly via labile dissolved organic matter (DOM). In the upper ocean an important source for labile DOM and subsequently for heterotrophic bacteria is the release of carbon-rich substances by phytoplankton, further referred to as exudation (Larsson and Hagstrom, 1979). The rate of exudation is linked to primary production and is highest under nutrient-poor conditions, when
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