CO<sub>2</sub> increases <sup>14</sup>C primary production in an Arctic plankton community

Abstract

Abstract. Responses to ocean acidification in plankton communities were studied during a CO2-enrichment experiment in the Arctic Ocean, accomplished from June to July 2010 in Kongsfjorden, Svalbard (78°56′ 2′′ N, 11°53′ 6′′ E). Enclosed in 9 mesocosms (volume: 43.9–47.6 m3), plankton was exposed to CO2 concentrations, ranging from glacial to projected mid-next-century levels. Fertilization with inorganic nutrients at day 13 of the experiment supported the accumulation of phytoplankton biomass, as indicated by two periods of high chl a concentration. This study tested for CO2 sensitivities in primary production (PP) of particulate organic carbon (PPPOC) and of dissolved organic carbon (PPDOC). Therefore, 14C-bottle incubations (24 h) of mesocosm samples were performed at 1 m depth receiving about 60% of incoming radiation. PP for all mesocosms averaged 8.06 ± 3.64 μmol C L−1 d−1 and was slightly higher than in the outside fjord system. Comparison between mesocosms revealed significantly higher PPPOC at elevated compared to low pCO2 after nutrient addition. PPDOC was significantly higher in CO2-enriched mesocosms before as well as after nutrient addition, suggesting that CO2 had a direct influence on DOC production. DOC concentrations inside the mesocosms increased before nutrient addition and more in high CO2 mesocosms. After addition of nutrients, however, further DOC accumulation was negligible and not significantly different between treatments, indicating rapid utilization of freshly produced DOC. Bacterial biomass production (BP) was coupled to PP in all treatments, indicating that 3.5 ± 1.9% of PP or 21.6 ± 12.5% of PPDOC provided on average sufficient carbon for synthesis of bacterial biomass. During the later course of the bloom, the response of 14C-based PP rates to CO2 enrichment differed from net community production (NCP) rates that were also determined during this mesocosm campaign. We conclude that the enhanced release of labile DOC during autotrophic production at high CO2 exceedingly stimulated activities of heterotrophic microorganisms. As a consequence, increased PP induced less NCP, as suggested earlier for carbon-limited microbial systems in the Arctic.

Marine CDOM accumulation during a coastal Arctic mesocosm experiment: No response to elevated pCO2 levels

“…Considerable day-to-day variations of up to 30% were observed in both fjord and mesocosms (Figure 6a). Based on data from all nine mesocosms, Engel et al [2013] showed that DOC increased significantly until nutrient addition; however, this may not be apparent from three mesocosms considered here. At the same time, Engel et al [2013] suggested that this might be attributed to contamination of samples during sample collection, transport, or instrument deployment.…”

“…Based on data from all nine mesocosms, Engel et al [2013] showed that DOC increased significantly until nutrient addition; however, this may not be apparent from three mesocosms considered here. At the same time, Engel et al [2013] suggested that this might be attributed to contamination of samples during sample collection, transport, or instrument deployment. For this reason, we did not estimate trends in DOC concentration but used average DOC values (over the course of experiment) for interpretation of other results assuming that potential methodological errors occurred randomly .…”

“…Concentrations of chlorophyll a and pigment analyses were done by high-performance liquid chromatography (WATERS HPLC with a Varian Microsorb-MV 100-3 C8 column) according to Barlow et al [1997]. DOC samples were analyzed using the high-temperature combustion method (TOC-VCSH, Shimadzu) [Qian and Mopper, 1996] (details in Engel et al [2013]). Bacteria and viruses were counted using a bench-top Becton Dickinson FACSCalibur flow cytometer (FCM) equipped with a 488 nm argon laser (details in Brussaard et al [2013]).…”

Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO<sub>2</sub> tolerance in phytoplankton productivity

“…Kim et al, 2006;Hopkinson et al, 2010;Riebesell et al, 2013;Paul et al, 2015;Bach et al, 2016;Bunse et al, 2016). Studies in the Arctic reported increases in phytoplankton primary productivity, growth, and organic matter concentration at CO 2 levels ≥ 800 µatm under nutrient-replete conditions (Bellerby et al, 2008;Egge et al, 2009;Engel et al, 2013;Schulz et al, 2013), whilst the bacterial community was unaffected (Grossart et al, 2006;Allgaier et al, 2008;Paulino et al, 2008;Baragi et al, 2015). These studies also highlight the importance of nutrient availability in the community response to elevated CO 2 , with substantial differences in primary and bacterial productivity, chlorophyll a (Chl a), and elemental stoichiometry observed between nutrient-replete and nutrient-limited conditions Schulz et al, 2013;Sperling et al, 2013;Bach et al, 2016).…”

“…High CO 2 levels have been observed to have either no effect on abundance and productivity (Grossart et al, 2006;Allgaier et al, 2008;Paulino et al, 2008;Baragi et al, 2015;Wang et al, 2016) or increase growth rate and production only during the postbloom phase of an experiment (Grossart et al, 2006;Sperling et al, 2013;Westwood et al, 2018). Thus, bacterial communities appear to be relatively tolerant to ocean acidification, with bacterial growth indirectly affected by the ocean acidification responses of the phytoplankton community (Grossart et al, 2006;Allgaier et al, 2008;Engel et al, 2013;Piontek et al, 2013;Sperling et al, 2013;Bergen et al, 2016).…”