Effect of increased <i>p</i>CO<sub>2</sub> on the planktonic metabolic balance during a mesocosm experiment in an Arctic fjord

Abstract

Abstract. The effect of ocean acidification on the balance between gross community production (GCP) and community respiration (CR) (i.e., net community production, NCP) of plankton communities was investigated in summer 2010 in Kongsfjorden, west of Svalbard. Surface water, which was characterized by low concentrations of dissolved inorganic nutrients and chlorophyll a (a proxy of phytoplankton biomass), was enclosed in nine mesocosms and subjected to eight pCO 2 levels (two replicated controls and seven enhanced pCO 2 treatments) for one month. Nutrients were added to all mesocosms on day 13 of the experiment, and thereafter increase of chlorophyll a was provoked in all mesocosms. No clear trend in response to increasing pCO 2 was found in the daily values of NCP, CR, and GCP. For further analysis, these parameters were cumulated for the following three periods: phase 1 -end of CO 2 manipulation until nutrient addition (t4 to t13); phase 2 -nutrient addition until the second chlorophyll a minimum (t14 to t21); phase 3 -the second chlorophyll a minimum until the end of this study (t22 to t28). A significant response was detected as a decrease of NCP with increasing pCO 2 during phase 3. CR was relatively stable throughout the experiment in all mesocosms. As a result, the cumulative GCP significantly decreased with increasing pCO 2 during phase 3. After the nutrient addition, the ratios of cumulative NCP to cumulative consumption of NO 3 and PO 4 showed a significant decrease during phase 3 with increasing pCO 2 . The results suggest that elevated pCO 2 influenced cumulative NCP and stoichiometric C and nutrient coupling of the plankton community in a high-latitude fjord only for a limited period. However provided that there were some differences or weak correlations between NCP data based on different methods in the same experiment, this conclusion should be taken with caution.

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

“…Primary production estimates obtained with the 14 Cmethod at 1 m depth exceeded O 2 gross community production (GCP) determined at 4 m depth by a factor of ∼ 2 (Tanaka et al, 2013), and O 2 -and DIC-based net community production (NCP) by a factor of 3-4 . These discrepancies were mainly due to the different amount of light that cells received during the various measurements, and are comparable to differences observed for polar phytoplankton along comparable depth and light gradients (Yun et al, 2012).…”

“…mesocosms was highest at low CO 2 concentration during the later phase of the experiment, i.e., after day 21 (Tanaka et al, 2013;Silyakova et al, 2012). A different or even anti-correlated response of PP and NCP to increasing pCO 2 would have important implications for carbon and oxygen cycling in the surface ocean.…”

“…Several methods were applied during this mesocosm study to investigate the sensitivity of plankton productivity to CO 2 perturbation, including in vitro O 2 measurements at 4 m depth outside the mesocosms (Tanaka et al, 2013), as well as changes in dissolved inorganic carbon (DIC) concentration and uptake of 13 C-labelled DIC inside the mesocosms (de Kluijver et al, 2012).…”

“…All samples were counted the following day in a liquid scintillation analyzer (Packard TriCarb, model 1900 A). Tanaka et al, 2013), while changes in DIC concentration were calculated from depth-integrated water sampling (0-12 m; de Kluijver et al, 2012;Silyakova et al, 2012).…”

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

“…Tolerance of CO 2 levels up to ∼ 1000 µatm has often been observed in natural phytoplankton communities in regions exposed to fluctuating CO 2 levels. In these communities, increasing CO 2 often had no effect on primary productivity (Tortell et al, 2000;Tortell and Morel, 2002;Tortell et al, 2008b;Hopkinson et al, 2010;Tanaka et al, 2013;Sommer et al, 2015;Young et al, 2015;Spilling et al, 2016) or growth (Tortell et al, 2008b;Schulz et al, 2013), although an increase in primary production has been observed in some instances (Riebesell, 2004;Tortell et al, 2008b;Egge et al, 2009;Tortell et al, 2010;Hoppe et al, 2013;Holding et al, 2015). These differing responses may be due to differences in community composition, nutrient supply, or ecological adaptations of the phytoplankton community in the region studied.…”