Description: 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 pCO2 levels (two replicated controls and seven enhanced pCO2 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 pCO2 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 CO2 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 pCO2 during phase 3. CR was relatively stable throughout the experiment in all mesocosms. As a result, the cumulative GCP significantly decreased with increasing pCO2 during phase 3. After the nutrient addition, the ratios of cumulative NCP to cumulative consumption of NO3 and PO4 showed a significant decrease during phase 3 with increasing pCO2. The results suggest that elevated pCO2 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.

Description: The effect of CO2 on carbon fluxes in Arctic plankton communities was investigated during the 2010 EPOCA mesocosm study in Ny Ålesund, Svalbard. Nine mesocosms were set up with initial pCO2 levels ranging from 185 to 1420 μatm for 5 weeks. 13C labelled bicarbonate was added at the start of the experiment to follow the transfer of carbon from dissolved inorganic carbon (DIC) into phytoplankton, bacteria, total particulate organic carbon (POC), zooplankton, and settling particles. Polar lipid derived fatty acids (PLFA) were used to trace carbon dynamics of phytoplankton and bacteria and allowed distinction of two groups of phytoplankton: phyto I (autotrophs) and phyto II (mixotrophs). Nutrients were added on day 13. A nutrient-phytoplankton-zooplankton-detritus model amended with 13C dynamics was constructed and fitted to the data to quantify uptake rates and carbon fluxes in the plankton community during the phase prior to nutrient addition (phase 1, days 0–12). During the first 12 days, a phytoplankton bloom developed that was characterized by high growth rates (0.87 days−1) for phyto I and lower growth rates (0.18 days−1) for phyto II. A large part of the carbon fixed by phytoplankton (~31%) was transferred to bacteria, while mesozooplankton grazed only ~6% of the production. After 6 days, the bloom collapsed and part of the organic matter subsequently settled into the sediment traps. The sedimentation losses of detritus in phase 1 were low (0.008 days−1) and overall export was only ~7% of production. Zooplankton grazing and detritus sinking losses prior to nutrient addition were sensitive to CO2: grazing decreased with increasing CO2, while sinking increased. Phytoplankton production increased again after nutrient addition on day 13. Although phyto II showed initially higher growth rates with increasing CO2 (days 14–22), the overall production of POC after nutrient addition (phase 2, days 14–29) decreased with increasing CO2. Significant sedimentation occurred towards the end of the experiment (after day 24) and much more material settled down in the sediment traps at low CO2.

Description: 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.

Description: The Central Arctic Ocean is one of the most oligotrophic oceans on Earth because of its sea-ice cover and short productive season. Nonetheless, across the peaks of extinct volcanic seamounts of the Langseth Ridge (87°N, 61°E), we observe a surprisingly dense benthic biomass. Bacteriosponges are the most abundant fauna within this community, with a mass of 460 g C m-2 and an estimated carbon demand of around 110 g C m-2 yr-1, despite export fluxes from regional primary productivity only sufficient to provide 〈1% of this required carbon. Observed sponge distribution, bulk and compound-specific isotope data of fatty acids suggest that the sponge microbiome taps into refractory dissolved and particulate organic matter, including remnants of an extinct seep community. The metabolic profile of bacteriosponge fatty acids and expressed genes indicate that autotrophic symbionts contribute significantly to carbon assimilation. We suggest that this hotspot ecosystem is unique to the Central Arctic and associated with extinct seep biota, once fueled by degassing of the volcanic mounts.

Description: The effect of ocean warming and acidification was investigated on a natural plankton assemblage from an oligotrophic area, the bay of Villefranche (NW Mediterranean Sea). The assemblage was sampled in March 2012 and exposed to the following four treatments for 12 days: control (~360 μatm, 14°C), elevated p CO 2 (~610 μatm, 14°C), elevated temperature (~410 μatm, 17°C), and elevated p CO 2 and temperature (~690 μatm, 17°C). Nutrients were already depleted at the beginning of the experiment and the concentrations of chlorophyll a (chl a ), heterotrophic prokaryotes and viruses decreased, under all treatments, throughout the experiment. There were no statistically significant effects of ocean warming and acidification, whether in isolation or combined, on the concentrations of nutrients, particulate organic matter, chl a and most of the photosynthetic pigments. Furthermore, 13 C labelling showed that the carbon transfer rates from 13 C-sodium bicarbonate into particulate organic carbon were not affected by seawater warming nor acidification. Rates of gross primary production followed the general decreasing trend of chl a concentrations and were significantly higher under elevated temperature, an effect exacerbated when combined to elevated p CO 2 level. In contrast to the other algal groups, the picophytoplankton population (cyanobacteria, mostly Synechococcus ) increased throughout the experiment and was more abundant in the warmer treatment though to a lesser extent when combined to high p CO 2 level. These results suggest that under nutrient-depleted conditions in the Mediterranean Sea, ocean acidification has a very limited impact on the plankton community and that small species will benefit from warming with a potential decrease of the export and energy transfer to higher trophic levels.