Properties of seawater (partial pressure of carbon dioxide (pCO2)) from a ProOceanus CO2-Pro sensor mounted on the continuous surface water sampling system during the Tara Oceans expedition 2009-2013

The Tara Oceans Expedition (2009-2013) sampled the world oceans on board a 36 m long schooner, collecting environmental data and organisms from viruses to planktonic metazoans for later analyses using modern sequencing and state-of-the-art imaging technologies. Tara Oceans Data are particularly suited to study the genetic, morphological and functional diversity of plankton. The present data set provides continuous measurements of partial pressure of carbon dioxide (pCO2), using a ProOceanus CO2-Pro instrument mounted on the flowthrough system. This automatic sensor is fitted with an equilibrator made of gas permeable silicone membrane and an internal detection loop with a non-dispersive infrared detector of PPSystems SBA-4 CO2 analyzer. A zero-CO2 baseline is provided for the subsequent measurements circulating the internal gas through a CO2 absorption chamber containing soda lime or Ascarite. The frequency of this automatic zero point calibration was set to be 24 hours. All data recorded during zeroing processes were discarded with the 15-minute data after each calibration. The output of CO2-Pro is the mole fraction of CO2 in the measured water and the pCO2 is obtained using the measured total pressure of the internal wet gas. The fugacity of CO2 (fCO2) in the surface seawater, whose difference with the atmospheric CO2 fugacity is proportional to the air-sea CO2 fluxes, is obtained by correcting the pCO2 for non-ideal CO2 gas concentration according to Weiss (1974). The fCO2 computed using CO2-Pro measurements was corrected to the sea surface condition by considering the temperature effect on fCO2 (Takahashi et al., 1993). The surface seawater observations that were initially estimated with a 15 seconds frequency were averaged every 5-min cycle. The performance of CO2-Pro was adjusted by comparing the sensor outputs against the thermodynamic carbonate calculation of pCO2 using the carbonic system constants of Millero et al. (2006) from the determinations of total inorganic carbon (CT ) and total alkalinity (AT ) in discrete samples collected at sea surface. AT was determined using an automated open cell potentiometric titration (Haraldsson et al. 1997). CT was determined with an automated coulometric titration (Johnson et al. 1985; 1987), using the MIDSOMMA system (Mintrop, 2005). fCO2 data are flagged according to the WOCE guidelines following Pierrot et al. (2009) identifying recommended values and questionable measurements giving additional information about the reasons of the questionability.

Performance of the Arctic calanoid copepods Calanus glacialis and C. hyperboreus under elevated pCO2 and temperatures

The sensitivity of copepods to ocean acidification (OA) and warming may increase with time, however, studies >10 days and on synergistic effects are rare. We therefore incubated late copepodites and females of two dominant Arctic species, Calanus glacialis and Calanus hyperboreus, at 0 °C at 390 and 3000 µatm pCO2 for several months in fall/winter 2010. Respiration rates, body mass and mortality in both species and life stages did not change with pCO2. To detect synergistic effects, in 2011 C. hyperboreus females were kept at different pCO2 and temperatures (0, 5, 10 °C). Incubation at 10 °C induced sublethal stress, which might have overruled effects of pCO2. At 5 °C and 3000 µatm, body carbon was significantly lowest indicating a synergistic effect. The copepods, thus, can tolerate pCO2 predicted for a future ocean, but in combination with increasing temperatures they could be sensitive to OA.

Elevated pCO2 from late summer coastal upwellings in the western Baltic Sea enhances vegetative growth in the brown algae Fucus serratus: a laboratory assessment

The brown algae Fucus serratus is one of the major meadow forming algae of the Western Baltic Sea nearshore ecosystem. At the end of summer, those meadows are exposed to local upwelling suddenly increasing the pCO2 and DIC up to 2500 µatm and 2250 µmol/kg resp., for period of days to weeks. This study investigates the growth response of summer's vegetative Fucus serratus to elevated pCO2 (1350 and 4080 µatm) during a 40 days laboratory incubation. After 10 days, increases of growth rates of 20 % and 47 % of the control were observed in the 1350 and 4080 µatm pCO2 treatments respectively. Beyond 20 days, the growth rates collapsed in all treatments due to nutrients shortage, as demonstrated by high C:N ratios (95:1) and low N tissue content (0.04 % of dry weight). The collapse occurs faster at higher pCO2. On day 30, growth rates were reduced by 40 % and 100 % relative to the control at 1350 and 4080 µatm respectively. These results are consistent with a fertilizing effect of elevated pCO2 on Fucus serratus presumably linked to the transition from active HCO3- to passive CO2(aq) uptake. This positive effect is limited by nutrients resources, low seawater dissolved inorganic N and P and shortage of the nutrients reserves accumulated over the previous autumn and winter.