Assessing seasonal changes in carbonate parameters across small spatial gradients in the Northeastern Chukchi Sea

Observations of the marine carbonate system were made in 2010 in the northeastern Chukchi Sea to constrain the seasonal progression of carbonate mineral saturation states (Ω) throughout the water column and determine the air-sea flux of carbon dioxide (CO₂). As sea ice retreats from the Chukchi Shelf, primary production consumes dissolved inorganic carbon (DIC) in the euphotic zone causing pH and carbonate mineral saturation states to increase. Throughout the summer and early autumn months of 2010, saturation states for calcite and aragonite ranged from 2.5–4.0 and 1.5–2.5, respectively, well about the saturation horizon of 1.0. Much of the organic matter produced during the bloom was vertically exported from the relatively small study area leading to an uptake of CO₂ from the atmosphere of at least 340,000 kg-C. The exported organic matter settled near the bottom and was remineralized back into DIC, causing concentrations to increase sharply, particularly in autumn months, driving down pH to as low as 7.75 and suppressing the concentrations of important carbonate minerals to the point that aragonite became undersaturated. The data showed a definitive seasonal progression of this process with aragonite becoming partially undersaturated along the bottom in September, and broadly undersaturated in October. While carbonate saturation states would naturally be suppressed by the high rates of export production and the accumulation of DIC near the bottom, the penetration of anthropogenic CO₂ into water column (ocean acidification) has caused these observed undersaturations, which will likely expand as CO₂ levels in the atmosphere continue to rise in the coming decades.

Effect of CO2 enrichment on bacterial metabolism in an Arctic fjord (update)

The anthropogenic increase of carbon dioxide (CO₂) alters the seawater carbonate chemistry, with a decline of pH and an increase in the partial pressure of CO₂ (pCO₂). Although bacteria play a major role in carbon cycling, little is known about the impact of rising pCO₂ on bacterial carbon metabolism, especially for natural bacterial communities. In this study, we investigated the effect of rising pCO₂ on bacterial production (BP), bacterial respiration (BR) and bacterial carbon metabolism during a mesocosm experiment performed in Kongsfjorden (Svalbard) in 2010. Nine mesocosms with pCO₂ levels ranging from ca. 180 to 1400 μatm were deployed in the fjord and monitored for 30 days. Generally BP gradually decreased in all mesocosms in an initial phase, showed a large (3.6-fold average) but temporary increase on day 10, and increased slightly after inorganic nutrient addition. Over the wide range of pCO₂ investigated, the patterns in BP and growth rate of bulk and free-living communities were generally similar over time. However, BP of the bulk community significantly decreased with increasing pCO₂ after nutrient addition (day 14). In addition, increasing pCO₂ enhanced the leucine to thymidine (Leu : TdR) ratio at the end of experiment, suggesting that pCO₂ may alter the growth balance of bacteria. Stepwise multiple regression analysis suggests that multiple factors, including pCO₂, explained the changes of BP, growth rate and Leu : TdR ratio at the end of the experiment. In contrast to BP, no clear trend and effect of changes of pCO₂ was observed for BR, bacterial carbon demand and bacterial growth efficiency. Overall, the results suggest that changes in pCO₂ potentially influence bacterial production, growth rate and growth balance rather than the conversion of dissolved organic matter into CO₂.

Implications of elevated CO2 on pelagic carbon fluxes in an Arctic mesocosm study – an elemental mass balance approach (update)

Recent studies on the impacts of ocean acidification on pelagic communities have identified changes in carbon to nutrient dynamics with related shifts in elemental stoichiometry. In principle, mesocosm experiments provide the opportunity of determining temporal dynamics of all relevant carbon and nutrient pools and, thus, calculating elemental budgets. In practice, attempts to budget mesocosm enclosures are often hampered by uncertainties in some of the measured pools and fluxes, in particular due to uncertainties in constraining air–sea gas exchange, particle sinking, and wall growth. In an Arctic mesocosm study on ocean acidification applying KOSMOS (Kiel Off-Shore Mesocosms for future Ocean Simulation), all relevant element pools and fluxes of carbon, nitrogen and phosphorus were measured, using an improved experimental design intended to narrow down the mentioned uncertainties. Water-column concentrations of particulate and dissolved organic and inorganic matter were determined daily. New approaches for quantitative estimates of material sinking to the bottom of the mesocosms and gas exchange in 48 h temporal resolution as well as estimates of wall growth were developed to close the gaps in element budgets. However, losses elements from the budgets into a sum of insufficiently determined pools were detected, and are principally unavoidable in mesocosm investigation. The comparison of variability patterns of all single measured datasets revealed analytic precision to be the main issue in determination of budgets. Uncertainties in dissolved organic carbon (DOC), nitrogen (DON) and particulate organic phosphorus (POP) were much higher than the summed error in determination of the same elements in all other pools. With estimates provided for all other major elemental pools, mass balance calculations could be used to infer the temporal development of DOC, DON and POP pools.

Future elevated pCO₂ was found to enhance net autotrophic community carbon uptake in two of the three experimental phases but did not significantly affect particle elemental composition. Enhanced carbon consumption appears to result in accumulation of dissolved organic carbon under nutrient-recycling summer conditions. This carbon over-consumption effect becomes evident from mass balance calculations, but was too small to be resolved by direct measurements of dissolved organic matter. Faster nutrient uptake by comparatively small algae at high CO₂ after nutrient addition resulted in reduced production rates under future ocean CO₂ conditions at the end of the experiment. This CO₂ mediated shift towards smaller phytoplankton and enhanced cycling of dissolved matter restricted the development of larger phytoplankton, thus pushing the system towards a retention type food chain with overall negative effects on export potential.

The real limits to marine life: a further critique of the Respiration Index (update)

The recently proposed “Respiration Index” (RI = log PO₂/PCO₂) suggests that aerobic metabolism is limited by the ratio of reactants (oxygen) to products (carbon dioxide) according to the thermodynamics of cellular respiration. Here, we demonstrate further that, because of the large standard free energy change for organic carbon oxidation (ΔG° = −686 kcal mol⁻¹), carbon dioxide can never reach concentrations that would limit the thermodynamics of this reaction. A PCO₂ to PO₂ ratio of 10⁵⁰³ would be required to reach equilibrium (equilibrium constant, K_eq = 10⁵⁰³), where ΔG = 0. Thus, a Respiration Index of −503 would be the real thermodynamic limit to aerobic life. Such a Respiration Index is never reached, either in the cell or in the environment. Moreover, cellular respiration and oxygen provision are kinetically controlled such that, within limits, environmental oxygen and CO₂ concentrations have little to do with intracellular concentrations. The RI is fundamentally different from the aragonite saturation state, a thermodynamic index used to quantify the potential effect of CO₂ on calcification rates, because of its failure to incorporate the equilibrium constant of the reaction. Not only is the RI invalid, but its use leads to incorrect and misleading predictions of the threat of changing oxygen and carbon dioxide to marine life. We provide a physiological framework that identifies oxygen thresholds and allows for synergistic effects of ocean acidification and global warming.

Synergism between elevated pCO2 and temperature on the Antarctic sea ice diatom Nitzschia lecointei

Polar oceans are particularly susceptible to ocean acidification and warming. Diatoms play a significant role in sea ice biogeochemistry and provide an important food source to grazers in ice-covered oceans, especially during early spring. However, the ecophysiology of ice living organisms has received little attention in terms of ocean acidification. In this study, the synergism between temperature and partial pressure of CO₂ (pCO₂) was investigated in relationship to the optimal growth temperature of the Antarctic sea ice diatom Nitzschia lecointei. Diatoms were kept in cultures at controlled levels of pCO₂ (∼390 and ∼960 μatm}) and temperature (−1.8 and 2.5 °C) for 14 days. Synergism between temperature and pCO₂ was detected in growth rate and acyl lipid fatty acid content. Carbon enrichment only promoted (3%) growth rate closer to the optimal growth, but not at the control temperature (−1.8 °C). Optimal growth rate was observed around 5 °C in a separate experiment. Polyunsaturated fatty acids (PUFA) comprised up to 98% of the total acyl lipid fatty acid pool at −1.8 °C. However, the total content of fatty acids was reduced by 39% at elevated pCO₂, but only at the control temperature. PUFAs were reduced by 30% at high pCO₂. Effects of carbon enrichment may be different depending on ocean warming scenario or season, e.g. reduced food quality for higher trophic levels during spring. Synergy between temperature and pCO₂ may be particularly important in polar areas since a narrow thermal window generally limits cold-water organisms.

Response of halocarbons to ocean acidification in the Arctic (update)

The potential effect of ocean acidification (OA) on seawater halocarbons in the Arctic was investigated during a mesocosm experiment in Spitsbergen in June–July 2010. Over a period of 5 weeks, natural phytoplankton communities in nine ~ 50 m³ mesocosms were studied under a range of pCO₂ treatments from ~ 185 μatm to ~ 1420 μatm. In general, the response of halocarbons to pCO₂ was subtle, or undetectable. A large number of significant correlations with a range of biological parameters (chlorophyll a, microbial plankton community, phytoplankton pigments) were identified, indicating a biological control on the concentrations of halocarbons within the mesocosms. The temporal dynamics of iodomethane (CH₃I) alluded to active turnover of this halocarbon in the mesocosms and strong significant correlations with biological parameters suggested a biological source. However, despite a pCO₂ effect on various components of the plankton community, and a strong association between CH₃I and biological parameters, no effect of pCO₂ was seen in CH₃I. Diiodomethane (CH₂I₂) displayed a number of strong relationships with biological parameters. Furthermore, the concentrations, the rate of net production and the sea-to-air flux of CH₂I₂ showed a significant positive response to pCO₂. There was no clear effect of pCO₂ on bromocarbon concentrations or dynamics. However, periods of significant net loss of bromoform (CHBr₃) were found to be concentration-dependent, and closely correlated with total bacteria, suggesting a degree of biological consumption of this halocarbon in Arctic waters. Although the effects of OA on halocarbon concentrations were marginal, this study provides invaluable information on the production and cycling of halocarbons in a region of the world’s oceans likely to experience rapid environmental change in the coming decades.

Role of mode and intermediate waters in future ocean acidification: analysis of CMIP5 models

Consistently with the past decades observations, CMIP5 Earth System Models project highest acidification rates in subsurface waters. Using 7 ESMs, we find that high acidification rates in mode and intermediate waters (MIW) on centennial timescales (-0.0008 ± 4 × 10^–5 yr^–1 to -0.0023 ± 0.0001 yr^–1 depending on the scenario) are predominantly explained by the geochemical effect of increasing atmospheric CO₂, whereas physical and biological climate change feedbacks explain less than 10% of the simulated changes. MIW are characterized by a larger surface area to volume ratio than deep and bottom waters leading to 5 to 10 times larger carbon uptake. In addition, MIW geochemical properties result in a sensitivity to increasing carbon concentration twice larger than surface waters (Δ[H⁺] of +1.2 mmol.m^–3 for every mmol.m^–3 of dissolved carbon in MIW vs. +0.6 in surface waters). Low pH transported by mode and intermediate waters are likely to influence surface pH in upwelling regions decades after their isolation from the atmosphere.

Rising CO2 interacts with growth light and growth rate to alter photosystem II photoinactivation of the coastal diatom Thalassiosira pseudonana

We studied the interactive effects of pCO₂ and growth light on the coastal marine diatom Thalassiosira pseudonana CCMP 1335 growing under ambient and expected end-of-the-century pCO₂ (750 ppmv), and a range of growth light from 30 to 380 µmol photons·m⁻²·s⁻¹. Elevated pCO₂ significantly stimulated the growth of T. pseudonana under sub-saturating growth light, but not under saturating to super-saturating growth light. Under ambient pCO₂ susceptibility to photoinactivation of photosystem II (σ_i) increased with increasing growth rate, but cells growing under elevated pCO₂ showed no dependence between growth rate and σ_i, so under high growth light cells under elevated pCO₂ were less susceptible to photoinactivation of photosystem II, and thus incurred a lower running cost to maintain photosystem II function. Growth light altered the contents of RbcL (RUBISCO) and PsaC (PSI) protein subunits, and the ratios among the subunits, but there were only limited effects on these and other protein pools between cells grown under ambient and elevated pCO₂.

Evolutionary responses of a coccolithophorid Gephyrocapsa oceanica to ocean acidification

The ongoing ocean acidification associated with a changing carbonate system may impose profound effects on marine planktonic calcifiers. Here, we show that a coccolithophore, Gephyrocapsa oceanica, evolved in response to an elevated CO2 concentration of 1000 μatm (pH reduced to 7.8) in a long term (∼ 670 generations) selection experiment. The high CO2 selected cells showed increases in photosynthetic carbon fixation, growth rate, cellular particulate organic carbon (POC) or nitrogen (PON) production and a decrease in C:N elemental ratio, indicating a greater up-regulation of PON than of POC production under the ocean acidification condition. Cells from the low CO2 selection process shifted to high CO2 exposure showed an enhanced cellular POC and PON production rates. Our data suggest that the coccolithophorid could adapt to ocean acidification with enhanced assimilations of carbon and nitrogen but decreased C:N ratios.

Contrasting responses of DMS and DMSP to ocean acidification in Arctic waters (update)

Increasing atmospheric CO₂ is decreasing ocean pH most rapidly in colder regions such as the Arctic. As a component of the EPOCA (European Project on Ocean Acidification) pelagic mesocosm experiment off Spitzbergen in 2010, we examined the consequences of decreased pH and increased pCO₂ on the concentrations of dimethylsulphide (DMS). DMS is an important reactant and contributor to aerosol formation and growth in the Arctic troposphere. In the nine mesocosms with initial pH_T 8.3 to 7.5, equivalent to pCO₂ of 180 to 1420 μatm, highly significant but inverse responses to acidity (hydrogen ion concentration [H⁺]) occurred following nutrient addition. Compared to ambient [H⁺], average concentrations of DMS during the mid-phase of the 30 d experiment, when the influence of altered acidity was unambiguous, were reduced by approximately 60% at the highest [H⁺] and by 35% at [H⁺] equivalent to 750 μatm pCO₂, as projected for 2100. In contrast, concentrations of dimethylsulphoniopropionate (DMSP), the precursor of DMS, were elevated by approximately 50% at the highest [H⁺] and by 30% at [H⁺] corresponding to 750 μatm pCO₂. Measurements of the specific rate of synthesis of DMSP by phytoplankton indicate increased production at high [H⁺], in parallel to rates of inorganic carbon fixation. The elevated DMSP production at high [H⁺] was largely a consequence of increased dinoflagellate biomass and in particular, the increased abundance of the species Heterocapsa rotundata. We discuss both phytoplankton and bacterial processes that may explain the reduced ratios of DMS:DMSPt (total dimethylsulphoniopropionate) at higher [H⁺]. The experimental design of eight treatment levels provides comparatively robust empirical relationships of DMS and DMSP concentration, DMSP production and dinoflagellate biomass versus [H⁺] in Arctic waters.