Seawater Carbonate System Research Articles

Seasonal variability of carbonate chemistry and decadal changes in waters of a marine sanctuary in the Northwestern Gulf of Mexico

Abstract We report seasonal water column carbonate chemistry data collected over a three-year period (late 2013 to 2016) at Flower Garden Banks National Marine Sanctuary (FGBNMS) located on the subtropical shelf edge of the northwestern Gulf of Mexico. The FGBNMS hosts the northernmost tropical coral species in the contiguous United States, with over 50% living coral cover. Presented here are results from samples of the upper 25 m of the water column collected from September 2013 to November 2016. Additionally, following a localized mortality event likely associated with major continental flooding in summer 2016, water samples from up to ~250 m depth were collected in the broader FGBNMS area on a rapid response cruise to examine the seawater carbonate system. Both surface (

The spatial and interannual dynamics of the surface water carbonate system and air–sea CO<sub>2</sub> fluxes in the outer shelf and slope of the Eurasian Arctic Ocean

Abstract. The Arctic is undergoing dramatic changes which cover the entire range of natural processes, from extreme increases in the temperatures of air, soil, and water, to changes in the cryosphere, the biodiversity of Arctic waters, and land vegetation. Small changes in the largest marine carbon pool, the dissolved inorganic carbon pool, can have a profound impact on the carbon dioxide (CO2) flux between the ocean and the atmosphere, and the feedback of this flux to climate. Knowledge of relevant processes in the Arctic seas improves the evaluation and projection of carbon cycle dynamics under current conditions of rapid climate change. Investigation of the CO2 system in the outer shelf and continental slope waters of the Eurasian Arctic seas (the Barents, Kara, Laptev, and East Siberian seas) during 2006, 2007, and 2009 revealed a general trend in the surface water partial pressure of CO2 (pCO2) distribution, which manifested as an increase in pCO2 values eastward. The existence of this trend was defined by different oceanographic and biogeochemical regimes in the western and eastern parts of the study area; the trend is likely increasing due to a combination of factors determined by contemporary change in the Arctic climate, each change in turn evoking a series of synergistic effects. A high-resolution in situ investigation of the carbonate system parameters of the four Arctic seas was carried out in the warm season of 2007; this year was characterized by the next-to-lowest historic sea-ice extent in the Arctic Ocean, on satellite record, to that date. The study showed the different responses of the seawater carbonate system to the environment changes in the western vs. the eastern Eurasian Arctic seas. The large, open, highly productive water area in the northern Barents Sea enhances atmospheric CO2 uptake. In contrast, the uptake of CO2 was strongly weakened in the outer shelf and slope waters of the East Siberian Arctic seas under the 2007 environmental conditions. The surface seawater appears in equilibrium or slightly supersaturated by CO2 relative to atmosphere because of the increasing influence of river runoff and its input of terrestrial organic matter that mineralizes, in combination with the high surface water temperature during sea-ice-free conditions. This investigation shows the importance of processes that vary on small scales, both in time and space, for estimating the air–sea exchange of CO2. It stresses the need for high-resolution coverage of ocean observations as well as time series. Furthermore, time series must include multi-year studies in the dynamic regions of the Arctic Ocean during these times of environmental change.

The stoichiometric dissociation constants of carbonic acid in seawater brines from 298 to 267 K

The stoichiometric dissociation constants of carbonic acid (K1C∗ and K2C∗) were determined by measurement of all four measurable parameters of the carbonate system (total alkalinity, total dissolved inorganic carbon, pH on the total proton scale, and CO2 fugacity) in natural seawater and seawater-derived brines, with a major ion composition equivalent to that of Reference Seawater, to practical salinity (SP) 100 and from 25 °C to the freezing point of these solutions and −6 °C temperature minimum. These values, reported in the total proton scale, provide the first such determinations at below-zero temperatures and for SP > 50. The temperature (T, in Kelvin) and SP dependence of the current pK1C∗ and pK2C∗ (as negative common logarithms) within the salinity and temperature ranges of this study (33 ≤ SP ≤ 100, −6 °C ≤ t ≤ 25 °C) is described by the following best-fit equations: pK1C∗ = −176.48 + 6.14528 SP0.5−0.127714 SP + 7.396×10−5SP2 + (9914.37 − 622.886 SP0.5 + 29.714 SP) T−1 + (26.05129 − 0.666812 SP0.5) lnT (σ = 0.011, n = 62), and pK2C∗ = −323.52692 + 27.557655 SP0.5 + 0.154922 SP − 2.48396×10−4SP2 + (14763.287−1014.819 SP0.5−14.35223 SP) T−1 + (50.385807−4.4630415 SP0.5) lnT (σ = 0.020, n = 62). These functions are suitable for application to investigations of the carbonate system of internal sea ice brines with a conservative major ion composition relative to that of Reference Seawater and within the temperature and salinity ranges of this study.

P CO 2 variability in the surface waters of the ultra-oligotrophic Levantine Sea: Exploring the air–sea CO 2 fluxes in a fast warming region

Abstract Seasonal p CO 2 variability was studied in the Southeast Levantine (SE-Levantine) during 2009–2015 with the aim of quantifying air–sea CO 2 fluxes in this ultra-oligotrophic, warm and highly evaporative marginal sea. Mixed layer p CO 2 varied significantly between 560 ± 9.0 μatm in August (summer) and 350 ± 8.7 μatm in March (winter). Comparison of p CO 2 to Sea Surface Temperature (SST) yielded a strong positive correlation ( n = 135, r 2 = 0.94), suggesting that the seasonal variations are the result of a thermodynamic effect on the carbonate system in seawater. Using the coupling between p CO 2 and SST, we calculated the mean monthly values and the air-sea fluxes in this region. These calculations indicated that this region is a net source of CO 2 to the atmosphere over an annual cycle, with an average flux of 845 ± 270 mmol C m 2 y − 1 (~ 0.98 Tg C y − 1 ). Flux estimation for three other sub-basins of the East Mediterranean (South of Crete) with similar oceanographic conditions, were calculated from mean seasonal SST and wind data (MEDATLAS). The results suggest that these regions may either be a weak source or sink for atmospheric CO 2 . However, considering the relatively rapid warming trends (~+1 °C/decade) observed in the Levantine basin and the anticipated thermodynamic effect on p CO 2 levels in seawater, these sub basins could become more pronounced sources for atmospheric CO 2 . Thus, with the fast warming and increased stratification of its surface water the CO 2 source of the Eastern Mediterranean is expected to expend spatially.

Impact of seawater carbonate variables on post-larval bivalve calcification

Several studies have demonstrated that shellfish calcification rate has been impacted by ocean acidification. However, the carbonate system variables responsible for regulating calcification rate are controversial. To distinguish the key variables, we manipulated a seawater carbonate system by regulating seawater pH and dissolved inorganic carbon (DIC). Calcification rates of juvenile blue mussel (Mytilus edulis) and Zhikong scallop (Chlamys farreri) were measured in different carbonate systems. Our results demonstrated that neither [HCOˉ3], DIC, or pH ([H+]) were determining factors for the shellfish calcification rate of blue mussel or Zhikong scallop. However, a significant correlation was detected between calcification rate and DIC/[H+] and [CO32ˉ] in both species.

A novel membrane inlet‐infrared gas analysis (MI‐IRGA) system for monitoring of seawater carbonate system

AbstractIncreased atmospheric CO2 concentrations are driving changes in ocean chemistry at unprecedented rates resulting in ocean acidification, which is predicted to impact the functioning of marine biota, in particular of marine calcifiers. However, the precise understanding of such impacts relies on an analytical system that determines the mechanisms and impact of elevated pCO2 on the physiology of organisms at scales from species to entire communities. Recent work has highlighted the need within experiments to control all aspects of the carbonate system to resolve the role of different inorganic carbon species on the physiological responses observed across taxa in real‐time. Presently however, there are limited options available for continuous quantification of physiological responses, coupled with real‐time calculation of the seawater carbonate chemistry system within microcosm environments. Here, we describe and characterise the performance of a novel pCO2 membrane equilibrium system (the Membrane Inlet Infra‐Red Gas Analyser, MI‐IRGA) integrated with a continuous pH and oxygen monitoring platform. The system can detect changes in the seawater carbonate chemistry and determine organism physiological responses, while providing the user with real‐time control over the microcosm system. We evaluate the systems control, response time and associated error, and demonstrate the flexibility of the system to operate under field conditions and within a laboratory. We use the system to measure physiological parameters (photosynthesis and respiration) for the corals Pocillipora damicornis and Porites cylindrica; in doing so we present a novel dataset examining the interactive role of temperature, light and pCO2 on the physiology of P. cylindrica.

Significant discharge of CO2 from hydrothermalism associated with the submarine volcano of El Hierro Island.

The residual hydrothermalism associated with submarine volcanoes, following an eruption event, plays an important role in the supply of CO2 to the ocean. The emitted CO2 increases the acidity of seawater. The submarine volcano of El Hierro, in its degasification stage, provided an excellent opportunity to study the effect of volcanic CO2 on the seawater carbonate system, the global carbon flux, and local ocean acidification. A detailed survey of the volcanic edifice was carried out using seven CTD-pH-ORP tow-yo studies, localizing the redox and acidic changes, which were used to obtain surface maps of anomalies. In order to investigate the temporal variability of the system, two CTD-pH-ORP yo-yo studies were conducted that included discrete sampling for carbonate system parameters. Meridional tow-yos were used to calculate the amount of volcanic CO2 added to the water column for each surveyed section. The inputs of CO2 along multiple sections combined with measurements of oceanic currents produced an estimated volcanic CO2 flux = 6.0 105 ± 1.1 105 kg d−1 which is ~0.1% of global volcanic CO2 flux. Finally, the CO2 emitted by El Hierro increases the acidity above the volcano by ~20%.

Effect of Ocean Acidification and pH Fluctuations on the Growth and Development of Coralline Algal Recruits, and an Associated Benthic Algal Assemblage.

Coralline algae are susceptible to the changes in the seawater carbonate system associated with ocean acidification (OA). However, the coastal environments in which corallines grow are subject to large daily pH fluctuations which may affect their responses to OA. Here, we followed the growth and development of the juvenile coralline alga Arthrocardia corymbosa, which had recruited into experimental conditions during a prior experiment, using a novel OA laboratory culture system to simulate the pH fluctuations observed within a kelp forest. Microscopic life history stages are considered more susceptible to environmental stress than adult stages; we compared the responses of newly recruited A. corymbosa to static and fluctuating seawater pH with those of their field-collected parents. Recruits were cultivated for 16 weeks under static pH 8.05 and 7.65, representing ambient and 4× preindustrial pCO2 concentrations, respectively, and two fluctuating pH treatments of daily (daytime pH = 8.45, night-time pH = 7.65) and daily (daytime pH = 8.05, night-time pH = 7.25). Positive growth rates of new recruits were recorded in all treatments, and were highest under static pH 8.05 and lowest under fluctuating pH 7.65. This pattern was similar to the adults’ response, except that adults had zero growth under fluctuating pH 7.65. The % dry weight of MgCO3 in calcite of the juveniles was reduced from 10% at pH 8.05 to 8% at pH 7.65, but there was no effect of pH fluctuation. A wide range of fleshy macroalgae and at least 6 species of benthic diatoms recruited across all experimental treatments, from cryptic spores associated with the adult A. corymbosa. There was no effect of experimental treatment on the growth of the benthic diatoms. On the community level, pH-sensitive species may survive lower pH in the presence of diatoms and fleshy macroalgae, whose high metabolic activity may raise the pH of the local microhabitat.

Long-term acclimation to elevated pCO2 alters carbon metabolism and reduces growth in the Antarctic diatom Nitzschia lecointei.

Increasing atmospheric CO2 levels are driving changes in the seawater carbonate system, resulting in higher pCO2 and reduced pH (ocean acidification). Many studies on marine organisms have focused on short-term physiological responses to increased pCO2, and few on slow-growing polar organisms with a relative low adaptation potential. In order to recognize the consequences of climate change in biological systems, acclimation and adaptation to new environments are crucial to address. In this study, physiological responses to long-term acclimation (194 days, approx. 60 asexual generations) of three pCO2 levels (280, 390 and 960 µatm) were investigated in the psychrophilic sea ice diatom Nitzschia lecointei. After 147 days, a small reduction in growth was detected at 960 µatm pCO2. Previous short-term experiments have failed to detect altered growth in N. lecointei at high pCO2, which illustrates the importance of experimental duration in studies of climate change. In addition, carbon metabolism was significantly affected by the long-term treatments, resulting in higher cellular release of dissolved organic carbon (DOC). In turn, the release of labile organic carbon stimulated bacterial productivity in this system. We conclude that long-term acclimation to ocean acidification is important for N. lecointei and that carbon overconsumption and DOC exudation may increase in a high-CO2 world.

English

Ocean acidification (OA) research seeks to understand how marine ecosystems and global elemental cycles will respond to changes in seawater carbonate chemistry in combination with other environmental perturbations such as warming, eutrophication, and deoxygenation. Here, we discuss the effectiveness and limitations of current research approaches used to address this goal. A diverse combination of approaches is essential to decipher the consequences of OA to marine organisms, communities, and ecosystems. Consequently, the benefits and limitations of each approach must be considered carefully. Major research challenges involve experimentally addressing the effects of OA in the context of large natural variability in seawater carbonate system parameters and other interactive variables, integrating the results from different research approaches, and scaling results across different temporal and spatial scales.

Evaluating the utility of B/Ca ratios in planktic foraminifera as a proxy for the carbonate system: A case study of Globigerinoides ruber

AbstractB/Ca ratios in foraminifera have attracted considerable scientific attention as a proxy for past ocean carbonate system. However, the carbonate system controls on B/Ca ratios are not straightforward, with Δ[ ] ([ ]in situ – [ ]at saturation) correlating best with B/Ca ratios in benthic foraminifera, rather than pH, , or (as a simple model of boron speciation in seawater and incorporation into CaCO3 would predict). Furthermore, culture experiments have shown that in planktic foraminifera properties such as salinity and [B]sw can have profound effects on B/Ca ratios beyond those predicted by simple partition coefficients. Here, we investigate the controls on B/Ca ratios in G. ruber via a combination of culture experiments and core‐top measurements, and add to a growing body of evidence that suggests B/Ca ratios in symbiont‐bearing foraminiferal carbonate are not a straightforward proxy for past seawater carbonate system conditions. We find that while B/Ca ratios in culture experiments covary with pH, in open ocean sediments this relationship is not seen. In fact, our B/Ca data correlate best with [ ] (a previously undocumented association) and in most regions, salinity. These findings might suggest a precipitation rate or crystallographic control on boron incorporation into foraminiferal calcite. Regardless, our results underscore the need for caution when attempting to interpret B/Ca records in terms of the ocean carbonate system, at the very least in the case of mixed‐layer planktic foraminifera.

Characteristic features of the dynamics of carbonate parameters in the eastern part of the Laptev Sea

This report presents the results of studies of the sea water carbonate system performed in September 2011 at the inner, middle, and outer shelf of the east of the Laptev Sea. It was shown that the key factor controlling the carbonate chemistry of the shelf waters during the warm season is the significant terrigenous runoff. In the autumn season, the surface waters of the inner and middle shelf of the eastern part of the Laptev Sea are a source of carbon dioxide to the atmosphere; those of the outer shelf act as a sink of atmospheric CO2. It is found that the waters of the inner shelf of the east of the Laptev Sea over the entire water column are now corrosive against calcium carbonate. The surface and near-bottom waters of the middle shelf are also undersaturated with respect to aragonite. It was shown that the main factors determining the undersaturation are the significant volume of freshwaters supplied with the riverine runoff and the ice melting, as well as the large amount of labile allochthonous organic matter involved into the modern biogeochemical cycle at the shelf. The performed studies showed that the shelf waters of the eastern part of the Laptev Sea constitute one of the most corrosive zones with respect to calcium carbonate in the World Ocean.

The response of the carbonate system to a green algal bloom during the post-bloom period in the southern Yellow Sea

Since 2007, the green algal bloom occurred along the coast of Qingdao city every summer. In this study, we focused on how the carbonate system responded to the green algal bloom. Dissolved inorganic carbon (DIC), total alkalinity (TA) and pH were measured in two cruises during the green algal post-bloom period in summer 2008 in the southern Yellow Sea. Data showed that the average values of alkalinity calculated from DIC and pH were 2217 and 2241μmolkg−1 during the first and second cruises, respectively. The alkalinity measured by Gran titration, however, was abnormal, with average values of 3333 and 3280μmolkg−1. The abnormal measured TA values were probably due to both direct contribution of organic matter and its side reaction in determining TA using Gran titration. Another significant phenomenon was an increase of DIC and a decrease of pH after the bloom. The changes of DIC and pH during the two cruises were +87μmolkg−1 and −0.18, respectively. Normalized DIC and TA showed that DIC was not conserved after the bloom while TA could be still conserved. In contrast to green algal bloom period, during the post-bloom period, pCO2 values were significantly raised in seawater, with average values of 596 and 981μatm, resulting in CO2 outgassing from seawater. These results showed that carbonate system in seawater could be greatly altered by green algal bloom.

Increasing coccolith calcification during CO2 rise of the penultimate deglaciation (Termination II)

Abstract Glacial to interglacial environmental changes have a strong impact on coccolithophore assemblage composition. At the same time, glacial terminations are characterised by an increase in atmospheric CO 2 concentration. In order to determine how these two processes influence the calcite production of coccolithophores, we compared coccolith weight estimates obtained with the automated coccolith recognition system SYRACO with SEM assemblage counts covering the penultimate glacial Termination (T II) from two sediment cores in the North Atlantic Ocean. At the temperate Rockall Plateau (ODP Site 980), mean coccolith weight peaks around Heinrich event 11. This is paralleled by a shift within the coccolith assemblage related to the changes of the oceanic frontal system during Termination II. In the tropical Florida Strait, far from the influences of frontal zones, mean Noelaerhabdaceae coccolith weight doubles during Termination II. This is partly due to an assemblage shift towards larger and heavier calcifying morphotypes, but mainly an effect of increasing coccolithophore calcification. This increase is exactly mirroring the rise in atmospheric CO 2 , contradicting previous findings from Termination I. Reconstructions of DIC, alkalinity and calcite saturation at the Florida Strait during Termination II produce higher estimates of these parameters compared to previous studies for which coccolith weight estimates are available, and therefore a change of the carbonate system is the most likely cause for the coccolithophore calcification increase during atmospheric CO 2 rise. Our results illustrate that even during rising atmospheric CO 2 the conditions of the seawater carbonate system can be favourable for coccolithophore calcification. The total CaCO 3 production of a coccolithophore assemblage under increasing CO 2 therefore depends on regional seawater carbonate system characteristics and the local assemblage composition.

The role of temperature in determining species' vulnerability to ocean acidification: a case study using Mytilus galloprovincialis.

Ocean acidification (OA) is occurring across a backdrop of concurrent environmental changes that may in turn influence species' responses to OA. Temperature affects many fundamental biological processes and governs key reactions in the seawater carbonate system. It therefore has the potential to offset or exacerbate the effects of OA. While initial studies have examined the combined impacts of warming and OA for a narrow range of climate change scenarios, our mechanistic understanding of the interactive effects of temperature and OA remains limited. Here, we use the blue mussel, Mytilus galloprovincialis, as a model species to test how OA affects the growth of a calcifying invertebrate across a wide range of temperatures encompassing their thermal optimum. Mussels were exposed in the laboratory to a factorial combination of low and high pCO2 (400 and 1200 µatm CO2) and temperatures (12, 14, 16, 18, 20, and 24°C) for one month. Results indicate that the effects of OA on shell growth are highly dependent on temperature. Although high CO2 significantly reduced mussel growth at 14°C, this effect gradually lessened with successive warming to 20°C, illustrating how moderate warming can mediate the effects of OA through temperature's effects on both physiology and seawater geochemistry. Furthermore, the mussels grew thicker shells in warmer conditions independent of CO2 treatment. Together, these results highlight the importance of considering the physiological and geochemical interactions between temperature and carbonate chemistry when interpreting species' vulnerability to OA.

In situ study of boron partitioning between calcite and fluid at different crystal growth rates

The boron isotopic and chemical content of carbonates (expressed as δ11B and B/Ca ratios) have been proposed as proxies for seawater carbonate system parameters. Thermodynamic and kinetic effects on δ11B and B partitioning are not yet fully constrained, underscoring the importance of exploring possible effects of growth rate on boron incorporation in synthetic calcium carbonate minerals, which is the focus of this study. Secondary Ion Mass Spectrometry (SIMS) measurements of B/Ca and δ11B were performed on single crystal of calcite synthesized by diffusion of CO2 under controlled conditions from a solution of NH4Cl–CaCl2 doped with boron. Growth rates of calcite (V, nm/s) within crystals grown isothermally at 24.6°C were monitored by sequentially spiking calcite-precipitating fluids with rare earth element (REE) dopants. The REE were analyzed with SIMS at spots that match those locations where B/Ca and δ11B ratios were measured. Values for the boron Nernst partition coefficient, defined as DB=B(calcite)/B(fluid), increase from 0.5 (pHNBS=8.15) to 0.9 (pH=8.00) with values of V increasing from 0.04 to 0.13nm/s. For δ11B behavior, it was not possible to draw conclusive results due to the analytical error (2.2‰; 2σ). pH effects on DB during calcite precipitation, associated with an increase in pH from 8.00 to 8.15, appear to be masked by the competing effects of changing V. We conclude that over the range of growth rates and other conditions investigated in this study, growth rate effects on B partitioning need to be accounted for when using B/Ca ratios in biogenic calcite as a proxy for seawater carbonate system parameters.

Technical Note: Precise quantitative measurements of total dissolved inorganic carbon from small amounts of seawater using a gas chromatographic system

Abstract. Total dissolved inorganic carbon (CT) is one of the most frequently measured parameters used to calculate the partial pressure of carbon dioxide in seawater. Its determination has become increasingly important because of the rising interest in the biological effects of ocean acidification. Coulometric and infrared detection methods are currently favored in order to precisely quantify CT. These methods however are not sufficiently validated for CT measurements of biological experiments manipulating seawater carbonate chemistry with an extended CT measurement range (~1250–2400 μmol kg–1) compared to natural open ocean seawater (~1950–2200 μmol kg−1). The requirement of total sample amounts between 0.1–1 L seawater in the coulometric- and infrared detection methods potentially exclude their use for experiments working with much smaller volumes. Additionally, precise CT analytics become difficult with high amounts of biomass (e.g., phytoplankton cultures) or even impossible in the presence of planktonic calcifiers without sample pre-filtration. Filtration however, can alter CT concentration through gas exchange induced by high pressure. Addressing these problems, we present precise quantification of CT using a small, basic and inexpensive gas chromatograph as a CT analyzer. Our technique is able to provide a repeatability of ±3.1 μmol kg−1, given by the pooled standard deviation over a CT range typically applied in acidification experiments. 200 μL of sample is required to perform the actual CT measurement. The total sample amount needed is 12 mL. Moreover, we show that sample filtration is applicable with only minor alteration of the CT. The method is simple, reliable and with low cumulative material costs. Hence, it is potentially attractive for all researchers experimentally manipulating the seawater carbonate system.

Engineering challenges of ocean liming

The relationship between the level of atmospheric CO2 (carbon dioxide) and the impacts of climate change is uncertain, but a safe concentration may be surpassed this century. Therefore, it is necessary to develop technologies that can accelerate CO2 removal from the atmosphere. This paper explores the engineering challenges of a technology that manipulates the carbonate system in seawater by the addition of calcium oxide powder (CaO; lime), resulting in a net sequestration of atmospheric CO2 into the ocean (ocean liming; OL). Every tonne of CO2 sequestered requires between 1.4 and 1.7 t of limestone to be crushed, calcined, and distributed. Approximately 1 t of CO2 would be created from this activity, of which >80% is a high purity gas (pCO2 > 98%) amenable to geological storage. It is estimated that the thermal and electrical energy requirements for OL would be 0.6–5.6 and 0.1–1.2 GJ tCO2−1 captured respectively. A preliminary economic assessment suggests that OL could cost approximately US$72–159 t−1 of CO2. The additional CO2 burden of OL makes it a poor alternative to point source mitigation. However, it may provide a means to mitigate some diffuse emissions and reduce atmospheric concentrations.

Conservative and non-conservative variations of total alkalinity on the southeastern Bering Sea shelf

Recent observations of calcium carbonate (CaCO3) mineral undersaturations on the Bering Sea shelf have prompted new interest in the physical and biological factors that control the inorganic carbon system in the region. Understanding of the dynamics that influence the spatio-temporal variability of total alkalinity (TA) – one major component of the seawater carbonate system – has been constrained by limited historical data collected across the shelf, and the consensus has been that TA is largely conservative. However, the recently documented undersaturated conditions have the potential to cause substantial non-conservative variability in TA in this region through the dissolution of carbonate minerals. In order to quantify the contribution of carbonate mineral precipitation and dissolution to variability in TA on the southeastern Bering Sea shelf, we examined seasonal observations of TA that were made between 2008 and 2010 as part of the BEST-BSIERP Bering Sea Project. Conservative influences accounted for most of the variability in TA concentrations, with well-constrained mixing dominating in spring and summer of 2008. Bering Shelf Water (BSW) contained a constant ratio of TA to salinity, while river discharge (RW) added TA relative to salinity at a predictable rate. Although substantial organic carbon production and denitrification can cause some non-conservative variation in TA concentrations (a maximum of ~15μmolkgSW−1 combined), carbonate mineral dissolution and precipitation were shown to be the most important processes responsible for non-conservative TA–salinity relationships. CaCO3 uptake by the dominant pelagic phytoplankton calcifier (i.e., coccolithophores) was shown to alter TA concentrations by as much as 59μmolkgSW−1. Evidence for shallow-water CaCO3 mineral dissolution was also observed, which caused TA concentrations to increase by as much as 36μmolkgSW−1. Therefore, contrary to our previous understanding, the non-conservative physico-biogeochemical factors observed in this study play an important role in controlling the ocean carbon cycle of the Bering Sea shelf.

Effects of ocean acidification on growth and physiology of Ulva lactuca (Chlorophyta) in a rockpool‐scenario

SummaryRising atmospheric CO2‐concentrations will have severe consequences for a variety of biological processes. We investigated the responses of the green alga Ulva lactuca (Linnaeus) to rising CO2‐concentrations in a rockpool scenario. U. lactuca was cultured under aeration with air containing either preindustrial pCO2 (280 μatm) or the pCO2 predicted by the end of the 21st century (700 μatm) for 31 days. We addressed the following question: Will elevated CO2‐concentrations affect photosynthesis (net photosynthesis, maximum relative electron transport rate (rETR(max)), maximum quantum yield (Fv/Fm), pigment composition) and growth of U. lactuca in rockpools with limited water exchange? Two phases of the experiment were distinguished: In the initial phase (day 1–4) the Seawater Carbonate System (SWCS) of the culture medium could be adjusted to the selected atmospheric pCO2 condition by continuous aeration with target pCO2 values. In the second phase (day 4–31) the SWCS was largely determined by the metabolism of the growing U. lactuca biomass. In the initial phase, Fv/Fm and rETR(max) were only slightly elevated at high CO2‐concentrations, whereas growth was significantly enhanced. After 31 days the Chl a content of the thalli was significantly lower under future conditions and the photosynthesis of thalli grown under preindustrial conditions was not dependent on external carbonic anhydrase. Biomass increased significantly at high CO2‐concentrations. At low CO2‐concentrations most adult thalli disintegrated between day 14 and 21, whereas at high CO2‐concentrations most thalli remained integer until day 31. Thallus disintegration at low CO2‐concentrations was mirrored by a drastic decline in seawater dissolved inorganic carbon and HCO3−. Accordingly, the SWCS differed significantly between the treatments. Our results indicated a slight enhancement of photosynthetic performance and significantly elevated growth of U. lactuca at future CO2‐concentrations. The accelerated thallus disintegration at high CO2‐concentrations under conditions of limited water exchange indicates additional CO2 effects on the life cycle of U. lactuca when living in rockpools.