Low Dissolved Inorganic Carbon Research Articles

The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory

Shale covers about 25% of the land surface, and is therefore an important rock type that consumes CO2 during weathering. We evaluated the potential of gray shale to take up CO2 from the atmosphere by investigating the evolution of dissolved inorganic carbon (DIC) concentrations and its carbon isotopic ratio (δ13CDIC) along water flow paths in a well-characterized critical zone observatory (Susquehanna Shale Hills catchment). In this catchment, chemical weathering in shallow soils is dominated by clay transformation as no carbonates are present, and soil pore waters are characterized by low DIC and pH. In shallow soil porewaters, the DIC, dominated by dissolved CO2, is in chemical and isotopic equilibrium with CO2 in the soil atmosphere where pCO2 varies seasonally to as high as 40 times that of the atmosphere. The degradation of ancient organic matter is negligible in contributing to soil CO2. The chemistry of groundwater varies along different flowpaths as soil pore water recharges to the water table and then dissolves ankerite or secondary calcite under the valley floor. Weathering of carbonate leads to much higher concentrations of DIC (∼2500μmol/L) and divalent cations (Ca2+ and Mg2+) in groundwaters than soil waters. The depth to the ankerite weathering front is hypothesized to be roughly coincident with the water table but it varies due to heterogeneities in the protolith composition. Groundwater chemistry therefore shows different saturation indices with respect to ankerite depending upon location along the valley. The δ13CDIC values of these groundwaters document mixing between the ankerite and soil CO2. The major element concentrations, DIC, and δ13CDIC in the first-order stream incising the valley of the catchment are derived from groundwater and soil waters in proportions that vary both spatially and temporally. The CO2 degassed slightly in the stream but little evidence of C isotopic equilibration with the atmosphere is observed, due to the short length of the stream and short contact time with air.The ankerite reaction front also lies close to the pyrite dissolution front. Pyrite oxidation in bedrock likely released sulfuric acid and played a minor role in the ankerite dissolution, shifting groundwater δ13CDIC slightly above the expected mixing values. At the catchment scale, the stream SO42− is also dominantly derived from wet deposition, as stream has δ34SSO4 values around 3‰, well within the range of acid deposition.A mass balance calculation shows that silicate and ankerite dissolution of the Rose Hill shale at Shale Hills consumes CO2 at a rate of ∼44 and ∼42–48molm−2ky−1 respectively, while degradation of ancient organic matter releases CO2 at a rate of ∼1.3molm−2ky−1. Silicate dissolution at the shallow soils is facilitated by low pH and high soil pCO2. As ankerite dissolution and organic matter oxidation are shown to occur early during shale alteration, CO2 consumption by shale weathering is thus limited by initiation of rock disintegration (e.g., fractures) and exposure of fresh surface area to infiltrating CO2- and O2-rich water.

Ecophysiology of gelatinous Nostoc colonies: unprecedented slow growth and survival in resource-poor and harsh environments.

The cyanobacterial genus Nostoc includes several species forming centimetre-large gelatinous colonies in nutrient-poor freshwaters and harsh semi-terrestrial environments with extended drought or freezing. These Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates. The objective of this review is to evaluate the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources. Simple models predict the decline in uptake of dissolved inorganic carbon (DIC) and a decline in the growth rate of spherical freshwater colonies of N. pruniforme and N. zetterstedtii and sheet-like colonies of N. commune in response to a thicker diffusion boundary layer, lower external DIC concentration and higher organic carbon mass per surface area (CMA) of the colony. Measured growth rates of N. commune and N. pruniforme at high DIC availability comply with general empirical predictions of maximum growth rate (i.e. doubling time 10-14 d) as functions of CMA for marine macroalgae and as functions of tissue thickness for aquatic and terrestrial plants, while extremely low growth rates of N. zetterstedtii (i.e. doubling time 2-3 years) are 10-fold lower than model predictions, either because of very low ambient DIC and/or an extremely costly colony matrix. DIC uptake is limited by diffusion at low concentrations for all species, although they exhibit efficient HCO3(-) uptake, accumulation of respiratory DIC within the colonies and very low CO2 compensation points. Long light paths and light attenuation by structural substances in large Nostoc colonies cause lower quantum efficiency and assimilation number and higher light compensation points than in unicells and other aquatic macrophytes. Extremely low growth and mortality rates of N. zetterstedtii reflect stress-selected adaptation to nutrient- and DIC-poor temperate lakes, while N. pruniforme exhibits a mixed ruderal- and stress-selected strategy with slow growth and year-long survival prevailing in sub-Arctic lakes and faster growth and shorter longevity in temperate lakes. Nostoc commune and its close relative N. flagelliforme have a mixed stress-disturbance strategy not found among higher plants, with stress selection to limiting water and nutrients and disturbance selection in quiescent dry or frozen stages. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range (0-35 °C; maximum at 25 °C). Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.

Positive and negative responses of coral calcification to elevated pCO2: case studies of two coral species and the implications of their responses

Fragments of 2 coral species (Acropora nasuta and Pocillopora damicornis), collected from the South China Sea, were incubated for 94 d under controlled conditions of pCO(2) = 389, 700, and 1214 mu atm. Our incubation experiments showed that the net calcification rate of A. nasuta responded negatively to elevated pCO(2) in both short and mid-term incubations. In contrast, the net calcification rate of P. damicornis increased under elevated pCO(2) during the first 17 d, but then returned to background rates. Based on previous models, our modified models showed that the different responses of these 2 coral species depended on the dissolved inorganic carbon (DIC) and pH levels in coral calcifying fluid. In the studied models, the positive responses of coral calcification to higher pCO(2) could be explained by either low DIC due to a higher photosynthesis rate or a stronger H+ pump. If DIC in calcifying fluid is greatly reduced by photosynthesis, the de creased DIC in calcifying fluid may be benefited by enhanced external seawater CO2, which will partially offset the dissolution due to ocean acidification. Therefore, in addition to our experimental results, our model provides a theoretical basis showing how coral may respond negatively and positively to ocean acidification.

Thermal responses of Symbiodinium photosynthetic carbon assimilation

The symbiosis between hermatypic corals and their dinoflagellate endosymbionts, genus Symbiodinium, is based on carbon exchange. This symbiosis is disrupted by thermally induced coral bleaching, a stress response in which the coral host expels its algal symbionts as they become physiologically impaired. The disruption of the dissolved inorganic carbon (DIC) supply or the thermal inactivation of Rubisco have been proposed as sites of initial thermal damage that leads to the bleaching response. Symbiodinium possesses a highly unusual Form II ribulose bisphosphate carboxylase/oxygenase (Rubisco), which exhibits a lower CO2:O2 specificity and may be more thermally unstable than the Form I Rubiscos of other algae and land plants. Components of the CO2 concentrating mechanism (CCM), which supplies inorganic carbon for photosynthesis, may also be temperature sensitive. Here, we examine the ability of four cultured Symbiodinium strains to acquire and fix DIC across a temperature gradient. Surprisingly, the half-saturation constant of photosynthesis with respect to DIC concentration (K P), an index of CCM function, declined with increasing temperature in three of the four strains, indicating a greater potential for photosynthetic carbon acquisition at elevated temperatures. In the fourth strain, there was no effect of temperature on K P. Finding no evidence for thermal inhibition of the CCM, we conclude that CCM components are not likely to be the primary sites of thermal damage. Reduced photosynthetic quantum yields, a hallmark of thermal bleaching, were observed at low DIC concentrations, leaving open the possibility that reduced inorganic carbon availability is involved in bleaching.

Photorespiration and carbon limitation determine productivity in temperate seagrasses.

The gross primary productivity of two seagrasses, Zostera marina and Ruppia maritima, and one green macroalga, Ulva intestinalis, was assessed in laboratory and field experiments to determine whether the photorespiratory pathway operates at a substantial level in these macrophytes and to what extent it is enhanced by naturally occurring shifts in dissolved inorganic carbon (DIC) and O2 in dense vegetation. To achieve these conditions in laboratory experiments, seawater was incubated with U. intestinalis in light to obtain a range of higher pH and O2 levels and lower DIC levels. Gross photosynthetic O2 evolution was then measured in this pretreated seawater (pH, 7.8–9.8; high to low DIC:O2 ratio) at both natural and low O2 concentrations (adjusted by N2 bubbling). The presence of photorespiration was indicated by a lower gross O2 evolution rate under natural O2 conditions than when O2 was reduced. In all three macrophytes, gross photosynthetic rates were negatively affected by higher pH and lower DIC. However, while both seagrasses exhibited significant photorespiratory activity at increasing pH values, the macroalga U. intestinalis exhibited no such activity. Rates of seagrass photosynthesis were then assessed in seawater collected from the natural habitats (i.e., shallow bays characterized by high macrophyte cover and by low DIC and high pH during daytime) and compared with open baymouth water conditions (where seawater DIC is in equilibrium with air, normal DIC, and pH). The gross photosynthetic rates of both seagrasses were significantly higher when incubated in the baymouth water, indicating that these grasses can be significantly carbon limited in shallow bays. Photorespiration was also detected in both seagrasses under shallow bay water conditions. Our findings indicate that natural carbon limitations caused by high community photosynthesis can enhance photorespiration and cause a significant decline in seagrass primary production in shallow waters.

Calcification and ocean acidification: new insights from the coccolithophore Emiliania huxleyi

Calcification and ocean acidification: new insights from the coccolithophore <i>Emiliania huxleyi</i>

Dissecting the impact of CO2 and pH on the mechanisms of photosynthesis and calcification in the coccolithophore Emiliania huxleyi

Coccolithophores are important calcifying phytoplankton predicted to be impacted by changes in ocean carbonate chemistry caused by the absorption of anthropogenic CO2 . However, it is difficult to disentangle the effects of the simultaneously changing carbonate system parameters (CO2 , bicarbonate, carbonate and protons) on the physiological responses to elevated CO2 . Here, we adopted a multifactorial approach at constant pH or CO2 whilst varying dissolved inorganic carbon (DIC) to determine physiological and transcriptional responses to individual carbonate system parameters. We show that Emiliania huxleyi is sensitive to low CO2 (growth and photosynthesis) and low bicarbonate (calcification) as well as low pH beyond a limited tolerance range, but is much less sensitive to elevated CO2 and bicarbonate. Multiple up-regulated genes at low DIC bear the hallmarks of a carbon-concentrating mechanism (CCM) that is responsive to CO2 and bicarbonate but not to pH. Emilianiahuxleyi appears to have evolved mechanisms to respond to limiting rather than elevated CO2 . Calcification does not function as a CCM, but is inhibited at low DIC to allow the redistribution of DIC from calcification to photosynthesis. The presented data provides a significant step in understanding how E.huxleyi will respond to changing carbonate chemistry at a cellular level.

岩溶区不同植被下土壤水溶解无机碳含量及其稳定碳同位素组成特征

Carbon plays a vital role in karst ecosystem.A proportion of carbon dioxide produced by microbial activities and respiration of roots can be dissolved and carried by the infiltration water and then dissolve carbonate materials in the karst soil system.This process can produce a lot of dissolved inorganic carbon(DIC) into the soil water and groundwater.The stable carbon composition of DIC(δ13CDIC) can be used as a tracer for studying biogeochemical processes in soil water because the potential biogenic,atmospheric and geologic sources of C have distinct δ13C signatures.In order to reveal the characteristics of temporal and spatial variations of DIC fluxes,which can help us to get an insight into the carbonate dissolution and its related carbon cycling,and evaluate the carbon isotopic evolution of soil water in karst areas,soil water derived from different vegetations in Qingmuguan karst areas,Chongqing,SW China,were collected monthly from July,2010 to July,2011,and the DIC concentrations and δ13CDIC values of soil water were analyzed.The results show that:(1) DIC concentrations and δ13CDIC of soil water showed distinct temporal and spatial variations in the study area.DIC concentrations and δ13CDIC values of soil water varied from 21.56 mg/L to 265.35 mg/L,and from-7.73‰ to-20.68‰,respectively.(2) Soil water derived from grass and coniferous forest land had lower DIC concentrations and δ13CDIC values,with a mean value of 59.12 mg/L and-17.22‰,31.47 mg/L and-16.37‰,respectively.The δ13CDIC indicated that these DIC were mainly derived from the dissolution of soil CO2.(3) Soil water derived from the dry land,shrub land and afforestation land had higher DIC concentrations and δ13CDIC values,with the averages of 153.88 mg/L and-12.2‰,221.82 mg/L and-11.9‰,97.30 mg/L and-11.23‰,respectively.Meanwhile,these δ13CDIC values of soil water were generally 4‰—5‰ higher in the rainy season than those in the dry season.Moreover,these δ13CDIC values showed positive correlations with corresponding DIC concentrations.These indicated that DIC in these soil water were primarily derived from dissolution of carbonate materials by soil CO2,suggesting that great carbonate dissolution occurs in the karst soil system which could contributes to the reduction of soil CO2 released to the atmosphere to some extent.Nevertheless,our research also indicated the carbonate was dissolved partly by sulphuric acid which derived from the atmospheric acid precipitation in the rainy season,supported by the stoichiometric analysis,elevated δ13CDIC and δ34S of sulfate in the soil water.Besides,organic acids in the soil system could also facilitate the dissolution of carbonate materials,and result in the elevated δ13CDIC of soil water.Hydrion produced by oxygenation of inorganic nitrogenous fertilizers from agricultural activities also probably participated in the dissolution of carbonate and elevated δ13CDIC of soil water.This study revealed the variations of DIC concentrations and δ13CDIC values in the karst soil water and their influencing factors,which will aid in understanding of the migration and transformation of soil CO2 in karst soil system,especially the reduction effects of soil CO2 released to the atmosphere.However,the accurate estimation of soil CO2 reduction requires further research.

Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from δ13C values of DIC

The cycling of carbon on the Arctic shelves, including outgassing of CO2 to the atmosphere, is not clearly understood. Degradation of terrestrial organic carbon (OCter) has recently been shown to be pronounced over the East Siberian Arctic Shelf (ESAS), i.e. the Laptev and East Siberian Seas, producing dissolved inorganic carbon (DIC). To further explore the processes affecting DIC, an extensive suite of shelf water samples were collected during the summer of 2008, and assessed for the stable carbon isotopic composition of DIC (δ13CDIC). The δ13CDIC values varied between −7.2‰ to +1.6‰ and strongly deviated from the compositions expected from only mixing between river water and seawater. Model calculations suggest that the major processes causing these deviations from conservative mixing were addition of (DIC) by degradation of OCter, removal of DIC during primary production, and outgassing of CO2. All waters below the halocline in the ESAS had δ13CDIC values that appear to reflect mixing of river water and seawater combined with additions of on average 70±20μM of DIC, originating from degradation of OCter in the coastal water column. This is of the same magnitude as the recently reported deficits of DOCter and POCter for the same waters. The surface waters in the East Siberian Sea had higher δ13CDIC values and lower DIC concentrations than expected from conservative mixing, consistent with additions of DIC from degradation of OCter and outgassing of CO2. The outgassing of CO2 was equal to loss of 123±50μM DIC. Depleted δ13CPOC values of −29‰ to −32‰ in the mid to outer shelf regions are consistent with POC from phytoplankton production. The low δ13CPOC values are likely due to low δ13CDIC of precursor DIC, which is due to degradation of OCter, rather than reflecting terrestrial input compositions. Overall, the δ13CDIC values confirm recent suggestions of substantial degradation of OCter over the ESAS, and further show that a large part of the CO2 produced from degradation has been outgassed to the atmosphere.

Effects of inorganic carbon concentration on carbon formation, nitrate utilization, biomass and oil accumulation of Nannochloropsis oculata CS 179

This investigation examined the effects of the inorganic carbon concentration (4, 0.8 and 0g/L NaHCO3) on the carbon formation, nitrate utilization, growth and fatty acids compositions of Nannochloropsis oculata. The dissolved inorganic carbon (DIC) concentration in the three treatments decreased sharply during the first 6days, and the percentage of dissolved organic carbon (DOC) (% of total organic carbon (TOC)) decreased with the depletion of the DIC. The NO3- assimilation of the algae was correlated with the DIC concentration. The algae in the highest DIC treatment had the highest specific grow rate (0.0843d−1) (P<0.0001), and their biomass and fatty acid methyl esters (FAME) productivity were 84.00 and 9.69mg/L/d, respectively (P<0.0001). Contents of C16 and C18 series (% of FAME) were high and the C16:0 increased with the decrease of C18:1 during the cultivation. The iodine value (IV) of the algae was low at the low DIC media.

Effects of dissolved inorganic carbon on competition of the bloom-forming cyanobacterium Microcystis aeruginosa with the green alga Chlamydomonas microsphaera

The availability of dissolved inorganic carbon (DIC) may have a considerable impact on species competition in phytoplankton communities. The growth, photosynthetic characteristics and competition of three strains, the toxic Microcystis aeruginosa FACHB 912, nontoxic M. aeruginosa FACHB 469 and Chlamydomonas microsphaera FACHB 52, were investigated under two different DIC concentrations (0.365 and 7.658 mmol l−1 KHCO3). In monoculture, DIC concentration did not affect the specific growth rates of any of the three strains. However, when grown in mixed culture with C. microsphaera, both toxic and nontoxic Microcystis strains showed increased percentages under low DIC concentration but decreased percentages under high DIC concentration. After 12 days’ mixed culture, the percentage of M. aeruginosa FACHB 912 or FACHB 469 decreased by 9−22% in high DIC medium, but increased by 6–11% in low DIC medium. Low DIC concentration decreased the cell size, cellular chlorophyll fluorescence and photosynthetic capacity of C. microsphaera, but it had little effect on those of M. aeruginosa FACHB 912 and FACHB 469. The apparent photosynthetic affinities for DIC were significantly higher in the two M. aeruginosa strains than in C. microsphaera. From the higher DIC affinity, more efficient photosynthetic capacity and increased percentages in competition experiments under low DIC concentration, it can be concluded that the bloom-forming M. aeruginosa possesses a competitive advantage in DIC-limited conditions.

Carbon system changes in the East China Sea induced by Typhoons Tina and Winnie in 1997

A simple chemical scheme was introduced into the Meteorological Research Institute Community Ocean Model (MRI.COM) for investigating the rapid CO2 flux increase at a moored buoy in the East China Sea caused by the passages of Typhoons Tina and Winnie in 1997. MRI.COM well reproduced variations in the observed water temperature and pCO2sea at 1 m depth and in the air‐sea CO2 flux at the buoy. Even though the analyzed magnitudes of the air‐sea CO2 fluxes were of the same order, the sudden CO2 flux change occurred nearly 2 days after Tina and nearly 3 days after Winnie. The increase in pCO2sea normalized to a temperature of 29°C induced by Winnie appeared longer than that induced by Tina. The numerical simulation results showed clear differences in the oceanic response between the two typhoons. In the case of Tina, upwelling along the continental shelf played a crucial role in rapidly decreasing the water temperature and transporting seawater with low pCO2sea and high dissolved inorganic carbon (DIC) to the upper ocean. In contrast, water with relatively high pCO2sea and low DIC transported by strong horizontal advection of the Kuroshio Current was vertically mixed by the passage of Winnie, resulting in a cool, deep mixed layer at the buoy. The surface seawater hydrogen ion concentration (pH) temporarily decreased during the passages of Tina and Winnie. In the case of Tina, pH decreased by at most 0.03 and for only 1 day.

Δ14C and Δ13C of Seawater DIC as Tracers of Coastal Upwelling: A 5-Year Time Series from Southern California

Marine radiocarbon (14C) is a widely used tracer of past ocean circulation, but very few high-resolution records have been obtained. Here, we report a time series of carbon isotope abundances of dissolved inorganic carbon (DIC) in surface seawater collected from the Newport Beach pier in Orange County, within the Southern California Bight, from 2005 to 2010. Surface seawater was collected bimonthly and analyzed for Δ14C, δ13C, and salinity. Results from May 2005 to November 2010 show no long-term changes in δ13C DIC values and no consistent variability that can be attributed to upwelling. Δ14C DIC values have lowered from ∼34‰ to about ∼16‰, an 18‰ decrease from the beginning of this project in 2005, and is consistent with the overall 14C depletion from the atmospheric thermonuclear bomb pulse at the end of the 1950s. Δ14C DIC values, paired with salinity, do appear to be suitable indicators of upwelling strength with periods of upwelling characterized by more saline and lower DIC Δ14C values. However, a similar signal was not observed during the strong upwelling event of 2010. These results were obtained in the Southern California Bight where upwelling is fairly weak and there is a complex occanographic circulation in comparison with the remaining western USA coastline. It is therefore likely that the link between DIC Δ14C, salinity, and upwelling would be even stronger at other sites. These data represent the longest time series of Δ14C data from a coastal Southern California site performed to date.

The carbonate system in the East China Sea in winter

Measurements of dissolved inorganic carbon (DIC), pH, total alkalinity (TA), and partial pressure of CO 2 (pCO 2) were conducted at a total of 25 stations along four cross shelf transects in the East China Sea (ECS) in January 2008. Results showed that their distributions in the surface water corresponded well to the general circulation pattern in the ECS. Low DIC and pCO 2 and high pH were found in the warm and saline Kuroshio Current water flowing northeastward along the shelf break, whereas high DIC and pCO 2 and low pH were mainly observed in the cold and less saline China Coastal Current water flowing southward along the coast of Mainland China. Difference between surface water and atmospheric pCO 2 (ΔpCO 2), ranging from ~ 0 to − 111 μatm, indicated that the entire ECS shelf acted as a CO 2 sink during winter with an average flux of CO 2 of −13.7 ± 5.7 (mmol C m − 2 day − 1 ), and is consistent with previous studies. However, pCO 2 was negatively correlated with temperature for surface waters lower than 20 °C, in contrast to the positive correlation found in the 1990s. Moreover, the wintertime ΔpCO 2 in the inner shelf near the Changjiang River estuary has appreciably decreased since the early 1990s, suggesting a decline of CO 2 sequestration capacity in this region. However, the actual causes for the observed relationship between these decadal changes and the increased eutrophication over recent decades are worth further study.

Controls on the origin and cycling of riverine dissolved inorganic carbon in the Brazos River, Texas

Rivers draining watersheds that include carbonate bedrock or organic matter (OM)-rich sedimentary rocks frequently have 14C-depleted dissolved inorganic carbon (DIC) relative to rivers draining carbonate- and OM-free watersheds, due to dissolution of carbonate and/or decomposition of ancient OM. However, our results from a subtropical river, the Brazos River in Texas, USA, show that in this watershed human activities appear to dominate basin lithology in controlling the origin and metabolism of DIC. The middle Brazos flows through limestone and coal-bearing bedrock, but DIC isotope data suggest no limestone dissolution or respiration of ancient OM, and instead reflect efficient air–water CO2 exchange, degradation of relatively young OM and photosynthesis in the river as a result of river damming and urban treated wastewater input. The lower Brazos drains only small areas of carbonate and coal-bearing bedrock, but DIC isotope data suggest the strong influence of carbonate dissolution, with a potentially minor contribution from decomposition of old soil organic matter (SOM). Oyster shells and crushed carbonate minerals used in road construction are likely sources of carbonate in the lower Brazos, in addition to natural marl and pedogenic carbonate. Additionally, the generally low pCO2 and high DIC concentration in the Brazos River lead to a low CO2 outgassing:DIC export ratio, distinguishing the Brazos River from other rivers.

Surface CO 2 parameters and air–sea CO 2 flux distribution in the eastern equatorial Atlantic Ocean

Six cruises have been carried out in the eastern equatorial Atlantic between June 2005 and September 2007 to study the distribution of the surface total alkalinity (TA) and dissolved inorganic carbon (DIC). During these cruises a strong north–south gradient is observed in sea surface temperature (SST), sea surface salinity (SSS), DIC and TA. Low surface salinity waters are associated with low TA and low DIC. They are observed north of the equator (0°–6°N) in the Guinea current, whereas higher concentrations are observed south of the equator. TA–SSS and DIC–SST–SSS relationships are determined and are used to calculate surface fCO 2. Monthly maps of fCO 2 are obtained, on a one degree grid, using SST and SSS fields for the region 10°W–10°E, 6°N–10°S from June to November. The monthly mean flux of CO 2 ranges from 1.69 ± 1.94 mmol m − 2 day − 1 in November to 2.78 ± 1.65 mmol m − 2 day − 1 in August over the region. Using the same gas exchange coefficient, the CO 2 flux was calculated with the climatological ΔfCO 2. Our estimates show a stronger gradient between northern and southern waters than the climatology, probably due to a lack of data in the climatology as well as its 4° latitude by 5° longitude resolution.

Effect of Cl− on photosynthetic bicarbonate uptake in two cyanobacteria Microcystis aeruginosa and Synechocystis PCC6803

Photosynthetic inorganic carbon utilization was investigated in two cyanobacteria Microcystis aeruginosa and Synechocystis PCC6803 grown in standing culture. Photosynthetic rates for the two algae reached about 10 times the theoretical CO2 supply rate at low dissolved inorganic carbon (DIC) of 100 μmol/L, and the rates were unaffected by the addition of 20 mmol/L Na+, indicating that the two algae possessed Na+-independent HCO 3 − utilization for photosynthesis under low DIC. Their photosynthetic rates at low DIC were inhibited by higher Cl− and the degrees of inhibition were increased with the rise of Cl− concentration, and in the presence of Diphenylamine-2-carboxylate (DPC), a reported Cl− channel inhibitor, the rates decreased by 74%–82%, implying that putative DPC-sensitive Cl− channels participate in Na+-independent HCO 3 − uptake for photosynthesis. The experiment of intracellular 14C-DIC accumulation for photosynthesis showed that internal DIC pools decreased by about 80% with 200 μmol/L DPC and by 64%–70% with 100 mmol/L Cl−. The experiment of chlorophyll a fluorescence quenching showed that initial rates and extents of fluorescence quenching obviously decreased with 200 μmol/L DPC or 100 mmol/L Cl−. The two experiments gave further evidence that putative DPC-sensitive Cl− channels participate in Na+-independent HCO 3 − uptake for photosynthesis in the two algae grown in standing culture.

Episodic riverine influence on surface DIC in the coastal Gulf of Maine

Anomalously high precipitation and river discharge during the spring of 2005 caused considerable freshening and depletion of dissolved inorganic carbon (DIC) in surface waters along the coastal Gulf of Maine. Surface pCO 2 and total alkalinity (TA) were monitored by repeated underway sampling of a cross-shelf transect in the western Gulf of Maine (GOM) during 2004–05 to examine how riverine fluxes, mixing, and subsequent biological activity exert control on surface DIC in this region. Most of the variability in surface DIC concentration was attributable to mixing of low DIC river water with higher DIC, saline GOM waters, but net biological uptake of DIC was significant especially during the spring and summer seasons. The extent and persistence of the coastal freshwater intrusion exerted considerable influence on net carbon dynamics. Integrated over the 10-m surface layer of our study region (∼5 × 10 4 km 2), net biological DIC uptake was 0.48 × 10 8 mol C during April–July of 2004 compared to 1.33 × 10 8 mol C during April–July of 2005. We found the temporal signature and magnitude of DIC cycling to be different in adjacent plume-influenced and non-plume regions. Extreme events such as the freshwater anomaly observed in 2005 will affect mean estimates of coastal carbon fluxes, thus budgets based on short time series of observations may be skewed and should be viewed with caution.

Growth limitation due to high pH and low inorganic carbon concentrations in temperate species of the dinoflagellate genus Ceratium

The effects of high pH and low DIC (dissolved inorganic carbon) concentrations on growth rates of the dinoflagellate Ceratium were studied using 2 different approaches: (1) pH drift experiments, in which both pH and DIC change during the experiment, and (2) constant pH and DIC experiments at a range of fixed pH and DIC levels. The upper pH limits for growth were between 8.7 and 9.1 for the 3 species tested (C. furca, C. tripos and C. fusus) and thus within an environmentally relevant pH range. Hence, for Ceratium spp. and other dinoflagellates with upper pH tolerances for growth below or around 9, high pH will most likely periodically be a limiting factor in coastal areas. Different upper pH limits for growth were found using either the pH drift or the constant pH methods for the larger Ceratium species, but, for the smaller species, the 2 methods provided similar results. The results therefore suggest a combined pH/DIC limitation for large species such as C. tripos and C. furca in natural waters.

Submersed macrophyte growth at low pH: carbon source influences response to dissolved inorganic carbon enrichment

Summary1. Twenty‐one submersed macrophyte species were grown in the greenhouse at low and high dissolved inorganic carbon (DIC) concentrations and at pH 6 to test the hypothesis that growth response to DIC enrichment in the water column would be greatest for species able to use both bicarbonate (HCO) and free CO2 from the water column, intermediate for species restricted to free CO2 in the water column, and least for those species able to use free CO2 from both sediment porewater and the water column.2. Relative growth rates (RGR) increased significantly with DIC enrichment for 12 of the 21 species. Bicarbonate users as a group averaged a 4.6‐fold increase in RGR with DIC enrichment, significantly greater than the 2.5‐ and 1.7‐fold increases of the water column CO2 users and sediment CO2 users, respectively.3. The ability to use HCO, as measured by a pH drift technique, was positively correlated with the ratio of RGR at high DIC to RGR at low DIC for the 21 species.4. Bicarbonate users also exhibited significantly lower belowground : total biomass (R : T) ratios than the other two groups, and alone showed a significant increase in R : T ratio with DIC enrichment.5. Our results indicate that DIC availability may influence community structure in acidic and circumneutral lakes.