Posts Tagged 'biogeochemistry'

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

Highlights

• Temperature dominated seawater carbonate system variations in FGBNMS.
• Subsurface acidification is caused by anthropogenic CO2 uptake and higher respiration.
• The FGBNMS is currently a negligible atmospheric CO2 sink.

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 (<5 m) total alkalinity (TA) and total dissolved inorganic carbon (DIC) vary over small ranges (2391 ± 19 μmol kg−1 and 2060 ± 19 μmol kg−1, respectively) for all times-series samples. Temperature and salinity both played an important role in controlling the surface water carbonate system dynamics, although temperature was the sole significant factor when there was no flooding. The FGBNMS area acted as a sink for atmospheric CO2 in winter and a CO2 source in summer, while the time-integrated CO2 flux is close to zero (−0.14 ± 1.96 mmol-C m−2 yr−1). Results from three cruises, i.e., the Gulf of Mexico and East Coast Carbon Project (GOMECC-1) in 2007, the rapid response study, and the Gulf of Mexico Ecosystems and Carbon Cruise (GOMECC-3), revealed decreases in both pH and saturation state with respect to aragonitearag) in subsurface waters (~100–250 m) over time. These decreases are larger than those observed in other tropical and subtropical waters. Based on reaction stoichiometry, calculated anthropogenic CO2 contributed 30–41% of the overall DIC increase, while elevated respiration accounted for the rest.

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

Impacts of shifts in phytoplankton community on clouds and climate via the sulfur cycle

Dimethyl sulfide (DMS), primarily produced by marine organisms, contributes significantly to sulfate aerosol loading over the ocean after being oxidized in the atmosphere. In addition to exerting a direct radiative effect, the resulting aerosol particles act as cloud condensation nuclei, modulating cloud properties and extent, with impacts on atmospheric radiative transfer and climate. Thus, changes in pelagic ecosystems, such as phytoplankton physiology and community structure, may influence organosulfur production, and subsequently affect climate via the sulfur cycle. A fully coupled Earth system model, including explicit marine ecosystems and the sulfur cycle, is used here to investigate the impacts of changes associated with individual phytoplankton groups on DMS emissions and climate. Simulations show that changes in phytoplankton community structure, DMS production efficiency, and interactions of multielement biogeochemical cycles can all lead to significant differences in DMS transfer to the atmosphere. Subsequent changes in sulfate aerosol burden, cloud condensation nuclei number, and radiative effect are examined. We find the global annual mean cloud radiative effect shifts up to 0.21 W/m2, and the mean surface temperature increases up to 0.1 °C due to DMS production changes associated with individual phytoplankton group in simulations with radiative effects at the 2,100 levels under an 8.5 scenario. However, changes in DMS emissions, radiative effect, and surface temperature are more intensive on regional scales. Hence, we speculate that major uncertainties associated with future marine sulfur cycling will involve strong region‐to‐region climate shifts. Further understanding of marine ecosystems and the relevant phytoplankton‐aerosol‐climate linkage are needed for improving climate projections.

Continue reading ‘Impacts of shifts in phytoplankton community on clouds and climate via the sulfur cycle’

Influences of coral genotype and seawater pCO2 on skeletal Ba/Ca and Mg/Ca in cultured massive Porites spp. corals

Highlights

• KD Ba/Ca vary significantly between massive Porites spp. coral genotypes.
• Seawater pCO2 affects KD Ba/Ca significantly in 1 of 3 coral genotypes.
• KD Mg/Ca varies significantly between some duplicates of the same coral.

Abstract

Coral skeletal Ba/Ca is a proxy for seawater Ba/Ca, used to infer oceanic upwelling and terrigenous runoff while [Mg2+] is implicated in the control of coral biomineralisation. We cultured large individuals (>12 cm diameter) of 3 genotypes of massive adult Porites spp. corals over a range of seawater pCO2 to test how atmospheric CO2 variations affect skeletal Ba/Ca and Mg/Ca. We identified the skeleton deposited after a 5 month acclimation period and analysed the skeletal Ba/Ca and Mg/Ca by secondary ion mass spectrometry. Skeletal Mg/Ca varies significantly between some duplicate colonies of the same coral genotype hampering identification of genotype and seawater pCO2 effects. Coral aragonite:seawater Ba/Ca partition coefficients (KD Ba/Ca) do not vary significantly between duplicate colonies of the same coral genotype. We observe large variations in KD Ba/Ca between different massive Porites spp. coral genotypes irrespective of seawater pCO2. These variations do not correlate with coral calcification, photosynthesis or respiration rates or with skeletal KD Mg/Ca or KD Sr/Ca. Seawater pCO2 does not significantly affect KD Ba/Ca in 2 genotypes but KD Ba/Ca is significantly higher at 750 μatm seawater pCO2 than at 180 μatm in 1 P. lutea genotype. Genotype specific variations in KD Ba/Ca between different Porites spp. could yield large errors (~250%) in reconstructions of seawater Ba when comparing Ba/Ca between corals. Analysis of fossil coral specimens deposited at low seawater pCO2, may underestimate past seawater Ba/Ca and ocean upwelling/freshwater inputs when compared with modern specimens but the effect is small in comparison with the observed difference between coral genotypes.

Continue reading ‘Influences of coral genotype and seawater pCO2 on skeletal Ba/Ca and Mg/Ca in cultured massive Porites spp. corals’

Temporal and spatial fluctuations of groundwater-derived alkalinity fluxes to a semiarid coastal embayment

Highlights

• SGD rates vary spatially and seasonally.
• Fluxes of TA, DIC and DOC are driven by both SGD rates and porewater chemistry.
• Production of TA in summer, fall and winter is driven by SGD and denitrification.
• Consumption of TA, in spring, is related to CaCO3 precipitation.
• SGD and autochthonous biogeochemical process offset TA-salinity relationships.

Abstract

We conducted a comprehensive analysis of a variety of geochemical data including total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), major ions, stable isotopes, and submarine groundwater discharge, to understand biogeochemical and hydrologic processes driving the seasonal to annual estuarine buffering capacity in Nueces Bay, Texas. These measurements, together with statistical analysis and geochemical modeling, show large variability of freshwater influence. TA consumption, common to spring seasons, was mainly driven by CaCO3 precipitation and, to some extent, by aerobic respiration. TA production occurred in some parts of the bay during summer, fall and winter, likely driven by denitrification. CaCO3 dissolution is stimulated by input of undersaturated river waters following significant flooding events. Since consumption and production of TA was not necessarily associated with different salinity zones, SGD, identified to be significant year-round, likely offsets the effects of salinity changes. Net DIC and TA fluxes exceeded dissolved organic carbon flux by an order of magnitude, except for winter 2014 when it was in the same order of magnitude. In addition to generally larger SGD rates when compared to other studies, production of TA (DIC and DOC) in the bottom sediments, as observed in this study, leads to larger fluxes, especially for the driest season (winter 2014), in the mid-bay area (6.27 · 106 μM m−2 d−1). Consistently larger inputs occur along the shoreline stations (6.14 · 106 μM m−2 d−1) following the flood recession, when compared to mid-bay (1.26 · 106 μM m−2 d−1) and are associated with lower SGD following the summer 2015 flooding. This study demonstrates that the carbonate chemistry of estuaries in semiarid areas is affected by non-conservative processes because of seasonal variability of hydroclimatic conditions.

Continue reading ‘Temporal and spatial fluctuations of groundwater-derived alkalinity fluxes to a semiarid coastal embayment’

Effect of organic Fe-ligands, released by Emiliania huxleyi, on Fe(II) oxidation rate in seawater under simulated ocean acidification conditions: a modeling approach

The potential effect of ocean acidification on the exudation of organic matter by phytoplankton and, consequently, on the iron redox chemistry is largely unknown. In this study, the coccolithophorid Emiliania huxleyi was exposed to different pCO2 conditions (225–900 μatm), in order to determine the role of natural organic ligands on the Fe(II) oxidation rate. Oxidation kinetics of Fe(II) were studied as a function of pH (7.75–8.25) and dissolved organic carbon levels produced (0–141.11 μmol C L−1) during the different growth stages. The Fe(II) oxidation rate always decreased in the presence of exudates as compared to that in the exudates-free seawater. The organic ligands present in the coccolithophorid exudates were responsible for this decrease. The oxidation of Fe(II) in artificial seawater was also investigated at nanomolar levels over a range of pH (7.75–8.25) at 25°C in the presence of different glucuronic acid concentrations. Dissolved uronic acids (DUA) slightly increased the experimental rate compared to control artificial seawater (ASW) which can be ascribed to the stabilization of the oxidized form by chelation. This behavior was a function of the Fe(II):DUA ratio and was a pH dependent process. A kinetic model in ASW, with a single organic ligand, was applied for computing the equilibrium constant (log KFeCHO+ = 3.68 ± 0.81 M−1) and the oxidation rate (log kFeCHO+ = 3.28 ± 0.41 M−1 min−1) for the Fe(II)-DUA complex (FeCHO+), providing an excellent description of data obtained over a wide range of DUA concentrations and pH conditions. Considering the Marcus theory the Fe(III) complexing constant with DUA was limited to between 1013 and 1016. For the seawater enriched with exudates of E. huxleyi a second kinetic modeling approach was carried out for fitting the Fe(II) speciation, and the contribution of each Fe(II) species to the overall oxidation rate as a function of the pH/pCO2 conditions. The influence of organic ligands in the Fe(II) speciation diminished as pH decreased in solution. During the stationary growth phase, the FeCHO+ complex became the most important contributor to the overall oxidation rate when pH was lower than 7.95. Because CO2 levels modify the composition of excreted organic ligands, the redox behavior of Fe in solution may be affected by future acidification conditions.

Continue reading ‘Effect of organic Fe-ligands, released by Emiliania huxleyi, on Fe(II) oxidation rate in seawater under simulated ocean acidification conditions: a modeling approach’

Short-term variability of carbon chemistry in two contrasting seagrass meadows at Dongsha Island: implications for pH buffering and CO2 sequestration

The diurnal cycles of carbon chemistry parameters, i.e., dissolved inorganic carbon (DIC), total alkalinity (TA), partial pressure of CO2 (pCO2), and pH, were investigated in two hydrodynamically contrasting seagrass meadows at Dongsha Island in the northern South China Sea in August 2015. The results show that the pH and TA were higher and that the pCO2 was lower in the semi-enclosed inner lagoon (IL) than on the open north shore (NS). The analyses of carbon chemistry parameters vs. dissolved oxygen and TA vs. DIC relationships reveal that the CO2 dynamics was dominated by photosynthesis/respiration (P/R) alone on the NS but by the combined effect of P/R and sedimentary anaerobic pathways in the IL. We suggest that the observed divergent behaviors in carbon chemistry between the two sites could be attributed to differences in hydrodynamic regimes. The less energetic hydrodynamics and longer residence time in the IL would be favorable for the occurrence of sedimentary anaerobic TA generation and the subsequent TA accumulation in the overlying waters. The elevated TA may lead to a pH increase and a pCO2 decrease, thus providing a buffering effect against ocean acidification (OA) and enhancing atmospheric CO2 sequestration at local scales. The present results demonstrate that hydrodynamic regime may play an important role in regulating biogeochemical processes in seagrass meadows, and thereby modulating their capacities in OA buffering and CO2 uptaking.

Continue reading ‘Short-term variability of carbon chemistry in two contrasting seagrass meadows at Dongsha Island: implications for pH buffering and CO2 sequestration’

Responses of the large centric diatom Coscinodiscus sp. to interactions between warming, elevated CO2, and nitrate availability

Marine ecosystems are facing multiple anthropogenic global changes, including ocean acidification, warming, and reduced nutrient supplies. Together, these will challenge phytoplankton including large centric diatoms such as Coscinodiscus sp., a group that is important to ocean food webs and carbon export. We investigated the interactive effects of warming, elevated CO2, and nitrate availability on Coscinodiscus growth, elemental stoichiometry, and Fe and C uptake rates in a four‐treatment factorial experiment combining two CO2 levels (∼400 ppm and 800 ppm) and two temperatures (16°C and 20°C) across seven nitrate concentrations (1–100 μmol L−1). Higher temperatures led to higher maximum growth rates (μmax), but also higher half‐saturation constants for nitrate (K1/2), while elevated CO2 increased K1/2 only at the warmer temperature. Lower μmax/K1/2 ratios under warming and rising CO2 indicated a higher nitrate requirement at these conditions. High temperature decreased cellular P and Si contents and consequently increased N : P and C : Si ratios, especially at ambient CO2. Fe : C uptake ratios responded positively to lower nitrate levels, lower CO2, and warming. Significant interactions between nitrate availability and temperature or CO2 were observed for specific growth rates, chlorophyll a and Si contents, Fe : C, N : P, and Si : C, while temperature and CO2 interactions were only significant for μmax/K1/2 and cellular P content. The mutual interactions among CO2 concentrations, temperature, and nitrate supply may all affect future growth, physiology, and carbon export by Coscinodiscus sp., however, in general warming and nitrate availability appear to be more influential than CO2.

Continue reading ‘Responses of the large centric diatom Coscinodiscus sp. to interactions between warming, elevated CO2, and nitrate availability’


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Ocean acidification in the IPCC AR5 WG II

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