Posts Tagged 'biogeochemistry'

What drives the latitudinal gradient in open-ocean surface dissolved inorganic carbon concentration?

Previous work has not led to a clear understanding of the causes of spatial pattern in global surface ocean dissolved inorganic carbon (DIC), which generally increases polewards. Here, we revisit this question by investigating the drivers of observed latitudinal gradients in surface salinity-normalized DIC (nDIC) using the Global Ocean Data Analysis Project version 2 (GLODAPv2) database. We used the database to test three different hypotheses for the driver producing the observed increase in surface nDIC from low to high latitudes. These are (1) sea surface temperature, through its effect on the CO2 system equilibrium constants, (2) salinity-related total alkalinity (TA), and (3) high-latitude upwelling of DIC- and TA-rich deep waters. We find that temperature and upwelling are the two major drivers. TA effects generally oppose the observed gradient, except where higher values are introduced in upwelled waters. Temperature-driven effects explain the majority of the surface nDIC latitudinal gradient (182 of the 223 µmol kg−1 increase from the tropics to the high-latitude Southern Ocean). Upwelling, which has not previously been considered as a major driver, additionally drives a substantial latitudinal gradient. Its immediate impact, prior to any induced air–sea CO2 exchange, is to raise Southern Ocean nDIC by 220 µmol kg−1 above the average low-latitude value. However, this immediate effect is transitory. The long-term impact of upwelling (brought about by increasing TA), which would persist even if gas exchange were to return the surface ocean to the same CO2 as without upwelling, is to increase nDIC by 74 µmol kg−1 above the low-latitude average.

Continue reading ‘What drives the latitudinal gradient in open-ocean surface dissolved inorganic carbon concentration?’

Dynamics of benthic metabolism, O2, and pCO2 in a temperate seagrass meadow

Seagrass meadows play an important role in “blue carbon” sequestration and storage, but their dynamic metabolism is not fully understood. In a dense Zostera marina meadow, we measured benthic O2 fluxes by aquatic eddy covariance, water column concentrations of O2, and partial pressures of CO2 (pCO2) over 21 full days during peak growing season in April and June. Seagrass metabolism, derived from the O2 flux, varied markedly between the 2 months as biomass accumulated and water temperature increased from 16°C to 28°C, triggering a twofold increase in respiration and a trophic shift of the seagrass meadow from being a carbon sink to a carbon source. Seagrass metabolism was the major driver of diurnal fluctuations in water column O2 concentration and pCO2, ranging from 173 to 377 μmol L−1 and 193 to 859 ppmv, respectively. This 4.5‐fold variation in pCO2 was observed despite buffering by the carbonate system. Hysteresis in diurnal water column pCO2 vs. O2 concentration was attributed to storage of O2 and CO2 in seagrass tissue, air–water exchange of O2 and CO2, and CO2 storage in surface sediment. There was a ~ 1:1 mol‐to‐mol stoichiometric relationship between diurnal fluctuations in concentrations of O2 and dissolved inorganic carbon. Our measurements showed no stimulation of photosynthesis at high CO2 and low O2 concentrations, even though CO2 reached levels used in IPCC ocean acidification scenarios. This field study does not support the notion that seagrass meadows may be “winners” in future oceans with elevated CO2 concentrations and more frequent temperature extremes.

Continue reading ‘Dynamics of benthic metabolism, O2, and pCO2 in a temperate seagrass meadow’

Seasonal interactive effects of pCO2 and irradiance on the ecophysiology of brown macroalga Fucus vesiculosus L.

Stochastic upwelling of seawater in the Baltic Sea from the deep, anoxic bottoms may bring low-pH water rich in CO2 close to the surface. Such events may become more frequent with climate change and ongoing ocean acidification (OA). Photoautotrophs, such as macroalgae, which are important foundation species, have been proposed to benefit from increased carbon availability due to reduced energetic cost in carbon acquisition. However, the exact effects of CO2 fertilization may depend on the ambient light environment, as photosynthesis rates depend on available irradiance. In this experimental study, interacting effects of CO2 addition and irradiance on the habitat-forming macroalga Fucus vesiculosus were investigated during two seasons – winter and summer – in the northern Baltic Sea. Growth rates remained unaffected by CO2 or irradiance during both seasons, suggesting that direct effects of elevated CO2 on mature F. vesiculosus are small. Increases in CO2 affected algal elemental ratios by increasing carbon and decreasing nitrogen content, with resulting changes in the C:N ratio, but only in winter. In summer, chlorophyll a content increased under low irradiance. Increases in CO2 caused a decline in light-harvesting efficiency (decrease in Fv/Fm and α) under high irradiance in summer, and conversely increased α under low irradiance. High irradiance caused increases in the maximum relative electron transport rate (rETRmax) in summer, but not in winter. Differences between winter and summer indicate that F. vesiculosus responses to CO2 and irradiance are season-specific. Increases in carbon content during winter could indicate slightly positive effects of CO2 addition in the long run if the extra carbon gained may be capitalized in growth. The results of this study suggest that increases in CO2, either through upwelling or OA, may have positive effects on F. vesiculosus, but these effects are probably small.

Continue reading ‘Seasonal interactive effects of pCO2 and irradiance on the ecophysiology of brown macroalga Fucus vesiculosus L.’

Spatiotemporal variability in seawater carbonate chemistry at two contrasting reef locations in Bocas del Toro, Panama

There is a growing concern for how coral reefs may fare in a high-CO2 world. The majority of laboratory and mesocosm experiments have revealed negative effects on the growth and calcification of reef builders exposed to elevated CO2 conditions. However, coral reefs are highly dynamic systems and the interplay between different biogeochemical and physical processes on reefs results in large variability of seawater carbonate chemistry on different functional scales. This can create localized seawater conditions that can either enhance or alleviate the effects of ocean acidification (OA). Consequently, in order to predict how coral reef ecosystems may respond to OA in the future, it is necessary to first establish a baseline of natural carbonate chemistry conditions. This includes characterizing the range and variability of carbonate chemistry and the physical and biogeochemical controls across a broad range of environments over both space and time. Here, we have characterized the spatial and temporal physiochemical variability of two contrasting coral reef locations in Bocas del Toro, Panama that differed in their benthic community composition, reef morphology, and exposure to open ocean conditions, using a combination of research approaches including stationary autonomous sensors and spatial surveys during the month of November 2015. Mean and diurnal temporal variability in both physical and chemical seawater parameters were remarkably similar between sites and sampling depths, although, the magnitude of spatial variability was quite different between the sites. Spatial gradients in physiochemical parameters at Punta Caracol reflected the cumulative modification from terrestrial runoff and benthic metabolism. Based on graphical vector analysis of salinity normalized TA-DIC data, reef metabolism was dominated by organic carbon cycling over inorganic carbon cycling at both sites, where the outer reef reflected net heterotrophy likely owing to remineralization of organic matter from terrestrial inputs. Altogether, the results of this study highlight the strong influence of terrigenous runoff on reef metabolism and seawater chemistry conditions and demonstrate the importance of considering external inputs of alkalinity in reefs when interpreting TA-DIC data in systems with large freshwater inputs. Predicting future changes to coral reef ecosystems requires an understanding of the natural complexity of these systems in which various physical, ecological and biogeochemical drivers interact creating large variability in seawater chemistry over space and time.

Continue reading ‘Spatiotemporal variability in seawater carbonate chemistry at two contrasting reef locations in Bocas del Toro, Panama’

The effect of Agulhas eddies on absorption and transport of anthropogenic carbon in the South Atlantic Ocean

The South Atlantic Ocean is currently undergoing significant alterations due to climate change. This region is important to the global carbon cycle, but marine carbon data are scarce in this basin. Additionally, this region is influenced by Agulhas eddies. However, their effects on ocean biogeochemistry are not yet fully understood. Thus, we aimed to model the carbonate parameters in this region and investigate the anthropogenic carbon (Cant) content in 13 eddies shed by the Agulhas retroflection. We used in situ data from the CLIVAR/WOCE/A10 section to elaborate total dissolved inorganic carbon (CT) and total alkalinity (AT) models and reconstruct those parameters using in situ data from two other Brazilian initiatives. Furthermore, we applied the Tracer combining Oxygen, inorganic Carbon, and total Alkalinity (TrOCA) method to calculate the Cant, focusing on the 13 identified Agulhas eddies. The CT and AT models presented root mean square errors less than 1.66 and 2.19 μmol kg−1, indicating Global Ocean Acidification Observing Network climate precision. The Cant content in the Agulhas eddies was 23% higher than that at the same depths of the surrounding waters. We observed that Agulhas eddies can play a role in the faster acidification of the South Atlantic Central Water.

Continue reading ‘The effect of Agulhas eddies on absorption and transport of anthropogenic carbon in the South Atlantic Ocean’

Characteristics of the carbonate system in a semiarid estuary that experiences summertime hypoxia

In oceanic environments, two sources of CO2 have been found to contribute to acidification of stratified water bodies, i.e., CO2 invasion due to anthropogenic atmospheric CO2 increase and respiration-produced CO2 from organic matter remineralization. Acidification caused by these CO2 sources has been observed frequently in numerous environments spanning from open continental shelves to enclosed estuaries. Here, we report observations on carbonate system dynamics in a relatively well-buffered lagoonal estuary, Corpus Christi Bay (CCB), in a semiarid subtropical region that is influenced by summertime hypoxia as well as strong evaporation and seagrass vegetation in the vicinity. While the relationship between dissolved oxygen (DO) and pH in the bottom waters of CCB was positive as in other coastal and estuarine environments prone to hypoxia, the slope was significantly less than in other systems. We attribute the high buffering capacity in CCB to the presence of abundant seagrass meadows adjacent to CCB and strong evaporation-produced density flow that delivers low CO2 waters to the bottom of CCB. Thus, despite the occurrence of hypoxia, neither bottom water carbonate saturation state with respect to aragonite (Ωarg) nor CO2 partial pressure (pCO2) reached critical levels, i.e., undersaturation (i.e., Ωarag1000 µatm), respectively.

Continue reading ‘Characteristics of the carbonate system in a semiarid estuary that experiences summertime hypoxia’

Seasonal dynamics of the marine CO2 system in Adventfjorden, a west Spitsbergen fjord

Time series of the marine CO2 system and related parameters at the IsA Station, by Adventfjorden, Svalbard, were investigated between March 2015 and November 2017. The physical and biogeochemical processes that govern changes in total alkalinity (TA), total dissolved inorganic carbon (DIC) and the saturation state of the calcium carbonate mineral aragonite (ΩAr) were assessed on a monthly timescale. The major driver for TA and DIC was changes in salinity, caused by river runoff, mixing and advection. This accounted for 77 and 45%, respectively, of the overall variability. It contributed minimally to the variability in ΩAr (5%); instead, biological activity was responsible for 60% of the monthly variations. For DIC, the biological activity was also important, contributing 44%. The monthly effect of air–sea CO2 fluxes accounted for 11 and 15% of the total changes in DIC and ΩAr, respectively. Net community production (NCP) during the productive season ranged between 65 and 85 g C m−2, depending on the year and the presence of either Arctic water or transformed Atlantic water (TAW). The annual NCP as estimated from DIC consumption was 34 g C m−2 yr−1 in 2016, which was opposite in direction but similar in magnitude to the integrated annual air–sea CO2 flux (i.e., uptake of carbon from the atmosphere) of −29 g C m−2 yr−1 for the same year. The results showed that increased intrusions of TAW into Adventfjorden in the future could possibly lower the NCP, with the potential to reduce the CO2buffer capacity and ΩAr over the summer season.

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Ocean acidification in the IPCC AR5 WG II

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