Posts Tagged 'biogeochemistry'

Carbon and nutrient cycling in Antarctic landfast sea ice from winter to summer

Seasonal cycling in carbon, alkalinity, and nutrients in landfast sea ice in Hangar Cove, Adelaide Island, West Antarctic Peninsula, were investigated during winter, spring, and summer 2014–2015. Temporal dynamics were driven by changes in the sea-ice physicochemical conditions, ice-algal community composition, and organic matter production. Winter sea ice was enriched with dissolved inorganic carbon (DIC) and inorganic nutrients from organic matter remineralization. Variations in alkalinity (Alk) and DIC indicated that abiotic calcium carbonate (ikaite) precipitation had taken place. Relative to other nutrients, low phosphate (PO4) concentrations potentially resulted from co-precipitation with ikaite. Seawater flooding and meltwater induced variability in the physical and biogeochemical properties in the upper ice in spring where nutrient resupply supported haptophyte productivity and increased particulate organic carbon (POC) in the interstitial layer. Rapid nitrate (NO3) and DIC (< 165 μmol kg−1) uptake occurred alongside substantial build-up of algal biomass (746 μg chlorophyll a L−1) and POC (6191 μmol L−1) during summer. Silicic acid drawdown followed NO3 depletion by approximately 1 month with a shift to diatom-dominated communities. Accumulation of PO4 in the lower ice layers in summer likely resulted from PO4 released during ikaite dissolution in the presence of biofilms. Increased Alk : DIC ratios in the lower ice and under-ice water suggested that ikaite dissolution buffered against meltwater dilution and enhanced the potential for atmospheric CO2 uptake. This study revealed strong seasonality in carbon and nutrient cycling in landfast sea ice and showed the importance of sea ice in biogeochemical cycling in seasonally ice-covered waters around Antarctica.

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Seasonal dynamics and annual budget of dissolved inorganic carbon in the northwestern Mediterranean deep convection region

Deep convection plays a key role in the circulation, thermodynamics and biogeochemical cycles in the Mediterranean Sea, considered as a hotspot of biodiversity and climate change. In the framework of the DEWEX (Dense Water Experiment) project, the seasonal cycle and annual budget of dissolved inorganic carbon in the deep convection area of the northwestern Mediterranean Sea are investigated over the period September 2012–September 2013, using a 3-dimensional coupled physical-biogeochemical-chemical modeling approach. We estimate that the northwestern Mediterranean Sea deep convection region was a moderate sink of CO2 for the atmosphere over the study period. The model results show the reduction of CO2 uptake during deep convection, and its increase during the abrupt spring phytoplankton bloom following the deep convection events. We highlight the dominant role of both biological and physical flows in the annual dissolved inorganic carbon budget. The upper layer of the northwestern deep convection region gained dissolved inorganic carbon through vertical physical supplies and, to a lesser extent, air-sea flux, and lost dissolved inorganic carbon through lateral transport and biological fluxes. The region, covering 2.5 % of the Mediterranean, acted as a source of dissolved inorganic carbon for the surface and intermediate water masses of the western and southern Western Mediterranean Sea and could contribute up to 10 and 20 % to the CO2 exchanges with the Eastern Mediterranean Sea and the Atlantic Ocean.

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Characteristics of calcium isotopes at different water depths and their palaeoenvironmental significance for carbonate rocks of the Permian-Triassic boundary in Chibi, southern China

Calcium isotopes of carbonate rocks can trace calcium cycles and record changes in the marine environment. As published calcium isotope profiles of carbonate rocks at the Permian-Triassic boundary are rare, comparative studies on deep-water profiles were lacking for the major extinction event that occurred during this time. We present sections of different water depths in the Chibi area of southern China that we have selected for a comparative study. We analyzed carbon isotopes, calcium isotopes, as well as major and trace elements of carbonates from two sections (Chibi North and Chibi West) to obtain information on the volcanic activity, ocean acidification, as well as sea level rise and fall in the Chibi area during the mass extinction period. All carbon and calcium isotopes of carbonates from both sections are all negative after the mass extinction boundary. Carbonates from the Chibi North section have higher δ44/40Ca values and lower Sr/Ca ratios than the rocks from the Chibi West section. We propose that the negative bias of the calcium isotopes in the two sections result from diagenesis. Diagenesis transforms primary aragonite into calcite, showing the characteristics of high δ44/40Ca value and low Sr/Ca. By comparing our data with three published profiles of shallow-water carbonate rock, we recognize that calcium isotopes record gradients at different water depths. In the slope environment, the enhancement of pore fluid action near the coast caused an increase of the fluid buffer alteration, and we propose that a regression event occurred in the Chibi region during the Late Permian.

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Reviews and syntheses: carbon biogeochemistry of Indian estuaries

The goal of this review is to provide a comprehensive overview of the magnitude and drivers of carbon cycling dynamics in the major estuaries of India. Data from a total of 32 estuaries along the Bay of Bengal (BB) and the Arabian Sea (AS) were compiled from the literature and re-analysed based on changes in season (wet vs. dry) and marine end-members (e.g., BB vs. AS). The estuaries are generally undersaturated in dissolved oxygen relative to the atmosphere and strongly influenced by local and regional precipitation patterns. Speciation of the dissolved inorganic carbon (DIC) pool is dominated by bicarbonate and primarily variability in DIC is controlled by a combination of carbonate weathering, the degree of precipitation, the length of the estuaries, in situ respiration, and mixing. Carbonate dissolution had the largest influence on DIC during the wet season, while respiration was the primary control of DIC variability in the estuaries connected with BB during the dry season. Interestingly, the influence of anaerobic metabolism on DIC is observed in the oxygenated mangrove dominated estuaries, which we hypothesize is driven by porewater exchange in intertidal sediments. Dissolved organic carbon (DOC) generally behaves non-conservatively in the studied estuaries. The DOC-particulate organic carbon (POC) inter-conversion and DOC mineralization are evident in the BB during the dry season and AS estuaries, respectively. The wet season δ13CPOC shows dominance of freshwater algae, C3 plant material, as well as marine organic matter in POC. However, anthropogenic inputs are evident in some estuaries in eastern India during the dry season. POC respiration was identified in the AS; however, a link between POC and CH4 is identified throughout both the regions. pCO2 is controlled principally by respiration with freshwater discharge only playing a marginal important role in the BB. The AS estuaries act as a CO2 source to the atmosphere; however, the BB estuaries vary between a source and sink. POC together with methanotrophy and dam abundance appear to control CH4 concentrations, and all of the studied estuaries act as a CH4 source to the atmosphere. Additionally, anthropogenic inputs and groundwater exchange also show potential influences in some cases. The Indian estuaries contribute 2.62 % and 1.09 % to the global riverine DIC and DOC exports to the ocean, respectively. The total CO2 and CH4 fluxes from Indian estuaries are estimated as ~9718 Gg yr-1 and 3.27 Gg yr-1, which contributes ~0.67 % and ~0.12 %, respectively, to global estimates of estuarine greenhouse gas emissions. While a qualitative idea on the major factors controlling the carbon biogeochemistry in India is presented through this work, a more thorough investigation including rate quantification of the above-mentioned mechanisms is essential for precise accounting of the C budget of Indian estuaries.

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A numerical reassessment of the Gulf of Mexico carbon system in connection with the Mississippi River and global ocean

Coupled physical–biogeochemical models can fill the spatial and temporal gap in ocean carbon observations. Challenges of applying a coupled physical–biogeochemical model in the regional ocean include the reasonable prescription of carbon model boundary conditions, lack of in situ observations, and the oversimplification of certain biogeochemical processes. In this study, we applied a coupled physical–biogeochemical model (Regional Ocean Modelling System, ROMS) to the Gulf of Mexico (GoM) and achieved an unprecedented 20-year high-resolution (5 km, 1/22°) hindcast covering the period of 2000 to 2019. The biogeochemical model incorporated the dynamics of dissolved organic carbon (DOC) pools and the formation and dissolution of carbonate minerals. The biogeochemical boundaries were interpolated from NCAR’s CESM2-WACCM-FV2 solution after evaluating the performance of 17 GCMs in the GoM waters. Model outputs included carbon system variables of wide interest, such as pCO2, pH, aragonite saturation state (ΩArag), calcite saturation state (ΩCalc), CO2 air–sea flux, and carbon burial rate. The model’s robustness is evaluated via extensive model–data comparison against buoys, remote-sensing-based machine learning (ML) products, and ship-based measurements. A reassessment of air–sea CO2 flux with previous modeling and observational studies gives us confidence that our model provides a robust and updated CO2 flux estimation, and NGoM is a stronger carbon sink than previously reported. Model results reveal that the GoM water has been experiencing a ∼ 0.0016 yr−1 decrease in surface pH over the past 2 decades, accompanied by a ∼ 1.66 µatm yr−1 increase in sea surface pCO2. The air–sea CO2 exchange estimation confirms in accordance with several previous models and ocean surface pCO2 observations that the river-dominated northern GoM (NGoM) is a substantial carbon sink, and the open GoM is a carbon source during summer and a carbon sink for the rest of the year. Sensitivity experiments are conducted to evaluate the impacts of river inputs and the global ocean via model boundaries. The NGoM carbon system is directly modified by the enormous carbon inputs (∼ 15.5 Tg C yr−1 DIC and ∼ 2.3 Tg C yr−1 DOC) from the Mississippi–Atchafalaya River System (MARS). Additionally, nutrient-stimulated biological activities create a ∼ 105 times higher particulate organic matter burial rate in NGoM sediment than in the case without river-delivered nutrients. The carbon system condition of the open ocean is driven by inputs from the Caribbean Sea via the Yucatan Channel and is affected more by thermal effects than biological factors.

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Slow-sinking particulate organic carbon and its attenuation in the mesopelagic water of the South China Sea

Coastal acidification has been widely investigated in terms of its rationale and ecological effects in the last decade. However, the driving mechanism for acidification in open seawater, especially in mesopelagic water, is still poorly understood. Here, the sinking velocity and flux attenuation of particulate organic carbon (POC) were examined based upon the radioactive 210Po-210Pb tracer to reveal the remineralization of POC in the mesopelagic zone in the northeastern South China Sea (SCS). Overall, the profiles of 210Po followed those of 210Pb, lending support to the particle sinking controlled top-down deficits of 210Po. Using an inverse model, the sinking velocity of particles, for the first time in the SCS, was estimated to vary from 3 to 34 m d-1 with the mean value of 15 ± 9 m d-1, indicating that the slow sinking particles largely contribute to the POC flux in the SCS. Beneath the euphotic zone, a consistent descending of the sinking speed implied continuous remineralization of sinking POC in the twilight zone. A preliminary estimate revealed that 1.9-5.4 mmol-C m-2 d-1 remineralized back to carbon dioxide within 100-500 m, representing about 70% of the exported autochthonous POC from the euphotic zone. In 100-1000 m, 2.4-6.6 mmol-C m-2 d-1 (i.e., 84%) remineralized. Thus, the upper twilight zone (i.e., 100-500 m) is the dominant layer of POC remineralization, and POC-induced acidification could be unneglectable there. These results provided insights into the POC-induced acidification mechanism in the mesopelagic water, especially in the upper mesopelagic layer.

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GLODAPv2.2022: the latest version of the global interior ocean biogeochemical data product

The Global Ocean Data Analysis Project (GLODAP) is a synthesis effort providing regular compilations of surface-to-bottom ocean biogeochemical bottle data, with an emphasis on seawater inorganic carbon chemistry and related variables determined through chemical analysis of seawater samples. GLODAPv2.2022 is an update of the previous version, GLODAPv2.2021 (Lauvset et al., 2021). The major changes are as follows: data from 96 new cruises were added, data coverage was extended until 2021, and for the first time we performed secondary quality control on all sulphur hexafluoride (SF6) data. In addition, a number of changes were made to data included in GLODAPv2.2021. These changes affect specifically the SF6 data, which are now subjected to secondary quality control, and carbon data measured onboard the RV Knorr in the Indian Ocean in 1994–1995 which are now adjusted using CRM measurements made at the time. GLODAPv2.2022 includes measurements from almost 1.4 million water samples from the global oceans collected on 1085 cruises. The data for the now 13 GLODAP core variables (salinity, oxygen, nitrate, silicate, phosphate, dissolved inorganic carbon, total alkalinity, pH, CFC-11, CFC-12, CFC-113, CCl4, and SF6) have undergone extensive quality control with a focus on systematic evaluation of bias. The data are available in two formats: (i) as submitted by the data originator but converted to World Ocean Circulation Experiment (WOCE) exchange format and (ii) as a merged data product with adjustments applied to minimize bias. For the present annual update, adjustments for the 96 new cruises were derived by comparing those data with the data from the 989 quality controlled cruises in the GLODAPv2.2021 data product using crossover analysis. SF6 data from all cruises were evaluated by comparison with CFC-12 data measured on the same cruises. For nutrients and ocean carbon dioxide (CO2) chemistry comparisons to estimates based on empirical algorithms provided additional context for adjustment decisions. The adjustments that we applied are intended to remove potential biases from errors related to measurement, calibration, and data handling practices without removing known or likely time trends or variations in the variables evaluated. The compiled and adjusted data product is believed to be consistent to better than 0.005 in salinity, 1 % in oxygen, 2 % in nitrate, 2 % in silicate, 2 % in phosphate, 4 μmol kg-1 in dissolved inorganic carbon, 4 μmol kg-1 in total alkalinity, 0.01–0.02 in pH (depending on region), and 5 % in the halogenated transient tracers. The other variables included in the compilation, such as isotopic tracers and discrete CO2 fugacity (fCO2), were not subjected to bias comparison or adjustments.

The original data, their documentation and DOI codes are available at the Ocean Carbon and Acidification Data System of NOAA NCEI (https://www.ncei.noaa.gov/access/ocean-carbon-acidification-data-system/oceans/GLODAPv2_2022/, last access: 15 August 2022). This site also provides access to the merged data product, which is provided as a single global file and as four regional ones – the Arctic, Atlantic, Indian, and Pacific oceans – under https://doi.org/10.25921/1f4w-0t92 (Lauvset et al., 2022). These bias-adjusted product files also include significant ancillary and approximated data, which were obtained by interpolation of, or calculation from, measured data. This living data update documents the GLODAPv2.2022 methods and provides a broad overview of the secondary quality control procedures and results.

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The influence of tides on the marine carbonate chemistry of a coastal polynya in the south-eastern Weddell Sea

Tides significantly affect polar coastlines by modulating ice shelf melt and modifying shelf water properties through transport and mixing. However, the effect of tides on the marine carbonate chemistry in such regions, especially around Antarctica, remains largely unexplored. We address this topic with two case studies in a coastal polynya in the south-eastern Weddell Sea, neighbouring the Ekström Ice Shelf. The case studies were conducted in January 2015 (PS89) and January 2019 (PS117), capturing semi-diurnal oscillations in the water column. These are pronounced in both physical and biogeochemical variables for PS89. During rising tide, advection of sea ice meltwater from the north-east created a fresher, warmer, and more deeply mixed water column with lower dissolved inorganic carbon (DIC) and total alkalinity (TA) content. During ebbing tide, water from underneath the ice shelf decreased the polynya’s temperature, increased the DIC and TA content, and created a more stratified water column. The variability during the PS117 case study was much smaller, as it had less sea ice meltwater input during rising tide and was better mixed with sub-ice shelf water. The contrasts in the variability between the two case studies could be wind and sea ice driven, and they underline the complexity and highly dynamic nature of the system.

The variability in the polynya induced by the tides results in an air–sea CO2 flux that can range between a strong sink (−24 mmol m−2 d−1) and a small source (3 mmol m−2 d−1) on a semi-diurnal timescale. If the variability induced by tides is not taken into account, there is a potential risk of overestimating the polynya’s CO2 uptake by 67 % or underestimating it by 73 %, compared to the average flux determined over several days. Depending on the timing of limited sampling, the polynya may appear to be a source or a sink of CO2. Given the disproportionate influence of polynyas on heat and carbon exchange in polar oceans, we recommend future studies around the Antarctic and Arctic coastlines to consider the timing of tidal currents in their sampling strategies and analyses. This will help constrain variability in oceanographic measurements and avoid potential biases in our understanding of these highly complex systems.

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Physical and biological controls on anthropogenic CO2 sink of the Ross Sea

The Antarctic continental shelf is known as a critical anthropogenic CO2 (Cant) sink due to its cold waters, high primary productivity, and unique circulation, which allow it to sequester large amounts of organic and inorganic carbon into the deep ocean. However, climate change is currently causing significant alteration to the Antarctic marine carbon cycle, with unknown consequences on the Cant uptake capacity, making model-based estimates of future ocean acidification of polar regions highly uncertain. Here, we investigated the marine carbonate system in the Ross Sea in order to assess the current anthropogenic carbon content and how physical–biological processes can control the Cant sequestration along the shelf-slope continuum. The Winter Water mass generated from convective events was characterized by high Cant level (28 µmol kg−1) as a consequence of the mixed layer break-up during the cold season, whereas old and less-ventilated Circumpolar Deep Water entering the Ross Sea revealed a very scarce contribution of anthropogenic carbon (7 µmol kg−1). The Cant concentration was also different between polynya areas and the shelf break, as a result of their specific hydrographic characteristics and biological processes: surface waters of the Ross Sea and Terra Nova Bay polynyas served as strong CO2 sink (up to −185 mmol m−2), due to the remarkable net community production, estimated from the summertime surface-dissolved inorganic carbon deficit. However, a large amount of the generated particulate organic carbon was promptly consumed by intense microbial activity, giving back carbon dioxide into the intermediate and deep layers of the continental shelf zone. Further Cant also derived from High-Salinity Shelf Water produced during winter sea ice formation (25 µmol kg−1), fueling dense shelf waters with additional input of Cant, which was finally stored into the abyssal sink through continental slope outflow (19 µmol kg−1). Our results suggest that summer biological activity over the Ross Sea shelf is pivotal for the shunt of anthropogenic CO2 between the organic and inorganic carbon pools, enhancing the ocean acidification of the upper mesopelagic zone and the long-term Cant sequestration into the deep ocean.

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The changing ocean carbon sink in the earth system

Eunice Foote, who was the first to measure the solar heating of CO2 in her early experiments already in the 1850s noted: “An atmosphere of that gas would give to our Earth a high temperature“ (Foote, 1856). Indeed, our planet is warming unprecedently fast due to rising anthropogenic CO2 emissions (Masson-Delmotte et al., 2021). Next to catastrophic floodings, wildfires and droughts on land, with tragic consequences for people, the ocean silently suffers from the ongoing heating, acidification, and deoxygenation with tragic impacts for marine systems.

The ocean plays an essential role in regulating Earth’s climate; it is also essential for regulating the Earth’s carbon cycle. The ocean contains around 38,000 Gt of carbon. This is 16 times more than the terrestrial biosphere (plant and the underlying soils), and about 60 times more than the pre-industrial atmosphere (Canadell et al., 2021). Therefore, even a small perturbation to the ocean carbon content by changing its capacity to store carbon would impact atmospheric CO2 concentrations (Fig.1.1), making the ocean carbon sink a major regulator of the Earth’s climate on a time scale of hundreds to thousands of years. As the ocean currently continuously absorbs anthropogenic carbon from the atmosphere, it thereby has a key role in moderating ongoing climate change.

Based on the Global Carbon Budget (GCB) estimates (Friedlingstein et al., 2020), the global ocean has already taken up about one third of the cumulative anthropogenic CO2 emissions (Fig.1.2). The strength of the ocean carbon sink is determined by chemical reactions in seawater (carbonate system), biological processes (photosynthesis, export flux, and remineralization by aerobic and anaerobic respiration), and physical processes (including ocean circulation and vertical mixing). But even though these key mechanisms are identified (Landschutzer et al., 2021), there are considerable uncertainties regarding their interannual and decadal variations, as well as their susceptibility to ongoing climate change. Here, a major uncertainty arises from the lack of knowledge regarding the contribution of the natural variability of the climate system (Ilyina, 2016).

In this essay, I present my research contributions based on my papers explicitly mentioned in the text. My research was guided by the following questions:

  1. How do ocean biogeochemical cycles accommodate perturbations brought about by anthropogenic activities or natural forcings?
  2. What are the predictability horizons of variations in the ocean carbon sink?
  3. What is the potential of the ocean carbon sink, artificially enhanced by ocean alkalinity additions, to mitigate climate change?

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Biogeochemistry of carbon, nitrogen and oxygen in the Bay of Bengal: new insights through re-analysis of data

Primary production is reported to be a fraction of heterotrophic carbon demand in the Bay of Bengal (BoB), and it is attributed to the unavailability of inorganic nutrients and faster sinking of organic matter in association with mineral particles. The contribution of nutrients through external sources to total primary production is low (<5%), suggesting internal cycling of nutrients is important in the BoB. Organic nutrients support primary production in the absence of inorganic nutrients in the BoB. It was noticed that about 45% of particulate organic carbon (POC) production is exudated as dissolved organic carbon (DOC). Therefore, the total organic carbon production is revised to twice that of the earlier estimate and it is sufficient to support heterotrophic carbon demand. Balance among the ventilation of oxygen by anticyclonic eddies, strengthening due to cyclonic eddies and salinity stratification controls the oxygen levels in the OMZ than hitherto hypothesized as ballasting of organic matter. The stable isotopic composition of nitrogen in nitrate and particulate organic nitrogen (PON) does not evidence a significant contribution of anthropogenic nitrogen in the BoB. This negates the hypothesis that anthropogenic inputs modify the biogeochemistry of BoB. The deposition of anthropogenic aerosols decreases the pH of surface waters in the western BoB, whereas a decrease in salinity due to an increase in freshwater flux due to warming of the Himalayan glacier may increase pH and decrease pCO2 levels. As a result, BoB is turning into more sink for atmospheric CO2, which is contrasting to that of elsewhere in the global ocean.

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Understanding the implications of hydrographic processes on the dynamics of the carbonate system in a sub-Antarctic marine-terminating glacier-fjord (53°S)

The biogeochemical dynamics of fjords in the southeastern Pacific Ocean are strongly influenced by hydrological and oceanographic processes occurring at a seasonal scale. In this study, we describe the role of hydrographic forcing on the seasonal variability of the carbonate system of the Sub-Antarctic glacial fjord, Seno Ballena, in the Strait of Magellan (53°S). Biogeochemical variables were measured in 2018 during three seasonal hydrographic cruises (fall, winter and spring) and from a high-frequency pCO2-pH mooring for 10 months at 10 ± 1 m depth in the fjord. The hydrographic data showed that freshwater input from the glacier influenced the adjacent surface layer of the fjord and forced the development of undersaturated CO2 (< 400 μatm) and low aragonite saturation state (ΩAr < 1) water. During spring, the surface water had relatively low pCO2 (mean = 365, range: 167 – 471 μatm), high pH (mean = 8.1 on the total proton concentration scale, range: 8.0 – 8.3), and high ΩAr (mean = 1.6, range: 1.3 – 4.0). Concurrent measurements of phytoplankton biomass and nutrient conditions during spring indicated that the periods of lower pCO2 values corresponded to higher phytoplankton photosynthesis rates, resulting from autochthonous nutrient input and vertical mixing. In contrast, higher values of pCO2 (range: 365 – 433 μatm) and relatively lower values of pHT (range: 8.0 – 8.1) and ΩAr (range: 0.9 – 2.0) were recorded in cold surface waters during winter and fall. The naturally low freshwater carbonate ion concentrations diluted the carbonate ion concentrations in seawater and decreased the calcium carbonate saturation of the fjord. In spring, at 10 m depth, higher primary productivity caused a relative increase in ΩAr and pHT. Assuming global climate change will bring further glacier retreat and ocean acidification, this study represents important advances in our understanding of glacier meltwater processes on CO2 dynamics in glacier–fjord systems.

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Metabolic alkalinity release from large port facilities (Hamburg, Germany) and impact on coastal carbon storage

Metabolic activities in estuaries, especially these of large rivers, exert profound impact on downstream coastal biogeochemistry. Here, we unravel the contribution of large industrial port facilities to these impacts and show that metabolic activity in the Hamburg port (Germany) increases total alkalinity (TA) and dissolved inorganic carbon (DIC) runoff to the North Sea. We explained this activity to be fueled by the imports of particulate inorganic and organic carbon (PIC, POC) and particulate organic nitrogen (PON) from the upstream Elbe River, resulting in maximum 90 % TA generation due to CaCO3 dissolution in the entire estuary. The remaining 10 % can be attributed to a TA generation by anaerobic metabolic processes such as denitrification of remineralized PON, or other pathways. The Elbe Estuary as a whole adds approximately 15 % to the overall DIC and TA runoff. Both the magnitude and partitioning among these processes appear to be sensitive to climate and anthropogenic changes, and affects coastal CO2 storage capacity.

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The differential responses of coastal diatoms to ocean acidification and warming: a comparison between Thalassiosira sp. and Nitzschia closterium f.minutissima

Marine diatoms are one of the marine phytoplankton functional groups, with high species diversity, playing important roles in the marine food web and carbon sequestration. In order to evaluate the species-specific responses of coastal diatoms to the combined effects of future ocean acidification (OA) and warming on the coastal diatoms, we conducted a semi-continuous incubation on the large centric diatom Thalassiosira sp. (~30 μm) and small pennate diatom Nitzschia closterium f.minutissima (~15 μm). A full factorial combination of two temperature levels (15 and 20°C) and pCO2 (400 and 1,000 ppm) was examined. The results suggest that changes in temperature played a more important role in regulating the physiology of Thalassiosira sp. and N. closterium f.minutissima than CO2. For Thalassiosira sp., elevated temperature significantly reduced the cellular particulate organic carbon (POC), particulate organic nitrogen (PON), particulate organic phosphate (POP), biogenic silica (BSi), chlorophyll a (Chl a), and protein contents, and the C:N ratio. CO2 only had significant effects on the growth rate and the protein content. However, for the smaller pennate diatom N. closterium f.minutissima, the growth rate, POC production rate, and the C:P ratio significantly increased with an elevated temperature, whereas the cellular POP and BSi contents significantly decreased. CO2 had significant effects on the POC production rate, cellular BSi, POC, and PON contents, the C:P, Si:C, N:P, and Si:P ratios, and sinking rate. The interaction between OA and warming showed mostly antagonistic effects on the physiology of both species. Overall, by comparison between the two species, CO2 played a more significant role in regulating the growth rate and sinking rate of the large centric diatom Thalassiosira sp., whereas had more significant effects on the elemental compositions of the smaller pennate diatom N. closterium f.minutissima. These results suggest differential sensitivities of different diatom species with different sizes and morphology to the changes in CO2/temperature regimes and their interactions.

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Effects of oyster aquaculture on carbon capture and removal in a tropical mangrove lagoon in southwestern Taiwan

Highlights

  • Mangroves acted as a key source of DIC and TA in tropical lagoon.
  • Oyster shell formation plays a significant role in decreasing TA.
  • DIC declined due to photosynthesis by phytoplankton.
  • Oyster benefit carbon capture by preying phytoplankton.
  • Oyster farming contributes to carbon capture in a mangrove-dominated lagoon.
Graphical abstract

Abstract

Blue carbon ecosystems (BCEs) are a promising resource for the mitigation of global warming; however, climate spectrums and anthropogenic activities could influence the fragile balance of BCEs as carbon sinks or sources. We assess how oyster farming affects dissolved inorganic carbon (DIC) and total alkalinity (TA) on CO2 fluxes in a mangrove-dominated lagoon. Water physical, chemical and biological parameters were recorded by in-situ buoys within the lagoon and at its inflow. Structural equation modeling was adopted to clarify the factors/processes controlling the partial pressure of CO2 (pCO2). A three-dimensional environmental model followed by a conceptual DIC model was used to quantify the spatiotemporal patterns of capture and release of DIC and TA by oyster production. The results showed that 49% of TA and DIC released from mangroves was depleted by oyster shell formation. DIC was reduced by algal photosynthesis and algal was served as a food source supporting oyster production. Annual oyster production through phytoplankton photosynthesis accounted for 11% of the atmosphere carbon inflows, suggesting that oyster production served as a significant atmospheric/terrestrial carbon sink in the lagoon. The results indicate that mangroves benefit local oyster production by acting as an important source of DIC and TA, and that the oyster aquaculture contributed to carbon capture in a mangrove-dominated lagoon ecosystem.

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The influence of climate change on marine bacterioplankton communities and greenhouse gases in New Zealand waters

Bacterioplankton communities play a fundamental role in the cycling of carbon and nitrogen in the oceans. Cycling of these nutrients by bacterioplankton also contributes to the production of nitrous oxide and methane, resulting in the oceans being a net source of both these greenhouse gases. Climate change is impacting the oceans through warming and acidification resulting in alteration of planktonic ecosystems, via changes in productivity, biomass, and species composition. The response of marine bacterioplankton communities to the direct effects of ocean warming and lowered pH, and to the indirect effects of changes in phytoplankton and zooplankton, has implications for biogeochemical cycling and therefore the production of nitrous oxide and methane. This thesis investigates the impact of both direct and indirect climate pressures by determining the influence of ocean warming and lowered pH on bacterioplankton and the production of methane and nitrous oxide in New Zealand coastal waters. It also assesses how open ocean bacterioplankton communities and dissolved methane and nitrous oxide are influenced by water mass properties and, in particular, how they may be affected by climate-induced changes in the distribution and abundance of salps, a dominant group of zooplankton.

To determine the impact of lower pH and warming on bacterioplankton community, production and abundance, coastal water was manipulated in three mesocosm experiments to projected future ocean temperature and pH. The experiments ran for 18-21 days using 4000-Litre mesocosms filled with coastal water and associated plankton communities, with pH and temperature continuously regulated. High-throughput sequencing of the 16S rRNA gene was used to determine bacterioplankton community composition and leucine incorporation was used to measure bacterial production during the experiments. Minor but significant increases in alpha diversity were seen under low pH and warming. However, overall results from the mesocosm experiments indicate resilience to ocean warming and low pH in coastal bacterioplankton communities, with no significant impacts on production, abundance or beta-diversity found. Bacterioplankton communities in coastal sites are likely to experience high natural variability, which may result in lack of sensitivity to projected climate change.

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Changing hydrographic, biogeochemical, and acidification properties in the Gulf of Maine as measured by the Gulf of Maine North Atlantic Time Series, GNATS, between 1998 and 2018

The Gulf of Maine North Atlantic Time Series (GNATS) has been run since 1998, across the Gulf of Maine (GoM), between Maine and Nova Scotia. GNATS goals are to provide ocean color satellite validation and to examine change in this coastal ecosystem. We have sampled hydrographical, biological, chemical, biogeochemical, and bio-optical variables. After 2008, warm water intrusions (likely North Atlantic Slope Water [NASW]) were observed in the eastern GoM at 50–180 m depths. Shallow waters (<50 m) significantly warmed in winter, summer, and fall but cooled during spring. Surface salinity and density of the GoM also significantly increased over the 20 years. Phytoplankton standing stock and primary production showed highly-significant decreases during the period. Concentrations of phosphate increased, silicate decreased, residual nitrate [N*; nitrate-silicate] increased, and the ratio of dissolved inorganic nitrogen:phosphate decreased, suggesting increasing nitrogen limitation. Dissolved organic carbon (DOC) and its optical indices generally increased over two decades, suggesting changes to the DOC cycle. Surface seawater carbonate chemistry showed winter periods where the aragonite saturation (Ωar) dropped below 1.6 gulf-wide due to upward winter mixing of cool, corrosive water. However, associated with increased average GoM temperatures, Ωar has significantly increased. These results reinforce the hypothesis that the observed decrease in surface GoM primary production resulted from a switch from Labrador Sea Water to NASW entering the GoM. A multifactor analysis shows that decreasing GoM primary production is most significantly correlated to decreases in chlorophyll and particulate organic carbon plus increases in N* and temperature.

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Chapter 13 – ocean systems

The ocean comprises ~71% of the Earth’s surface area and is in constant interaction with the atmosphere above and the land surface at the coastal interface, allowing a continuous exchange of greenhouse gases (GHGs) between the spheres. The ocean plays an important role in absorbing and storing carbon dioxide (CO2) from fossil fuel combustion, land-use change, and cement production. Since the industrial revolution, the ocean has stored ~31% of human emitted CO2 adding to a total storage of anthropogenic CO2 of 152 ± 20 Pg C (PgC = Petagrams of carbon) from 1850 to 2007 and is currently removing about 2.6 ± 0.6 Pg C of excess CO2 every year from the atmosphere. On longer timescales (i.e., centuries to millennia), the ocean carbon sink acts as a primary regulator of the Earth’s climate. While the ocean carbon sink mitigates climate change, absorption of anthropogenic CO2 leads to ocean acidification with potentially harmful effects for marine ecosystems. The ocean also contributes to the cycles of other greenhouse gases. Specifically, it is a weak source of methane. The contribution of the ocean to the net global methane budget, however, is substantially smaller than the oceanic uptake of CO2. The ocean was a net source of methane (CH4) to the atmosphere of ~13 Tg CH4 year−1 (Tg CH4 = Teragrams of CH4) with a possible range of 9–22 Tg CH4 year−1 over the period 2000–20. Hence, the methane fluxes from the ocean to the atmosphere are an order of magnitude smaller than the anthropogenic emissions over the same period. Likewise, the ocean comprises a natural source of nitrous oxide (N2O) of ~ 3.4 Tg N year−1 between 2007 and 2016, although with a substantial possible range between 2.5 and 4.3 Tg N year−1.

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Enhanced silica export in a future ocean triggers global diatom decline

Diatoms account for up to 40% of marine primary production and require silicic acid to grow and build their opal shell3. On the physiological and ecological level, diatoms are thought to be resistant to, or even benefit from, ocean acidification. Yet, global-scale responses and implications for biogeochemical cycles in the future ocean remain largely unknown. Here we conducted five in situ mesocosm experiments with natural plankton communities in different biomes and find that ocean acidification increases the elemental ratio of silicon (Si) to nitrogen (N) of sinking biogenic matter by 17 ± 6 per cent under pCO2 conditions projected for the year 2100. This shift in Si:N seems to be caused by slower chemical dissolution of silica at decreasing seawater pH. We test this finding with global sediment trap data, which confirm a widespread influence of pH on Si:N in the oceanic water column. Earth system model simulations show that a future pH-driven decrease in silica dissolution of sinking material reduces the availability of silicic acid in the surface ocean, triggering a global decline of diatoms by 13–26 per cent due to ocean acidification by the year 2200. This outcome contrasts sharply with the conclusions of previous experimental studies, thereby illustrating how our current understanding of biological impacts of ocean change can be considerably altered at the global scale through unexpected feedback mechanisms in the Earth system.

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Sinking diatoms trap silicon in deep seawater of acidified oceans

The seas are acidifying as a result of carbon dioxide emissions. It now emerges that this will alter the solubility of the shells of marine organisms called diatoms — and thereby change the distribution of nutrients and plankton in the ocean.

The ecologically dominant phytoplankton in much of the ocean are a group of unicellular organisms known as diatoms. Writing in Nature, Taucher et al. present a study that uses a combination of experimental, observational and modelling approaches to examine how the diatom-driven effects of ocean acidification — a consequence of rising carbon dioxide concentrations in seawater — will affect biogeochemical cycles. The separate lines of evidence suggest that ocean acidification will have far-reaching effects on the export of elements to the deep ocean.

Diatoms are highly efficient at converting dissolved CO2 into organic carbon through photosynthesis, whereupon this organic carbon becomes incorporated into particles that sink rapidly to the deep ocean. Diatoms therefore serve as primary engines of a ‘biological pump’ that exports carbon to the deep ocean for sequestration. Each diatom cell is enclosed in a shell of silica (SiO2, where Si is silicon), and the solubility of the silicon in this biomineral is pH-sensitive — it becomes less soluble as seawater acidity rises. Although these features of diatoms are familiar to marine scientists, their combined implications for future biogeochemical cycles in the context of ocean acidification had not been explored.

Enter Taucher and colleagues. They carried out a series of five experiments in various parts of the ocean in which natural phytoplankton communities were grown in large enclosures (with volumes of 35–75 cubic metres) known as mesocosms, which simulated future ocean acidification. When the authors measured the elemental composition of the diatom-derived debris at the bottom of the mesocosms, they observed much higher ratios of silicon to nitrogen than the ratios of particles suspended near the surface. This suggested that, at low seawater pH, diatom silica shells were dissolving much more slowly than nitrogen-containing compounds in the same sinking material. In other words, silicon was being exported from the surface to deeper waters preferentially to nitrogen. The authors validated this finding using records of silicon-to-nitrogen ratios in sinking biological detritus in the open ocean, measured as a function of seawater pH, and obtained from particle-collecting sediment traps deployed by research vessels.

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