Posts Tagged 'biogeochemistry'



Isotopic fractionation of carbon during uptake by phytoplankton across the South Atlantic subtropical convergence

The stable isotopic composition of particulate organic carbon (δ13CPOC) in the surface waters of the global ocean can vary with the aqueous CO2 concentration ([CO2(aq)]) and affects the trophic transfer of carbon isotopes in the marine food web. Other factors such as cell size, growth rate and carbon concentrating mechanisms decouple this observed correlation. Here, the variability in δ13CPOC is investigated in surface waters across the south subtropical convergence (SSTC) in the Atlantic Ocean, to determine carbon isotope fractionation (εp) by phytoplankton and the contrasting mechanisms of carbon uptake in the subantarctic and subtropical water masses. Our results indicate that cell size is the primary determinant of δ13CPOC across the Atlantic SSTC in summer. Combining cell size estimates with CO2 concentrations, we can accurately estimate εp within the varying surface water masses in this region. We further utilize these results to investigate future changes in εp with increased anthropogenic carbon availability. Our results suggest that smaller cells, which are prevalent in the subtropical ocean, will respond less to increased [CO2(aq)] than the larger cells found south of the SSTC and in the wider Southern Ocean. In the subantarctic water masses, isotopic fractionation during carbon uptake will likely increase, both with increasing CO2 availability to the cell, but also if increased stratification leads to decreases in average community cell size. Coupled with decreasing δ13C of [CO2(aq)] due to anthropogenic CO2 emissions, this change in isotopic fractionation and lowering of δ13CPOC may propagate through the marine food web, with implications for the use of δ13CPOC as a tracer of dietary sources in the marine environment.

Continue reading ‘Isotopic fractionation of carbon during uptake by phytoplankton across the South Atlantic subtropical convergence’

Net heterotrophy and carbonate dissolution in two subtropical seagrass meadows

The net ecosystem productivity (NEP) of two contrasting seagrass meadows within one of the largest seagrass ecosystems in the world, Florida Bay, was assessed using direct measurements over consecutive diel cycles. We report significant differences between NEP determined by dissolved inorganic carbon (NEPDIC) and by dissolved oxygen (NEPDO), likely driven by differences in air-water gas exchange and contrasting responses to variations in light intensity. In this first direct determination of NEPDIC in seagrasses, we found that both seagrass ecosystems were net heterotrophic, on average, despite large differences in seagrass net aboveground primary productivity. Net ecosystem calcification (NEC) was also negative, indicating that both sites were net dissolving of carbonate minerals. We suggest that a combination of carbonate dissolution and respiration in sediments exceeded seagrass primary production and calcification, supporting our negative NEP and NEC measurements. Furthermore, a simple budget analysis indicates that these two seagrass meadows have contrasting impacts on pH buffering of adjacent systems, due to variations in the TA : DIC export ratio. The results of this study highlight the need for better temporal resolution, as well as accurate carbonate chemistry accounting in future seagrass metabolism studies.

Continue reading ‘Net heterotrophy and carbonate dissolution in two subtropical seagrass meadows’

Development of a biogeochemical and carbon model related to ocean acidification indices with an operational ocean model product in the North Western Pacific

We developed a biogeochemical and carbon model (JCOPE_EC) coupled with an operational ocean model for the North Western Pacific. JCOPE_EC represents ocean acidification indices on the background of the risks due to ocean acidification and our model experiences. It is an off-line tracer model driven by a high-resolution regional ocean general circulation model (JCOPE2M). The results showed that the model adequately reproduced the general patterns in the observed data, including the seasonal variability of chlorophyll-a, dissolved inorganic nitrogen/phosphorus, dissolved inorganic carbon, and total alkalinity. We provide an overview of this system and the results of the model validation based on the available observed data. Sensitivity analysis using fixed values for temperature, salinity, dissolved inorganic carbon and total alkalinity helped us identify which variables contributed most to seasonal variations in the ocean acidification indices, pH and Ωarg. The seasonal variation in the pHinsitu was governed mainly by balances of the change in temperature and dissolved inorganic carbon. The seasonal increase in Ωarg from winter to summer was governed mainly by dissolved inorganic carbon levels.

Continue reading ‘Development of a biogeochemical and carbon model related to ocean acidification indices with an operational ocean model product in the North Western Pacific’

First ROV exploration of the Perth Canyon: canyon setting, faunal observations, and anthropogenic impacts

This study represents the first ROV-based exploration of the Perth Canyon, a prominent submarine valley system in the southeast Indian Ocean offshore Fremantle (Perth), Western Australia. This multi-disciplinary study characterizes the canyon topography, hydrography, anthropogenic impacts, and provides a general overview of the fauna and habitats encountered during the cruise. ROV surveys and sample collections, with a specific focus on deep-sea corals, were conducted at six sites extending from the head to the mouth of the canyon. Multi-beam maps of the canyon topography show near vertical cliff walls, scarps, and broad terraces. Biostratigraphic analyses of the canyon lithologies indicate Late Paleocene to Late Oligocene depositional ages within upper bathyal depths (200–700 m). The video footage has revealed a quiescent ‘fossil canyon’ system with sporadic, localized concentrations of mega- and macro-benthos (∼680–1,800 m), which include corals, sponges, molluscs, echinoderms, crustaceans, brachiopods, and worms, as well as plankton and nekton (fish species). Solitary (Desmophyllum dianthus, Caryophyllia sp., Vaughanella sp., and Polymyces sp.) and colonial (Solenosmilia variabilis) scleractinians were sporadically distributed along the walls and under overhangs within the canyon valleys and along its rim. Gorgonian, bamboo, and proteinaceous corals were present, with live Corallium often hosting a diverse community of organisms. Extensive coral graveyards, discovered at two disparate sites between ∼690–720 m and 1,560–1,790 m, comprise colonial (S. variabilis) and solitary (D. dianthus) scleractinians that flourished during the last ice age (∼18 ka to 33 ka BP). ROV sampling (674–1,815 m) spanned intermediate (Antarctic Intermediate Water) and deep waters (Upper Circumpolar Deep Water) with temperatures from ∼2.5 to 6°C. Seawater CTD profiles of these waters show consistent physical and chemical conditions at equivalent depths between dive sites. Their carbonate chemistry indicate supersaturation (Ωcalcite ∼ 1.3–2.2) with respect to calcite, but mild saturation to undersaturation (Ωaragonite ∼ 0.8–1.4) of aragonite; notably some scleractinians were found living below the aragonite saturation horizon (∼1,000 m). Seawater δ13C and nuclear bomb produced Δ14C compositions decrease in the upper canyon waters by up to ∼0.8‰ (<800 m) and 95‰ (<500 m), respectively, relative to measurements taken nearby in 1978, reflecting the ingress of anthropogenic carbon into upper intermediate waters.

Continue reading ‘First ROV exploration of the Perth Canyon: canyon setting, faunal observations, and anthropogenic impacts’

Anthropogenic ocean warming and acidification recorded by Sr/Ca, Li/Mg, δ11B and B/Ca in Porites coral from the Kimberley region of northwestern Australia

Highlights

• Ocean warming has accelerated since the 1970s in the nearshore Kimberley.

• Coral calcification remains less affected and ‘normal’ seasonal coral internal carbonate chemistry is observed.

• Under intensified warming, coral’s ability to concentrate metabolic DIC has been reduced.

• Ocean acidification has led to the secular reduction of pHcf.

Abstract

The impact of climate changes on corals living in naturally extreme environments is poorly understood but crucial to longer-term sustainability of coral reefs. Here we report century-long temperature (Sr/Ca and Li/Mg) and calcifying fluid (CF) carbonate chemistry (δ11B and B/Ca) records for a long-lived (1919 to 2016) Porites coral from the high thermally variable Kimberley region of northwestern Australia. We investigate how increasing temperatures and ocean acidification are manifested in the carbonate chemistry of coral’s CF and impacts of climate change on calcification. Using Sr/Ca and Li/Mg multiproxy we show that annual temperature in the nearshore Kimberley exhibited a gradual increase (0.009 ± 0.003 °C/yr) from the 1920s onward. However for the most recent years (2000–2015) more rapid summer warming (0.05 ± 0.01 °C/yr) are registered, indicative of intensified warming. Despite that, we find no significant trend for calcification rate of this coral over the past century, as well as ‘normal’ seasonal variability in coral’s CF carbonate chemistry. Importantly, the coral’s ability to concentrate inorganic carbon seems to be affected by recent warming, with reduced DICcf observed during 2008 to 2015, while the minimally-affected pHcf acts to compensate the decreases of DICcf with the calcification rate showing only slight decrease. Additionally, we also find that ocean acidification has clearly led to the long-term reduction in the pH of the CF.

Continue reading ‘Anthropogenic ocean warming and acidification recorded by Sr/Ca, Li/Mg, δ11B and B/Ca in Porites coral from the Kimberley region of northwestern Australia’

Carbon outwelling across the shelf following a massive mangrove dieback in Australia: insights from radium isotopes

Mangrove soil carbon stocks are known to decrease following forest loss due to respiration and enhanced soil CO2 emissions. However, changes in carbon outwelling to the coastal ocean due to mangrove forest disturbance have not been considered. In December 2015, an extremely large mangrove dieback event (∼7000 hectares, spanning 1000 km of coastline) occurred in the Gulf of Carpentaria, Australia. To assess the effect this dieback event had on carbon outwelling, we used radium isotopes and dissolved carbon measurements (dissolved organic carbon, DOC, dissolved inorganic carbon, DIC, and total alkalinity, TAlk) to estimate cross-shelf carbon transport from living and dead mangrove areas and to calculate the carbon losses from living and dead forest soils via SGD. Radium distributions imply cross shelf eddy diffusivity of 107.5 ± 26.9 and 104.6 ± 23.9 m−2 s−1 from dead and living areas and radium water ages reveal that mangrove carbon reaches 10 km offshore within 7 days. Outwelling rates from living and dead areas were explained by soil carbon losses via SGD. This study suggests a decrease in carbon outwelling to the ocean from dead forest areas compares to living areas by 0–12% for DOC, 50–52% for DIC and by 37–51% for TAlk ∼8 months after the dieback event occurred. Changes to oceanic carbon outwelling rates following mangrove loss are likely driven by a gradual depletion of carbon stocks from the sediment profile.

Continue reading ‘Carbon outwelling across the shelf following a massive mangrove dieback in Australia: insights from radium isotopes’

Coral reef calcification and production after the 2016 bleaching event at Lizard Island, Great Barrier Reef

Severe coral bleaching events have affected the Great Barrier Reef (GBR) causing massive losses of hard coral cover. Here, we use flow respirometry approaches to assess coral reef net ecosystem calcification (NEC) and net ecosystem production (NEP) following the 2015/2016 bleaching event at Lizard Island in the northern GBR, a heavily impacted area. Previous studies conducted in 2008 and 2009 [Silverman et al., 2014] were used as pre‐impact data. Lagrangian and Eulerian approaches provided varied results. Estimated NEC (29.1 – 137.7 mmol m‐2 day‐1) and NEP (‐876.7 – 50.5 mmol m‐2 day‐1) rates in 2016 were highly sensitive to assumptions about reef water residence times and oceanic endmember concentrations. Replicating the methodology used for the 2008 and 2009 study resulted in post‐bleaching NEC in 2016 at 32 ± 10.8 mmol m‐2 day‐1, 40 – 46% lower than pre‐bleaching estimates in 2008 (61 ± 12 mmol m‐2 day‐1) and 2009 (54 ± 13 mmol m‐2 day‐1). The slopes of a total alkalinity vs. dissolved inorganic carbon (TA – DIC) plot decreased from ~ 0.3 in 2008 and 2009 to 0.1 in 2016, indicating elevated organic production and a shift in community function. Changes in NEC relative to the previous study were not driven by changing Ω arag. Coral cover shifted from 8.3% and 7.1% in 2008 and 2009 to 3.0% in 2016. We demonstrate a clear decrease in coral reef NEC following bleaching and highlight that subtle assumptions/methodological differences may create bias in the interpretation of results. Therefore, comparing coral reef metabolism datasets and predicting long‐term coral reef calcification based on existing short‐term datasets needs to be done with care.

Continue reading ‘Coral reef calcification and production after the 2016 bleaching event at Lizard Island, Great Barrier Reef’


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OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

OUP book