Posts Tagged 'biogeochemistry'

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’

The Great Barrier Reef: A source of CO2 to the atmosphere

Highlights

• Seasonal variations in air-sea CO2 fluxes on the Great Barrier Reef reveal a strong CO2 release during the early-dry season.

• The Great Barrier Reef is overall a net source of CO2.

• CO2 fluxes are largely controlled by cross-shelf advection of oversaturated warm surface waters from the Coral Sea.

Abstract

The Great Barrier Reef (GBR) is the largest contiguous coral reef system in the world. Carbonate chemistry studies and flux quantification within the GBR have largely focused on reef calcification and dissolution, with relatively little work on shelf-scale CO2 dynamics. In this manuscript, we describe the shelf-scale seasonal variability in inorganic carbon and air-sea CO2 fluxes over the main seasons (wet summer, early dry and late dry seasons) in the GBR.

Our large-scale dataset reveals that despite spatial-temporal variations, the GBR as a whole is a net source of CO2 to the atmosphere, with calculated air–sea fluxes varying between −6.19 and 12.17 mmol m−2 d−1 (average ± standard error: 1.44 ± 0.15 mmol m−2 d−1), with the strongest release of CO2 occurring during the wet season. The release of CO2 to the atmosphere is likely controlled by mixing of Coral Sea surface water, typically oversaturated in CO2, with the warm shelf waters of the GBR. This leads to oversaturation of the GBR system relative to the atmosphere and a consequent net CO2 release.

Continue reading ‘The Great Barrier Reef: A source of CO2 to the atmosphere’

Riverine calcium end-members improve coastal saturation state calculations and reveal regionally variable calcification potential

Carbonate-rich groundwater discharged from springs, seeps, and spring-fed rivers on carbonate platforms creates environments of potential refuge for calcifying organisms in coastal waters by supplying higher [Ca2+] and [CO32-] along with typically lower nutrient concentrations. The benefits associated with carbonate terrains are maximized in the presence of submerged aquatic vegetation (SAV), especially seagrasses. To improve the accuracy of carbonate saturation state (Ω) determinations, calculated values of [CO32-] and Ksp∗ were paired with [Ca2+] values determined using a model that incorporates directly measured riverine calcium end-members (model A). This model results in Ω values larger than those calculated by assuming that [Ca2+] is directly proportional to salinity (model B; e.g., using CO2SYS, CO2calc). As an example, for salinity (S) between 13.5 and 24, improvements in saturation states calculated as differences (ΔΩ) between model A and model B saturation states in the tidal mixing zone of the Weeki Wachee River (Florida, United States) ranged from 0.39 to 1.00 (aragonite) and 0.61–1.65 (calcite). Saturation state ratios (Ω(A)/Ω(B)) for coastal waters with enhanced [Ca2+] originating from carbonate-rich groundwater can be calculated from end-member calcium concentrations and salinity. Applied to several river systems in the conterminous United States, Ω(A)/Ω(B) values calculated at S = 20 lead to Ω(A)/Ω(B) ratios of 1.12 (Weeki Wachee), 1.09 (Anclote), 1.06 (Mississippi), and 1.03 (Columbia). These increases in saturation states can be used to identify potential calcification refugia for subsequent high resolution field studies that focus on, for example, the long-term viability of oyster communities and other calcifying organisms in brackish coastal waters.

Continue reading ‘Riverine calcium end-members improve coastal saturation state calculations and reveal regionally variable calcification potential’


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Ocean acidification in the IPCC AR5 WG II

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