Posts Tagged 'biogeochemistry'

Organic carbon and carbonate system in the bottom sediments of shallow bights of the Peter the Great Bay (Sea of Japan)

The diagenesis of organic matter (OM) is studied in bottom sediments taken in February, 2018 from therapeutic mud deposits of the Uglovoi Bay and Voevoda and Ekspeditsiya bights (Peter the Great Bay, Sea of Japan). The carbonate system of bottom sediments and pore water were analyzed for the contents of nutrients, dissolved organic carbon, humic substance, and concentrations of sulfates and chlorides. The concentrations of organic carbon, chlorophyll-a, humic and fulvic acids, and mobile sulfide species are measured in a solid phase of sediment. Underwater photographing shows that sampling localities are covered by Zostera marina meadows in the Voevoda and Ekspeditsiya bights and by diatom mats in Uglovoi Bay. The proportions between dissolved inorganic carbon and alkalinity, as well as data on sulfate–chlorine ratios and mobile sulfide species indicate that the OM degradation in bottom sediments is mainly controlled by sulfate reduction. The Uglovoi Bay and Voevoda and Ekspeditsii bights are characterized by different values of bioturbation coefficients: 3.0, 107.6, and 14.5 cm2/day, respectively. The estimated fluxes of organic carbon from water into sediment and of dissolved inorganic carbon from sediment into water significantly differ. The disbalance between organic and inorganic carbons can be caused by the following reasons: (a) ignored CO2 flux released by marine organisms from bottom sediments through their siphonal system; (b) partial OM consumption in food with its subsequent deposition in it.

Continue reading ‘Organic carbon and carbonate system in the bottom sediments of shallow bights of the Peter the Great Bay (Sea of Japan)’

Along-path evolution of biogeochemical and carbonate system properties in the Intermediate Water of the Western Mediterranean

A basin-scale oceanographic cruise (OCEANCERTAIN2015) was carried out in the Western Mediterranean (WMED) in summer 2015 to study the evolution of hydrological and biogeochemical properties of the most ubiquitous water mass of the Mediterranean Sea, the Intermediate Water (IW). IW is a relatively warm water mass, formed in the Eastern Mediterranean (EMED) and identified by a salinity maximum all over the basin. While it flows westward, toward and across the WMED, it gradually loses its characteristics. This study describes the along-path changes of thermohaline and biogeochemical properties of the IW in the WMED, trying to discriminate changes induced by mixing and changes induced by interior biogeochemical processes. In the first part of the path (from the Sicily Channel to the Tyrrhenian Sea), respiration in the IW interior was found to have a dominant role in determining its biogeochemical evolution. Afterward, when IW crosses regions of enhanced vertical dynamics (Ligurian Sea, Gulf of Lion and Catalan Sea), mixing with surrounding water masses becomes the primary process. In the final part of the investigated IW path (the Menorca-Mallorca region), the role of respiration is further masked by the effects of a complex circulation of IW, indicating that short-term sub-regional hydrological processes are important to define IW characteristics in the westernmost part of the investigated area. A pronounced along-path acidification was detected in IW, mainly due to remineralization of organic matter. This induced a shift of the carbonate equilibrium toward more acidic species and makes this water mass increasingly less adequate for an optimal growth of calcifying organisms. The carbonate buffering capacity also decreases as IW flows through the WMED, making it more exposed to the adverse effects of a decreasing pH. The present analysis indicates that IW evolution in the sub-basins of the WMED is currently driven by complex hydrological and biogeochemical processes, which could be differently impacted by coming climate changes, in particular considering expected increases of extreme meteorological events, mainly due to the warming of the Mediterranean basin.

Continue reading ‘Along-path evolution of biogeochemical and carbonate system properties in the Intermediate Water of the Western Mediterranean’

Severe coastal hypoxia interchange with ocean acidification: an experimental perturbation study on carbon and nutrient biogeochemistry

Normally atmospheric CO2 is the major driver of ocean acidification (OA); however, local discharge/degradation of organic matter (OM) and redox reactions can exacerbate OA in coastal areas. In this work we study the response of nutrient and carbon systems to pH decrease in relation to hydrographically induced intermittent characteristics and examine scenarios for future ocean acidification in a coastal system. Laboratory microcosm experiments were conducted using seawater and surface sediment collected from the deepest part of Elefsis Bay; the pH was constantly being monitored while CO2 gas addition was adjusted automatically. In Elefsis Bay surface pCO2 is already higher than global present atmospheric values, while near the bottom pCO2 reaches 1538 μatm and carbonate saturation states were calculated to be around 1.5. During the experiment, in more acidified conditions, limited alkalinity increase was observed and was correlated with the addition of bicarbonates and OM. Ammonium oxidation was decelerated and a nitrification mechanism was noticed, despite oxygen deficiency, paralleled by reduction of Mn-oxides. Phosphate was found significantly elevated for the first time in lower pH values, without reprecipitating after reoxygenation; this was linked with Fe(II) oxidation and Fe(III) reprecipitation without phosphate adsorption affecting both available dissolved phosphate and (dissolved inorganic nitrogen) DIN:DIP (dissolved inorganic phosphate)ratio.

Continue reading ‘Severe coastal hypoxia interchange with ocean acidification: an experimental perturbation study on carbon and nutrient biogeochemistry’

The ability of fragmented kelp forests to mitigate ocean acidification and the effects of seasonal upwelling on kelp-purple sea urchin interactions

Bull kelp (Nereocystis leutkeana) forests along the coast for northern California have decreased dramatically as a result of a ‘perfect storm’ of multiple environmental stressors. The disappearance of a predatory sea star and subsequent increase in purple sea urchins (Strongylocentrotus purpuratus) and the recurrence of marine heat waves have caused these once diverse ecosystems to be rapidly converted into relative species-depauperate urchin barrens. By examining the interactive effects of both a rapidly changing abiotic environment and the increase in urchin grazing pressure that is affecting this vital ecosystem, we can better understand its ultimate fate and make better-informed decisions to manage and protect it. As once large and persistent kelp forests are converted into fragmented landscapes of small kelp patches, kelp’s ability to take up dissolved inorganic carbon and reduce nearby acidity and increase both dissolved oxygen and bio-available calcium carbonate may be reduced, preventing it from serving as an environmental stress-free ‘oasis’ of reduced environmental stresses for local marine organisms and affecting ecosystem dynamics. In my first chapter, I examined whether small, fragmented kelp patches are able to retain their ability to alter local seawater chemistry to the same extent a large persistent kelp forests that have been studied previously. I found that in the canopies of small kelp patches, multiple parameters of carbonate chemistry fluctuated more than in the kelp benthos and in adjacent urchin barrens, consistent with metabolic activity by the kelp. Further, kelp fragments increased pH and aragonite saturation and decreased pCO2 during the day to a similar degree as large, intact kelp forests. These results suggest that small kelp patches could mitigate OA stress during the day and serve as spatial and temporal refugia for canopy-dwelling organisms. I also found that the benthic environment in kelp forests and adjacent urchin barrens is subject to unbuffered decreases in temperature, dissolved oxygen and pH. Thus, in chapter two, I assessed how current-day and future-predicted fluctuations in the duration and magnitude of these upwelling-associated stressors would impact the grazing, growth, and survivorship of purple urchins from kelp forest and urchin barren habitats. With upwelling predicted to increase in both intensity and duration with global climate change, understanding whether urchins from different habitats are differentially affected by upwelling-related stressors will give insight into how current and future stressors may be able to help ‘tip the scales’ and convert the increasing number of urchin barrens back into healthy productive kelp forests. I found condition-dependent susceptibility in urchins to increased magnitude and duration upwelling-related stressors. Grazing and gonadal development in kelp forest urchins was most negatively affected by distant future upwelling conditions, whereas in urchin barren urchins, grazing and survival were sensitive to exposure to upwelling in general, and also to increase in magnitudes of acidity, hypoxia, and temperature across both upwelling and non-upwelling events in the future. These results have important implications for population dynamics of urchins and their interactions with bull kelp, which could strongly affect ecosystem dynamics and transitions between kelp forests and urchin barrens. Taken together, the two chapters my thesis provide valuable insight into the potential resilience of bull kelp, a critical foundation species in northeastern Pacific coastal habitats, in the face of a rapidly changing multi-stressor environment.

Continue reading ‘The ability of fragmented kelp forests to mitigate ocean acidification and the effects of seasonal upwelling on kelp-purple sea urchin interactions’

Sea-ice loss amplifies summertime decadal CO2 increase in the western Arctic Ocean

Rapid climate warming and sea-ice loss have induced major changes in the sea surface partial pressure of CO2 (pCO2pCO2). However, the long-term trends in the western Arctic Ocean are unknown. Here we show that in 1994–2017, summer pCO2pCO2 in the Canada Basin increased at twice the rate of atmospheric increase. Warming and ice loss in the basin have strengthened the pCO2pCO2 seasonal amplitude, resulting in the rapid decadal increase. Consequently, the summer air–sea CO2 gradient has reduced rapidly, and may become near zero within two decades. In contrast, there was no significant pCO2pCO2 increase on the Chukchi Shelf, where strong and increasing biological uptake has held pCO2pCO2 low, and thus the CO2 sink has increased and may increase further due to the atmospheric CO2 increase. Our findings elucidate the contrasting physical and biological drivers controlling sea surface pCO2pCO2 variations and trends in response to climate change in the Arctic Ocean.

Continue reading ‘Sea-ice loss amplifies summertime decadal CO2 increase in the western Arctic Ocean’

Changing nutrients, dissolved oxygen and carbonate system in the Bohai and Yellow Seas, China

The Bohai and Yellow Seas in the Northwest Pacific are semi-enclosed shallow marginal seas of ecological and economic significance. By reviewing and synthesizing literature data, basin-wide decadal changes in nutrients and bottom-water dissolved oxygen and carbonate system parameters in the two coastal oceans were investigated. Results showed that both of the two coastal oceans were subject to basin-wide increases in wintertime nitrate during the past 40 years. The present-day seawater N:P ratios are usually within the algae-favorable range of 14–19. Presumably due to these changes, the Bohai Sea exhibits a 33-year decline in summertime bottom-water dissolved oxygen and the associated suppression of pH and CaCO3 saturation states in summer. The historically lowest bottom-water dissolved oxygen in the Bohai Sea was recorded at 67 μmol O2 L−1 in early September 2015, which was very close to the threshold value of hypoxia. In the Yellow Sea, periodical suppression of pH and CaCO3 saturation states occurs in its central basin area, where the net community carbonate dissolution was detectable in bottom waters in late summer and autumn, threatening marine calcifiers inhabiting there and with potentially severe consequences for valuable shellfish fisheries.

Continue reading ‘Changing nutrients, dissolved oxygen and carbonate system in the Bohai and Yellow Seas, China’

Transient carbonate chemistry in the expanded Kuroshio region

The Kuroshio is the most significant current in the western North Pacific Ocean and affects a wide area. This work shows that the intrusion of the oligotrophic upper-layer West Philippine Sea seawater into the South China Sea (SCS) as the branch of Kuroshio reduced the productivity and hence the fluxes of sinking particles in the SCS between 2013 and 2017. Conversely, the productivity in the SCS increased during a large scale Kuroshio intrusion in 1998–2006, indicating that other factors also affected the productivity. Further, the western North Pacific Ocean is acidifying, with the surface seawaters to the west having lower acidification rates. This phenomenon is likely a consequence of enhanced productivity owing to more anthropogenic nutrient inputs from the continent in the west, but needs further investigation. In the East China Sea, the Kuroshio Intermediate Water has increased nutrient concentrations, but decreased in both dissolved oxygen (DO) concentration and pH, most likely owing to reduced ventilation in the North Pacific Intermediate Water. Further warming of the surface oceans would strengthen the stratification of the surface ocean, weakening ventilation. Consequently, DO and pH would continue to decline while nutrients level increases.

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Direct evidence of sediment carbonate dissolution in response to bottom-water acidification in the Gulf of St. Lawrence, Canada

Over the past century, dissolved oxygen concentrations (DO) have decreased and metabolic CO2 has accumulated within in the bottom waters of the Gulf of St. Lawrence (GSL) and Lower St. Lawrence Estuary (LSLE). Oxygen depletion has been attributed primarily to changes in ocean circulation in the northwest Atlantic Ocean, as well as an increase in the flux of organic matter at or near the seafloor and its accompanying biological oxygen demand. The accumulation of metabolic CO2 in these waters has led to their progressive acidification and a decrease in pH (0.3-0.4 pH unit) commensurate to the variation expected for global oceanic surface waters by the end of this century, albeit by a different mechanism (anthropogenic CO2 uptake from the atmosphere). The decrease in bottom-water pH of the GSL and LSLE is accompanied by a decrease in the carbonate ion concentration and the saturation state of the waters with respect to both calcite and aragonite (ΩC and ΩA). Although the Laurentian Trough sediments are mostly devoid of modern calcium carbonate fossils, detrital (Ordovician/Silurian) carbonates, eroded from Anticosti Island, accumulate on the seafloor. Evidence of carbonate mineral dissolution in the sediments of the Laurentian Trough is examined and supported by pore-water data and vertical variations of their inorganic carbon content. Historical, solid-phase profile data are used to estimate temporal variations of the sedimentary calcite dissolution rates and document the anthropogenic modification of the sediment record.

Continue reading ‘Direct evidence of sediment carbonate dissolution in response to bottom-water acidification in the Gulf of St. Lawrence, Canada’

Response of phytoplankton assemblages from naturally acidic coastal ecosystems to elevated pCO2

The interplay of coastal oceanographic processes usually results in partial pressures of CO2 (pCO2) higher than expected from the equilibrium with the atmosphere and even higher than those expected by the end of the century. Although this is a well-known situation, the natural variability of seawater chemistry at the locations from which tested organisms or communities originate is seldom considered in ocean acidification experiments. In this work, we aimed to evaluate the role of the carbonate chemistry dynamics in shaping the response of coastal phytoplankton communities to increased pCO2 levels. The study was conducted at two coastal ecosystems off Chile, the Valdivia River estuary and the coastal upwelling ecosystem in the Arauco Gulf. We characterized the seasonal variability (winter/summer) of the hydrographic conditions, the carbonate system parameters, and the phytoplankton community structure at both sites. The results showed that carbonate chemistry dynamics in the estuary were mainly related to seasonal changes in freshwater discharges, with acidic and corrosive conditions dominating in winter. In the Arauco Gulf, these conditions were observed in summer, mainly associated with the upwelling of cold and high pCO2 (>1,000 μatm) waters. Diatoms dominated the phytoplankton communities at both sites, yet the one in Valdivia was more diverse. Only certain phytoplankton groups in this latter ecosystem showed a significant correlations with the carbonate system parameters. When the impact of elevated pCO2 levels was investigated by pCO2 manipulation experiments, we did not observe any significant effect on the biomass of either of the two communities. Changes in the phytoplankton species composition and abundance during the incubations were related to other factors, such as competition and growth phases. Our findings highlight the importance of the natural variability of coastal ecosystems and the potential for local adaptation in determining responses of coastal phytoplankton communities to increased pCO2 levels.

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Evolutionary links between intra‐ and extracellular acid–base regulation in fish and other aquatic animals

The acid–base relevant molecules carbon dioxide (CO2), protons (H+), and bicarbonate (HCO3) are substrates and end products of some of the most essential physiological functions including aerobic and anaerobic respiration, ATP hydrolysis, photosynthesis, and calcification. The structure and function of many enzymes and other macromolecules are highly sensitive to changes in pH, and thus maintaining acid–base homeostasis in the face of metabolic and environmental disturbances is essential for proper cellular function. On the other hand, CO2, H+, and HCO3 have regulatory effects on various proteins and processes, both directly through allosteric modulation and indirectly through signal transduction pathways. Life in aquatic environments presents organisms with distinct acid–base challenges that are not found in terrestrial environments. These include a relatively high CO2 relative to O2 solubility that prevents internal CO2/HCO3 accumulation to buffer pH, a lower O2 content that may favor anaerobic metabolism, and variable environmental CO2, pH and O2 levels that require dynamic adjustments in acid–base homeostatic mechanisms. Additionally, some aquatic animals purposely create acidic or alkaline microenvironments that drive specialized physiological functions. For example, acidifying mechanisms can enhance O2 delivery by red blood cells, lead to ammonia trapping for excretion or buoyancy purposes, or lead to CO2 accumulation to promote photosynthesis by endosymbiotic algae. On the other hand, alkalinizing mechanisms can serve to promote calcium carbonate skeletal formation. This nonexhaustive review summarizes some of the distinct acid–base homeostatic mechanisms that have evolved in aquatic organisms to meet the particular challenges of this environment.

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