Posts Tagged 'biogeochemistry'

Impact of ocean acidification on the biogeochemistry and meiofaunal assemblage of carbonate-rich sediments: results from core incubations (Bay of Villefranche, NW Mediterranean Sea)

Highlights

• A sediment incubation experiment to assess the effect of ocean acidification
• Porewater concentration gradients and sediment-water fluxes (DIC, TA, pH, Ca2+, O2)
• Ocean acidification impacts early diagenesis in carbonate-rich sediments.
• CaCO3 dissolution and the TA release may increase the buffering capacity of bottom water.

Abstract

Marine sediments are an important carbonate reservoir whose partial dissolution could buffer seawater pH decreases in the water column as a consequence of anthropogenic CO2 uptake by the ocean. This study investigates the impact of ocean acidification on the carbonate chemistry at the sediment-water interface (SWI) of shallow-water carbonate sediments. Twelve sediment cores were sampled at one station in the Bay of Villefranche (NW Mediterranean Sea). Four sediment cores were immediately analyzed in order to determine the initial distribution (T0) of dissolved inorganic carbon (DIC), total alkalinity (TA), pH and dissolved oxygen (O2) in the porewaters and to quantify sediment-water fluxes. Four other cores were kept submerged in the laboratory for 25 days with ambient seawater (pHT = 8.12) and the remaining four cores were incubated with acidified seawater (average pH offset of −0.68). This acidification experiment was carried out in an open-flow system, in the dark and at in-situ temperature (15 °C). Every three days, sediment-water fluxes (DIC, TA, pH, O2 and nutrients) were determined using a whole core 12-h incubation technique. Additionally, vertical O2 and pH microprofiles were regularly recorded in the first 2 cm of the sediment during the entire experiment. At the end of the experiment, TA, DIC and Ca2+ concentrations were analyzed in the porewaters and the abundance and taxonomic composition of meiofaunal organisms were assessed. The saturation states of the porewaters with respect to calcite and aragonite were over-saturated but under-saturated with respect to 12 mol% Mg-calcite, in both acidified and non-acidified treatments. The sediment-water fluxes of TA and DIC increased in the acidified treatment, likely as a consequence of enhanced carbonate dissolution. In contrast, the acidification of the overlying water did not significantly affect the O2 and nutrients fluxes at the SWI. Meiofaunal abundance decreased in both treatments over the duration of the experiment, but the organisms seemed unaffected by the acidification. Our results demonstrate that carbonate dissolution increased under acidified conditions but other parameters, such as microbial redox processes, were apparently not affected by the pH decrease, at least during the duration of our experiment. The dissolution of sedimentary carbonates and the associated release of TA may potentially buffer bottom water, depending on the intensity of the TA flux, the TA/DIC ratio, vertical mixing and, therefore, the residence time of bottom water. Under certain conditions, this process may mitigate the effect of ocean acidification on benthic ecosystems.

Continue reading ‘Impact of ocean acidification on the biogeochemistry and meiofaunal assemblage of carbonate-rich sediments: results from core incubations (Bay of Villefranche, NW Mediterranean Sea)’

Primary producers may ameliorate impacts of daytime CO2 addition in a coastal marine ecosystem

Predicting the impacts of ocean acidification in coastal habitats is complicated by bio-physical feedbacks between organisms and carbonate chemistry. Daily changes in pH and other carbonate parameters in coastal ecosystems, associated with processes such as photosynthesis and respiration, often greatly exceed global mean predicted changes over the next century. We assessed the strength of these feedbacks under projected elevated CO2 levels by conducting a field experiment in 10 macrophyte-dominated tide pools on the coast of California, USA. We evaluated changes in carbonate parameters over time and found that under ambient conditions, daytime changes in pH, pCO2, net ecosystem calcification (NEC), and O2 concentrations were strongly related to rates of net community production (NCP). CO2 was added to pools during daytime low tides, which should have reduced pH and enhanced pCO2. However, photosynthesis rapidly reduced pCO2 and increased pH, so effects of CO2 addition were not apparent unless we accounted for seaweed and surfgrass abundances. In the absence of macrophytes, CO2 addition caused pH to decline by ∼0.6 units and pCO2 to increase by ∼487 µatm over 6 hr during the daytime low tide. As macrophyte abundances increased, the impacts of CO2 addition declined because more CO2 was absorbed due to photosynthesis. Effects of CO2addition were, therefore, modified by feedbacks between NCP, pH, pCO2, and NEC. Our results underscore the potential importance of coastal macrophytes in ameliorating impacts of ocean acidification.

Continue reading ‘Primary producers may ameliorate impacts of daytime CO2 addition in a coastal marine ecosystem’

Effect of elevated pCO2 on trace gas production during an ocean acidification mesocosm experiment

A mesocosm experiment was conducted in Wuyuan Bay (Xiamen), China to investigate the effects of elevated pCO2 on phytoplankton species and production of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) as well as four halocarbon compounds (CHBrCl2, CH3Br, CH2Br2, and CH3I). Over a period of 5 weeks, P. tricornutum outcompeted T. weissflogii and E. huxleyi, comprising more than 99 % of the final biomass. During the logarithmic growth phase (phase I), DMS concentrations in high pCO2 mesocosms (1000 µatm) were 28.2 % lower than those in low pCO2 mesocosms (400 µatm). Elevated pCO2 led to a delay in DMSP-consuming bacteria attached to T. weissflogii and P. tricornutum and finally resulted in the delay of DMS concentration in the HC treatment. Unlike DMS, the elevated pCO2 did not affect DMSP production ability of T. weissflogii or P. tricornutum throughout the 5 week culture. A positive relationship was detected between CH3I and T. weissflogii and P. tricornutum during the experiment, and there was a 40.2 % reduction in mean CH3I concentrations in the HC mesocosms. CHBrCl2, CH3Br, and CH2Br2 concentrations did not increase with elevated chlorophyll a (Chl a) concentrations compared with DMS(P) and CH3I, and there were no major peak in the HC or LC mesocosms. In addition, no effect of elevated pCO2 was identified for any of the three bromocarbons. Continue reading ‘Effect of elevated pCO2 on trace gas production during an ocean acidification mesocosm experiment’

Consideration of carbon dioxide release during shell production in LCA of bivalves

Purpose

Life cycle assessment (LCA) can be used to understand the environmental impacts of the shellfish aquaculture and wild harvest industries. To date, LCA of shellfish exclude carbon dioxide (CO2) release from bivalve shell production when quantifying global warming potential per functional unit. In this study, we explain the rationale for including CO2 released during shell production in LCA of bivalves, demonstrate a method for estimating this CO2 release, and apply the method to previous studies to demonstrate the importance of including CO2 from shell production in LCA.

Methods

A simple approach for calculating CO2 from bivalve shell production was developed utilizing the seacarb package in R statistical software. The approach developed allows for inclusion of site-specific environmental parameters such as water temperature, salinity, pH, and pCO2 when calculating CO2 release from shell production. We applied the method to previously published LCA of bivalve production systems to assess the impact of including this CO2 source in the LCA. The past studies include aquaculture and wild harvest production strategies and multiple bivalve species.

Results and discussion

When we recalculated the total kg CO2 released in past studies including CO2 release from shell production, the additional CO2 release increased the total global warming impact category (CO2 equivalents) in cradle-to-gate studies by approximately 250% of the original reported value. Discussion of our results focuses on the importance of different components of our calculations and site-specific environmental parameters. We make predictions on how the magnitude and importance of CO2 released during shell production could change due to climate change and ocean acidification, and provide suggestions on how CO2 release from shell production can be reduced through careful selection of aquaculture facility location and aquaculture practices.

Conclusions

We provide a method for including CO2 from shell release in LCA of bivalves and recommend that future LCA of bivalves include this CO2 as part of the global warming impact category.

Continue reading ‘Consideration of carbon dioxide release during shell production in LCA of bivalves’

Shallow water carbonate sediments of the Galapagos archipelago: ecologically sensitive biofacies in a transitional oceanographic environment

Shallow water carbonate producing organisms are directly controlled by their local oceanography. As a result, long-term environmental signals—stemming from the breakdown of calcareous organisms—can be read from time-averaged carbonate sediments. To better understand these complex biophysical interactions, it is important to study carbonate development within oceanographic transition zones and environments affected by disturbances, such as the El Niño—Southern Oscillation (ENSO). This dissertation represents the first investigation into modern shallow water, soft sediment, carbonate environments of the Galápagos Archipelago, eastern tropical Pacific (ETP). This region is notable for straddling an oceanographic transition zone from tropical oligotrophic to temperate eutrophic—caused by high nutrient and low pH upwelling—and for being directly impacted by ENSO. A top-down approach is followed, which analyzes the biogenic structure of Galápagos sediments and their connection to local and regional oceanography and climate, and then explores how these findings relate to benthic foraminifera—sensitive environmental indicators contained within the sediments. Sediment point counting and statistical models revealed that while these carbonate environments span a biogenic and oceanographic transition comparable to similar settings in the ETP, the proximity of the Galápagos to the ENSO region directly influences its sedimentary structure and distribution. Point counting also revealed a near-absence of benthic foraminifera, which is unusual for ETP, and tropical shallow water carbonates in general. Statistically comparing foraminiferal species composition and diversity to dominant oceanographic parameters revealed the low abundances and distribution of these testate (shelled) single-celled protists to be negatively influenced by the combination of repeated Holocene ENSO events, and the effects of protracted exposure to high nutrient and low pH waters of the southern archipelago. Ultimately, the results of this study may serve as a template for investigating the interaction of carbonates and oceanography within similar atypical tropical assemblages in the fossil record.

Continue reading ‘Shallow water carbonate sediments of the Galapagos archipelago: ecologically sensitive biofacies in a transitional oceanographic environment’

Phytoplankton do not produce carbon‐rich organic matter in high CO2 oceans

The ocean is a substantial sink for atmospheric carbon dioxide (CO2) released as a result of human activities. Over the coming decades the dissolved inorganic C concentration in the surface ocean is predicted to increase, which is expected to have a direct influence on the efficiency of C utilization (consumption and production) by phytoplankton during photosynthesis. Here, we evaluated the generality of C‐rich organic matter production by examining the elemental C:N ratio of organic matter produced under conditions of varying pCO2. The data used in this analysis were obtained from a series of pelagic in situ pCO2 perturbation studies that were performed in the diverse ocean regions and involved natural phytoplankton assemblages. The C:N ratio of the resulting particulate and dissolved organic matter did not differ across the range of pCO2 conditions tested. In particular, the ratio for particulate organic C and N was found to be 6.58 ± 0.05, close to the theoretical value of 6.6.

Continue reading ‘Phytoplankton do not produce carbon‐rich organic matter in high CO2 oceans’

pH as a primary control in environmental microbiology: 1. Thermodynamic perspective

pH influences the occurrence and distribution of microorganisms. Microbes typically live over a range of 3–4 pH units and are described as acidophiles, neutrophiles, and alkaliphiles, depending on the optimal pH for growth. Their growth rates vary with pH along bell- or triangle-shaped curves, which reflect pH limits of cell structural integrity and the interference of pH with cell metabolism. We propose that pH can also affect the thermodynamics and kinetics of microbial respiration, which then help shape the composition and function of microbial communities. Here we use geochemical reaction modeling to examine how environmental pH controls the energy yields of common redox reactions in anoxic environments, including syntrophic oxidation, iron reduction, sulfate reduction, and methanogenesis. The results reveal that environmental pH changes energy yields both directly and indirectly. The direct change applies to reactions that consume or produce protons whereas the indirect effect, which applies to all redox reactions, comes from the regulation of chemical speciation by pH. The results also show that energy yields respond strongly to pH variation, which may modulate microbial interactions and help give rise to the pH limits of microbial metabolisms. These results underscore the importance of pH as a control on microbial metabolisms and provide insight into potential impacts of pH variation on the composition and activity of microbial communities. In a companion paper, we continue to explore how the kinetics of microbial metabolisms responds to pH variations, and how these responses control the outcome of microbial interactions, including the activity and membership of microbial consortia.

Continue reading ‘pH as a primary control in environmental microbiology: 1. Thermodynamic perspective’


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Ocean acidification in the IPCC AR5 WG II

OUP book