Development of a colorimetric microfluidic pH sensor for autonomous seawater measurements

High quality carbonate chemistry measurements are required in order to fully understand the dynamics of the oceanic carbonate system. Seawater pH data with good spatial and temporal coverage are particularly critical to apprehend ocean acidification phenomena and their consequences. There is a growing need for autonomous in situ instruments that measure pH on remote platforms. Our aim is to develop an accurate and precise autonomous in situ pH sensor for long term deployment on remote platforms. The widely used spectrophotometric pH technique is capable of the required high-quality measurements. We report a key step toward the miniaturisation of a colorimetric pH sensor with the successful implementation of a simple microfluidic design with low reagent consumption. The system is particularly adapted to shipboard deployment: high quality data was obtained over a period of more than a month during a shipboard deployment in northwest European shelf waters, and less than 30 mL of indicator was consumed. The system featured a short term precision of 0.001 pH (n=20) and an accuracy within the range of a certified Tris buffer (0.004 pH). The quality of the pH system measurements have been checked using various approaches: measurements of certified Tris buffer, measurement of certified seawater for DIC and TA, comparison of measured pH against calculated pH from pCO₂, DIC and TA during the cruise in northwest European shelf waters. All showed that our measurements were of high quality. The measurements were made close to in situ temperature (+0.2 °C) in a sampling chamber which had a continuous flow of the ship’s underway seawater supply. The optical set up was robust and relatively small due to the use of an USB mini-spectrometer, a custom made polymeric flow cell and an LED light source. The use of a three wavelength LED with detection that integrated power across the whole of each LED output spectrum indicated that low wavelength resolution detectors can be used instead of the current USB mini spectrophotometer. Artefacts due to the polychromatic light source and inhomogeneity in the absorption cell are shown to have a negligible impact on the data quality. The next step in the miniaturisation of the sensor will be the incorporation of a photodiode as detector to replace the spectrophotometer.

Assessing seasonal changes in carbonate parameters across small spatial gradients in the Northeastern Chukchi Sea

Observations of the marine carbonate system were made in 2010 in the northeastern Chukchi Sea to constrain the seasonal progression of carbonate mineral saturation states (Ω) throughout the water column and determine the air-sea flux of carbon dioxide (CO₂). As sea ice retreats from the Chukchi Shelf, primary production consumes dissolved inorganic carbon (DIC) in the euphotic zone causing pH and carbonate mineral saturation states to increase. Throughout the summer and early autumn months of 2010, saturation states for calcite and aragonite ranged from 2.5–4.0 and 1.5–2.5, respectively, well about the saturation horizon of 1.0. Much of the organic matter produced during the bloom was vertically exported from the relatively small study area leading to an uptake of CO₂ from the atmosphere of at least 340,000 kg-C. The exported organic matter settled near the bottom and was remineralized back into DIC, causing concentrations to increase sharply, particularly in autumn months, driving down pH to as low as 7.75 and suppressing the concentrations of important carbonate minerals to the point that aragonite became undersaturated. The data showed a definitive seasonal progression of this process with aragonite becoming partially undersaturated along the bottom in September, and broadly undersaturated in October. While carbonate saturation states would naturally be suppressed by the high rates of export production and the accumulation of DIC near the bottom, the penetration of anthropogenic CO₂ into water column (ocean acidification) has caused these observed undersaturations, which will likely expand as CO₂ levels in the atmosphere continue to rise in the coming decades.

Ocean acidification from 1997 to 2011 in the subarctic western North Pacific Ocean

Rising atmospheric CO₂ contents have led to greater CO₂ uptake by the oceans, lowering both pH due to increasing hydrogen ions and CaCO₃ saturation states due to declining carbonate ion (CO₃²⁻). Here, we used previously compiled data sets and new data collected in 2010 and 2011 to investigate ocean acidification of the North Pacific western subarctic gyre. In winter, the western subarctic gyre is a source of CO₂ to the atmosphere because of convective mixing of deep waters rich in dissolved inorganic carbon (DIC). We calculated pH in winter mixed layer from DIC and total alkalinity (TA), and found that it decreased at the rate of −0.001 ± 0.0004 yr⁻¹ from 1997 to 2011. This decrease rate is slower than that expected under condition of seawater/atmosphere equilibration, and it is also slower than the rate in the subtropical regions (−0.002 yr⁻¹). The slow rate is caused by a reduction of CO₂ emission in winter due to an increase in TA. Below the mixed layer, the calcite saturation horizon (~185 m depth) shoaled at the rate of 2.9 ± 0.9 m yr⁻¹ as the result of the declining CO₃²⁻ concentration (−0.03 ± 0.01 μmol k⁻¹yr⁻¹). Between 200 m and 300 m depth, pH decline during the study period (−0.0051 ± 0.0010 yr⁻¹) was larger than ever reported in the open North Pacific. This enhanced acidification rate below the calcite saturation horizon reflected not only the uptake of anthropogenic CO₂ but also the increase in the decomposition of organic matter evaluated from the increase in AOU, which suggests that the dissolution of CaCO₃ particles increased.

Ecosystem effects of shell aggregations and cycling in coastal waters: an example of Chesapeake Bay oyster reefs

Disease, overharvesting, and pollution have impaired the role of bivalves on coastal ecosystems, some to the point of functional extinction. An underappreciated function of many bivalves in these systems is shell formation. The ecological significance of bivalve shell has been recognized; geochemical effects are now more clearly being understood. A positive feedback exists between shell aggregations and healthy bivalve populations in temperate estuaries, thus linking population dynamics to shell budgets and alkalinity cycling. On oyster reefs a balanced shell budget requires healthy long-lived bivalves to maximize shell input per mortality event thereby countering shell loss. Active and dense populations of filter-feeding bivalves couple production of organic-rich waste with precipitation of calcium carbonate minerals, creating conditions favorable for alkalinity regeneration. Although the dynamics of these processes are not well described, the balance between shell burial and metabolic acid production seems the key to the extent of alkalinity production vs. carbon burial as shell. We present an estimated alkalinity budget that highlights the significant role oyster reefs once played in the Chesapeake Bay inorganic-carbon cycle. Sustainable coastal and estuarine bivalve populations require a comprehensive understanding of shell budgets and feedbacks among population dynamics, agents of shell destruction, and anthropogenic impacts on coastal carbonate chemistry.

Ocean acidification state in western Antarctic surface waters: drivers and interannual variability

Each December during four years from 2006 to 2010, the surface water carbonate system was measured and investigated in the Amundsen Sea and Ross Sea, western Antarctica as part of the Oden Southern Ocean expeditions (OSO). The I/B Oden started in Punta Arenas in Chile and sailed southwest, passing through different regimes such as, the marginal/seasonal ice zone, fronts, coastal shelves, and polynyas. Discrete surface water was sampled underway for analysis of total alkalinity (A_T), total dissolved inorganic carbon (C_T) and pH. Two of these parameters were used together with sea-surface temperature (SST), and salinity to obtain a full description of the surface water carbonate system, including pH in situ and calcium carbonate saturation state of aragonite (Ω_Ar) and calcite (Ω_Ca). Multivariate analysis was used to investigate interannual variability and the major controls (sea-ice concentration, SST, salinity and chlorophyll a) on the variability in the carbonate system and Ω. This analysis showed that SST and chlorophyll a were the major drivers of the Ω variability in both the Amundsen and Ross seas. In 2007, the sea-ice edge was located further south and the area of the open polynya was relatively small compared to 2010. We found the lowest pH in situ (7.932) and Ω = 1 values in the sea-ice zone and in the coastal Amundsen Sea, nearby marine out flowing glaciers. In 2010, the sea-ice coverage was the largest and the areas of the open polynyas were the largest for the whole period. This year we found the lowest salinity and A_T, coinciding with highest chl a. This implies that the highest Ω_Ar in 2010 was likely an effect of biological CO₂ drawdown, which out-competed the dilution of carbonate ion concentration due to large melt water volumes. We predict and discuss future Ω values, using our data and reported rates of oceanic uptake of anthropogenic CO₂, suggesting that the Amundsen Sea will become undersaturated with regard to aragonite about 20 yr sooner than predicted by models.

Report of the SNOMS Project 2006 to 2012, SNOMS SWIRE NOCS Ocean Monitoring System. Part 1: Narrative description

The ocean plays a major role in controlling the concentration of carbon dioxide (CO2) in the atmosphere. Increasing concentrations of CO2 in the atmosphere are a threat to the stability of the earth’s climate. A better understanding of the controlling role of the ocean will improve predictions of likely future changes in climate and the impact of the uptake of CO2 itself on marine eco-systems caused by the associated acidification of the ocean waters. The SNOMS Project (SWIRE NOCS Ocean Monitoring System) is a ground breaking joint research project supported by the Swire Group Trust, the Swire Educational Trust, the China Navigation Company (CNCo) and the Natural Environment Research Council. It collects high quality data on concentrations of CO2 in the surface layer of the ocean. It contributes to the international effort to better quantify (and understand the driving processes controlling) the exchanges of CO2 between the ocean and the atmosphere. In 2006 and 2007 a system that could be used on a commercial ship to provide data over periods of several months with only limited maintenance by the ships crew was designed and assembled by NOCS. The system was fitted to the CNCo ship the MV Pacific Celebes in May 2007. The onboard system was supported by web pages that monitored the progress of the ship and the functioning of the data collection system. To support the flow of data from the ship to the archiving of the data at the Carbon Dioxide Information Analysis Center (CDIAC in the USA) data processing procedures were developed for the quality control and systematic handling of the data. Data from samples of seawater collected by the ships crew and analysed in NOC (730 samples) have been used to confirm the consistency of the data from the automated measurement system on the ship. To examine the data collected between 2007 and 2012 the movements of the ship are divided into 16 voyages. Initially The Celebes traded on a route circum-navigating the globe via the Panama and Suez Canals. In 2009 the route shifted to one between Australia and New Zealand to USA and Canada. Analysis of the data is an on going process. It has demonstrated that the system produces reliable data. Data are capable of improving existing estimates of seasonal variability. The work has improved knowledge of gas exchange processes. Data from the crew-collected-samples are helping improve our ability to estimate alkalinity in different areas. This helps with the study of ocean acidification. Data from the 9 round trips in the Pacific are currently being examined along with data made available by the NOAA-PMEL laboratory forming time series from 2004 to 2012. The data from the Pacific route are of considerable interest. One reason is that the data monitors variations in the fluxes of CO2 associated with the current that flows westwards along the equator. This is one of the major natural sources of CO2 from the ocean into the atmosphere.

Development and validation of an experimental life support system for assessing the effects of global climate change and environmental contamination on estuarine and coastal marine benthic communities

An experimental life support system (ELSS) was constructed to study, the interactive effects of multiple stressors on coastal and estuarine benthic communities, specifically perturbations driven by global climate change and anthropogenic environmental contamination. The ELSS allows researchers to control salinity, pH, temperature, ultraviolet radiation (UVR), tidal rhythms and exposure to selected contaminants. Unlike most microcosms previously described, our system enables true independent replication (including randomisation). In addition to this, it can be assembled using commercially available materials and equipment, thereby facilitating the replication of identical experimental setups in different geographical locations. Here, we validate the reproducibility and environmental quality of the system by comparing chemical and biological parameters recorded in our ELSS with those prevalent in the natural environment. Water, sediment microbial community and ragworm (the polychaete Hediste diversicolor) samples were obtained from four microcosms after 57 days of operation. In general, average concentrations of dissolved inorganic nutrients (NO₃^-; NH₄⁺ and PO₄ ⁻³) in the water-column of the ELSS experimental control units were within the range of concentrations recorded in the natural environment. While some shifts in bacterial community composition were observed between in situ and ELSS sediment samples, the relative abundance of most metabolically active bacterial taxa appeared to be stable. In addition, ELSS operation did not significantly affect survival, oxidative stress and neurological biomarkers of the model organism H. diversicolor. The validation data indicates that this system can be used to assess independent or interactive effects of climate change and environmental contamination on benthic communities. Researchers will be able to simulate the effects of these stressors on processes driven by microbial communities, sediment and seawater chemistry and to evaluate potential consequences to sediment toxicity using model organisms such as Hediste diversicolor.

Acid sulfate soil induced acidification of estuarine areas used for the production of Sydney Rock oysters, Saccostrea glomerata

This study investigated estuarine acidification, associated with drainage and excavation of acid sulfate soils, in areas used for commercial cultivation of Sydney rock oysters (Saccostrea glomerata). Regular measurements of pH and electrical conductivity were collected in oyster cultivation areas and acidified reaches of the Hastings River estuary and Port Stephens estuary located on the mid north coast of New South Wales, Australia. Water quality information from acidified floodplain drains was also collected in the Hastings River following heavy rainfall. Both estuaries experienced acidification of tributaries following periods of heavy rainfall. Drain outflow waters were acidic (pH < 3.5); contained elevated concentrations of iron, aluminium, manganese and zinc; and polluted areas used for oyster production. The extent and duration of estuarine acidification events was capable of causing a variety of short- and long-term impacts to oysters as well as other aquatic organisms in affected areas.

Evidence for changes in carbon isotopic fractionation by phytoplankton between 1960 and 2010

Rising CO₂ is expected to drive a myriad of environmental changes in the surface ocean. Deciphering the phytoplankton response to this complex change is difficult. Here we determine whether a trend in the biological fractionation of stable carbon isotopes (ε_p) has occurred over the past 50 years. ε_p is primarily controlled by the acquistion and intracellular transport of inorganic carbon and the rate of carbon fixation. In turn, these processes are sensitive to phytoplankton physiology, community composition, and, notably, inorganic carbon availability. ε_p may therefore carry a signal of biological response to climate change. Temporal and spatial records of ε_p can be deciphered from the difference between the stable carbon isotopic composition of particulate organic matter (δ¹³C_POC) and that of the ambient inorganic carbon pool (δ¹³C_CO2). Here we establish a global record of ε_p extending from the 1960s to today, extracted from a newly compiled dataset of global measured δ¹³C_POC and part measured/part climatology δ¹³C_CO2. We find that ε_p has changed significantly since the 1960s in the low- to mid-latitude surface ocean. The increase is most pronounced in the subtropics, where it is is estimated at > 0.015 ‰ per year. Our findings of such rates of change are further supported by a high resolution temporal record from a single sediment trap near Bermuda. Our results are consistent with the idea that ε_p is affected by increased inorganic carbon availability driven by the rise in atmospheric CO₂.

Long-term trends in calcifying plankton and pH in the North Sea

Relationships between six calcifying plankton groups and pH are explored in a highly biologically productive and data-rich area of the central North Sea using time-series datasets. The long-term trends show that abundances of foraminiferans, coccolithophores, and echinoderm larvae have risen over the last few decades while the abundances of bivalves and pteropods have declined. Despite good coverage of pH data for the study area there is uncertainty over the quality of this historical dataset; pH appears to have been declining since the mid 1990s but there was no statistical connection between the abundance of the calcifying plankton and the pH trends. If there are any effects of pH on calcifying plankton in the North Sea they appear to be masked by the combined effects of other climatic (e.g. temperature), chemical (nutrient concentrations) and biotic (predation) drivers. Certain calcified plankton have proliferated in the central North Sea, and are tolerant of changes in pH that have occurred since the 1950s but bivalve larvae and pteropods have declined. An improved monitoring programme is required as ocean acidification may be occurring at a rate that will exceed the environmental niches of numerous planktonic taxa, testing their capacities for acclimation and genetic adaptation.