NSFGEO-NERC: A Thermodynamic Chemical Speciation Model for the Oceans, Seas, and Estuaries

Ocean acidification due to the dissolution of anthropogenic CO2, and the effects of cumulative stressors (including acidification, pollution, warming, and anoxia) are among the top priorities for ocean research, requiring accurate and consistent measurements across the globe to monitor and understand present effects, and modelling to evaluate future scenarios and methods of remediation. The work of observational scientists and modellers is linked by the need for an accurate knowledge of the chemical speciation of the inorganic carbonate system, pH, and nutrient and contaminate trace metals, in both natural waters and the reference materials and solutions used for instrument calibration.

Chemical speciation is defined as the distribution of a chemical element between different molecular and ionic forms in seawater, and determines its reactivity and bioavailability. Speciation depends on the value of the relevant thermodynamic equilibrium constant, and on the activities of each of the dissolved ions and molecules. These are complex functions of temperature, pressure, and salinity (or, more generally, solution composition), and cannot be predicted from theory. Many of the important reactions in seawater involve acid-base equilibria, which introduces pH as an additional variable. Despite the importance of chemical speciation, the available calculation tools are often only simple empirical equations that yield equilibrium constants for reactions as functions of salinity and temperature. Such equations cannot be used for many important natural waters whose composition differs from that of normal seawater (e.g., polar brines, estuaries, pore-waters, enclosed seas, and paleo-oceans). Furthermore, human-driven changes in seawater pH and carbonate chemistry in shelf seas and estuaries are complicated by the effects of eutrophication, upwelling, the dissolved solutes contained in river water, and changes in metal toxicity accompanying pH change. Consequently, despite the best efforts of physical chemists over the last several decades, there is not yet the ability to calculate the equilibria controlling the chemical factors impacting shellfish and a broader range of marine fauna in the brackish/mesohaline environments typical of many estuaries and coasts.

We will create a step change in the capability of marine scientists to measure, interpret, and predict chemical speciation and pH in natural waters of varying composition by creating a speciation model based upon the Pitzer equations for the calculation of solute and water activities. The approach has a long track record of success in geochemistry. The equations are based upon the concept that interactions between pairs and triplets of dissolved solute species control activities. The values of the parameters for these interactions are determined from a wide range of measurements of solution properties. Work in this project will include measuring activities and heat capacities, and using recent literature data, to improve and test the model; the computer coding and validation of the model and the development of methods to quantify the relationship between uncertainties in model-predicted speciation and those in the underlying measurements; and engagement with oceanographers internationally to help design practical speciation modelling tools and associated guidance for specific applications.

The completed model will enable the activities and speciation of all seawater components to be calculated within a unified framework that, (i) includes the major and trace elements in seawater and its mixtures with freshwaters, (ii) includes other saline environments of differing composition, and (iii) encompasses the buffers that are used to calibrate pH and other instruments, and. Our results will this advance the quantitative understanding of chemical speciation - from ocean measurements to ecosystem models - for an expanded range of natural water bodies and marine environments.

The degree of saturation with respect to aragonite (a form of calcium carbonate) determines the ease with which molluscs are able to grow their shells. Farming and harvesting of oysters and other species are an industry worth tens of millions of pounds annually in the UK, and billions of dollars in the USA. The first economic and societal impact of our work will be through improved decision support tools in fisheries management, based on our speciation model, which will yield a more accurate quantification of the carbonate system, aragonite saturation, and toxicity of some metals based upon regular water sampling and monitoring. This will allow the shellfish stock to be protected and/or remediation action to be taken. This impact will be accelerated by working with the Marine Knowledge Exchange Network in the U.K., and project partners who are members of recently formed ocean acidification networks in the USA and Latin America.

The same elements of the chemical modelling noted above are also central to the assessment and control of water quality, ecosystem health, and ecosystem services in (often highly populated) estuaries and coastal areas. Our chemical speciation model has a role both in the interpretation of regular measurements used for monitoring purposes, and embedded in large scale marine ecosystem models - of the North Sea for example. It will begin an expansion of the capability of current marine ecosystem models to address policy and society relevant problems including the impact of contaminants (e.g. heavy metals) on biota and their accumulation along the trophic web, the role of micronutrients in bloom formation, and the global impact of multiple stressors. This is crucial to provide information to marine environment managers and to regulators for the achievement of the Good Environmental Status as required by the Marine Strategy Framework Directive. In order to achieve impact in this area we will work with our project partner at the Plymouth Marine Laboratory first, to ensure our models include the range of inorganic species and ambient conditions required; second, to assist coding of our models in modular form to be included in ERSEM (the European Regional Seas Ecosystem Model).

This project will also have societal and economic impacts at the level of global marine environmental change, focused on the area of ocean acidification: the need to monitor and understand it over short to very long timescales, and to plan to mitigate its effects on marine environments of particular social, economic, and ecological value. The elements of our project that relate to these effects range from work to characterise the chemical speciation in buffers used to calibrate pH instrumentation (a particular interest of one of our co-investigators) and the development of stable reference materials, to the interpretation of monitoring data, and the use of marine ecosystem models to assess impacts and evaluate remediation methods.