The PI request funding to undertake design improvements and complete the evaluation of an autonomous sensor for measurement of seawater total alkalinity (SAMI-alk). Total alkalinity (AT) is one of the four measureable dissolved inorganic carbon parameters and is therefore of paramount importance in the study of carbon cycling in the oceans. AT is commonly measured as part of shipboard hydrographic surveys and ocean time-series and has more recently become a critical parameter for ocean acidification studies. An autonomous AT sensor can be used in combination with other CO2 parameter sensors that are commercially available, i.e. pH and pCO2, to remotely quantify the inorganic carbon system. For example, if pH and AT are accurately measured, total dissolved inorganic carbon (DIC), carbonate, and CaCO3 saturation state can be calculated. Long-term mooring based quantification of these parameters is not yet possible. Through a previous NSF grant, we demonstrated that a novel titration methodology named Tracer Monitored Titration (TMT) could simplify AT analysis by eliminating the need for volumetric or gravimetric measurements required for conventional AT titrations. The SAMI-alk was tested both in the lab and during a cruise. The performance on the cruise was not as good as the laboratory tests. This proposal requests funds to complete the SAMI-alk development and evaluation.
Broader Impacts:
The ability to measure alkalinity is a key need for acidification research. If successful, the instrumentation development proposed here will provide an important tool for understanding the vulnerability and response of marine ecosystems to acidification. The PI has actively pursued commercialization of the autonomous instruments that he has developed in the past. Commercialization of the proposed alkalinity sensor through will greatly broaden the impacts of the technology, making it broadly available to the ocean sciences community. The project includes funding for on undergraduate student.
Overview: Atmospheric CO2 has increased from 280 ppm to 400 ppm in the past ~150 years. Approximately one third of the anthropogenic CO2 has been absorbed by the oceans.1 Although CO2 uptake by the oceans has reduced accumulation of greenhouse gases in the atmosphere, it has decreased seawater pH by more than 0.1 pH units by increasing levels of carbonic acid.2 The decreased pH is changing biological processes in the oceans, for example, it becomes more difficult for calcifying organisms to produce shells.3 The detrimental effects on calcifiers could propagate through the food chain altering entire ecosystems. The effects of ocean acidification are studied by first defining the inorganic carbon species. These species include the partial pressure of CO2 (pCO2), total hydrogen ion concentration (pH), total alkalinity (AT), total dissolved inorganic carbon (DIC or CT), and calcium carbonate saturation states (Ωcalcite and Ωaragonite). These species can all be quantified if two are measured, and the most accurate quantification comes from measurement of either pH or pCO2 in combination with either AT or DIC. In order to understand the effects of ocean acidification in many different environments, autonomous in situ instruments (instruments that can operate in the ocean without human assistance for long periods) are needed. We can currently measure pH and pCO2 autonomously, but the combination of these two measurements gives large errors for the other species4. Methods: AT measurements are laborious, taking ~20-30 minutes per sample, and generally require manual sample collection, limiting the number of samples that can be collected and analyzed. Although some benchtop, flow-through, automated systems have been produced, these cannot be moored in the ocean. Using a simplified titration method5 and technology from our autonomous instruments for measurement of pCO2 and pH, we produced the SAMI-alk (Figure 1) which measures AT autonomously and can be moored. The SAMI uses a colored indicator, Bromocresol purple, which changes form according to pH. Each form of the indicator absorbs light at a different wavelength. The indicator is mixed with an acid titrant and added to a sample, stepwise, until the sample reaches pH 4. After each addition of titrant, the concentration of the indicator and the pH of the titrant-sample mixture is measured by quantifying the amount of light absorbed at the two wavelengths that are absorbed by BCP in the pH range of interest. The concentration and pH are used to calculate the AT of the original sample. For each measurement, the SAMI-alk uses ~80 mL of sample and 4.5 mL of acid indicator, and takes ~12 minutes to measure AT. The instrument can be moored on a buoy or used on a research cruise ship. The current design supports ~750 AT measurements. Results: We deployed the SAMI-alk in a test tank of filtered water at the Hatfield Marine Sciences Center in Newport, Oregon for 10 days and in Kaneohe Bay, Oahu, Hawaii for 17 days (Figure 2). We assessed accuracy of the measurements by comparing them to benchtop, manual AT measurements on bottled samples that were collected frequently during each deployment (Figures 3 - 4). The overall accuracy was ± 2.2 µmol kg-1. The overall precision (average standard deviation of duplicate standard measurements) was ± 4.7 µmol kg-1. The performance of the instrument is good for deployments in areas where AT varies significantly throughout the day, for example on coral reefs. However, in the open ocean, the daily AT range might be as low as 5 – 10 µmol kg soln-1, which will require improved accuracy and precision. The ability to collect in situ AT data will greatly enhance our ability to study the effects of ocean acidification. References: (1) Khatiwala, S.; Tanhua, T.; Mikaloff Fletcher, S.; Gerber, M.; Doney, S. C.; Graven, H. D.; Gruber, N.; McKinley, G. a.; Murata, a.; Ríos, a. F.; et al. Global ocean storage of anthropogenic carbon. Biogeosciences 2013, 10, 2169–2191. (2) Doney, S. C.; Fabry, V. J.; Feely, R. a.; Kleypas, J. a. Ocean Acidification: The Other CO2 Problem. Ann. Rev. Mar. Sci. 2009, 1, 169–192. (3) Fabry, V. J.; Seibel, B. A.; Feely, R. A.; Orr, J. C. Impacts of ocean acidification on marine fauna and ecosystem processes. ICES J. Mar. Sci. J. du Cons. 2008, 65, 414–432. (4) Byrne, R. H.; Degrandpre, M. D.; Short, R. T.; Martz, T. R.; Merlivat, L.; Mcneil, C.; Sayles, F. L.; Bell, R.; Fietzek, P. SENSORS AND SYSTEMS FOR IN SITU OBSERVATIONS OF MARINE CARBON DIOXIDE SYSTEM VARIABLES. (5) Martz, T. R.; Dickson, A. G.; DeGrandpre, M. D. Tracer monitored titrations: measurement of total alkalinity. Anal. Chem. 2006, 78, 1817–1826.
