Development and qualification of autonomous measurement systems for the determination of the pH value and total alkalinity in seawater

In dieser Arbeit wird die Entwicklung hochgenauer autonomer Messsysteme für den pH-Wert und die Gesamtalkalinität zur vollständigen Charakterisierung des Carbonatsystems in Meerwasser aufgezeigt. Es wurde die Carbonatchemie in der Nordsee vollständig beschrieben und gezeigt, wie durch eine Überstimmung des Carbonatsystems systematische Fehler der Messungen identifiziert und eliminiert werden können und somit eine Qualitätssicherung der gemessenen Daten erfolgen kann.This study presents the development of high quality measurement systems for pH and total alkalinity for a full characterization of the carbonate system in seawater. The carbonate chemistry in the North Sea has been described on the basis of an over determined carbonate system. It has been shown how systematic errors can be identified and eliminated to provide quality assured data

Enhance Ocean Carbon Observations: Successful Implementation of a Novel Autonomous Total Alkalinity Analyzer on a Ship of Opportunity

Over recent decades, observations based on merchant vessels (Ships of Opportunity—SOOP) equipped with sensors measuring the CO2 partial pressure (pCO2) in the surface seawater formed the backbone of the global ocean carbon observation system. However, the restriction to pCO2 measurements alone is one severe shortcoming of the current SOOP observatory. Full insight into the marine inorganic carbon system requires the measurement of at least two of the four measurable variables which are pCO2, total alkalinity (TA), dissolved inorganic carbon (DIC), and pH. One workaround is to estimate TA values based on established temperature-salinity parameterizations, but this leads to higher uncertainties and the possibility of regional and/or seasonal biases. Therefore, autonomous SOOP-based TA measurements are of great interest. Our study describes the implementation of a novel autonomous analyzer for seawater TA, the CONTROS HydroFIAⓇ TA system (-4H-JENA engineering GmbH, Germany) for unattended routine TA measurements on a SOOP line operating in the North Atlantic. We present the installation in detail and address major issues encountered with autonomous measurements using this analyzer, e.g., automated cleaning and stabilization routines, and waste handling. Another issue during long-term deployments is the provision of reference seawater in large-volume containers for quality assurance measurements and drift correction. Hence, a stable large-volume seawater storage had to be found. We tested several container types with respect to their suitability to store seawater over a time period of 30 days without significant changes in TA. Only one gas sampling bag made of polyvinylidene fluoride (PVDF) satisfied the high stability requirement. In order to prove the performance of the entire setup, we compared the autonomous TA measurements with TA from discrete samples taken during the first two trans-Atlantic crossings. Although the measurement accuracy in unattended mode (about ± 5 μmol kg^-1) slightly deteriorated compared to our previous system characterization, its overall uncertainty fulfilled requirements for autonomous TA measurements on SOOP lines. A comparison with predicted TA values based on an established and often used parameterization pointed at regional and seasonal limitations of such TA predictions. Consequently, TA observations with better coverage of spatiotemporal variability are needed, which is now possible with the method described here

Impact of impurities in bromocresol green indicator dye on spectrophotometric total alkalinity measurements

Due to its accurate and precise character, spectrophotometric pH detection is a common technique applied in measurement methods for carbonate system parameters. However, impurities in the used pH indicator dyes can influence the measurements quality. During our work described here, we focused on impacts of impurities in the pH indicator dye bromocresol green (BCG) on spectrophotometric seawater total alkalinity (AT) measurements. In order to evaluate the extent of such influences, purified BCG served as a reference. First, a high-performance liquid chromatography (HPLC) purification method for BCG was developed as such a method did not exist at the time of this study. An analysis of BCG dye from four different vendors with this method revealed different types and quantities of impurities. After successful purification, AT measurements with purified and unpurified BCG were carried out using the novel autonomous analyzer CONTROS HydroFIA® TA. Long-term measurements in the laboratory revealed a direct influence of impurity types and quantities on the drift behavior of the analyzer. The purer the BCG, the smaller was the AT increase per measurement. The observed drift is generally caused by deposits in the optical pathway mainly generated by the impurities. However, the analyzers drift behavior could not be fully overcome. Furthermore, we could show that a certain impurity type in some indicator dyes changed the drift pattern from linear to nonlinear, which can impair long-term deployments of the system. Consequently, such indicators are impractical for these applications. Laboratory performance characterization experiments revealed no improvement of the measurement quality (precision and bias) by using purified BCG as long as the impurities of the unpurified dye do not exceed a quantity of 2 % (relationship of peak areas in the chromatogram). However, BCG with impurity quantities higher than 6 % provided AT values which failed fundamental quality requirements. In conclusion, to gain optimal AT measurements especially during long-term deployments, an indicator purification is not necessarily required as long as the purchased dye has a purity level of at least 98 % and is free of the named impurity type. Consequently, high-quality AT measurements do not require pure but the purest BCG that is purchasable

Seagrass beds as ocean acidification refuges for mussels? High resolution measurements of pCO2 and O2 in a Zostera marina and Mytilus edulis mosaic habitat

It has been speculated that macrophytes beds might act as a refuge for calcifiers from ocean acidification. In the shallow nearshores of the western Kiel Bay (Baltic Sea), mussel and seagrass beds are interlacing, forming a mosaic habitat. Naturally, the diverse physiological activities of seagrasses and mussels are affected by seawater carbonate chemistry and they locally modify it in return. Calcification by shellfishes is sensitive to seawater acidity; therefore the photosynthetic activity of seagrasses in confined shallow waters creates favorable chemical conditions to calcification at daytime but turn the habitat less favorable or even corrosive to shells at night. In contrast, mussel respiration releases CO2, turning the environment more favorable for photosynthesis by adjacent seagrasses. At the end of summer, these dynamics are altered by the invasion of high pCO2/low O2 coming from the deep water of the Bay. However, it is in summer that mussel spats settle on the leaves of seagrasses until migrating to the permanent habitat where they will grow adult. These early life phases (larvae/spats) are considered as most sensitive with regard to seawater acidity. So far, the dynamics of CO2 have never been continuously measured during this key period of the year, mostly due to the technological limitations. In this project we used a combination of state-of-the-art technologies and discrete sampling to obtain high-resolution time-series of pCO2 and O2 at the interface between a seagrass and a mussel patch in Kiel Bay in August and September 2013. From these, we derive the entire carbonate chemistry using statistical models. We found the monthly average pCO2 more than 50 % (approx. 640 μatm for August and September) above atmospheric equilibrium right above the mussel patch together with large diel variations of pCO2 within 24 h: 887 ± 331 μatm in August and 742 ± 281 μatm in September (mean ± SD). We observed important daily corrosiveness for calcium carbonates (Ωarag and Ωcalc < 1) centered on sunrise. On the positive side, the investigated habitat never suffered from hypoxia during the study period. We emphasize the need for more experiments on the impact of these acidic conditions on (juvenile) mussels with a focus on the distinct day-night variations observed

Novel Optical Oxygen Sensor for Profiling Observation Platforms: Fast Response Time Enables Higher Spatial and Temporal Data Resolution

Ocean warming has a severe impact on oxygen distribution because it reduces oxygen solubility and increases stratification in the upper ocean. Models predict a decline of the global oxygen inventory of about 1-7% over the next century and data show a decrease of more than 2% since 1960 (Schmidtko et al., Nature, 2017). Quantifying global as well as regional changes of oxygen will improve the understanding of chemical, biological and physical processes, especially in Oxygen Minimum Zones (OMZ) where consistent trends of intensification and spatial expansion exit (e.g., Stramma et al., Science, 2008). Although optical sensors, so-called optodes, are available to accurately measure changes in ocean oxygen levels, users still wish to obtain better spatial and temporal resolution on profiling observation platforms than can be currently achieved. Here we demonstrate the utility of a novel and fast, commerciallyavailable optode for in-situ and autonomous oxygen measurements, potentially closing this gap. This novel oxygen optode shows a temperature-dependent response time (t63%) of about 4 seconds and is thereby at least 50% faster compared to other optical oxygen sensors. We aim to fully characterize this optode with regard to accuracy, precision, pressure dependence, long-term stability and drift, response time as well as aircalibration compatibility. Results build on data from extensive laboratory experiments and field deployments in the Tropical North, South and Southern Atlantic (underway, mooring, float and CTD-cast applications). This promises high quality observations for detecting oxygen level changes on small and fast-changing scales in this ocean region. This novel optode could be used on a wide range of autonomous observation platforms such as ships for Repeat Hydrography, time-series stations and wave gliders, yet is especially promising on floats, gliders and fast-moving ships. In a changing ocean those applications eventually will contribute valuable information to the global oxygen budget

Improving Optical Measurements: Non-Linearity Compensation of Compact Charge-Coupled Device (CCD) Spectrometers

Charge-coupled device (CCD) spectrometers are widely used as detectors in analytical laboratory instruments and as sensors for in situ optical measurements. However, as the applications become more complex, the physical and electronic limits of the CCD spectrometers may restrict their applicability. The errors due to dark currents, temperature variations, and blooming can be readily corrected. However, a correction for uncertainty of integration time and wavelength calibration is typically lacking in most devices, and detector non-linearity may distort the signal by up to 5% for some measurements. Here, we propose a simple correction method to compensate for non-linearity errors in optical measurements where compact CCD spectrometers are used. The results indicate that the error due to the non-linearity of a spectrometer can be reduced from several hundred counts to about 40 counts if the proposed correction function is applied

Variability of USA East Coast surface total alkalinity distributions revealed by automated instrument measurements

Seawater total alkalinity (TA) is one important determinant used to monitor the ocean carbon cycle, whose spatial distributions have previously been characterized along the United States East Coast via discrete bottle samples. Using these data, several regional models for TA retrievals based on practical salinity (S) have been developed. Broad-scale seasonal or interannual variations, however, are not well resolved in these models and existing data are highly seasonally biased. This study reports findings from the first long duration deployment of a new, commercially available TA titrator aboard a research vessel and the continuous underway surface TA measurements produced. The instrument, operated on seven East Coast USA cruises during six months in 2017 and for two months in 2018 on the summertime East Coast Ocean Acidification survey (ECOA-2), collected a total of nearly 11,000 surface TA measurements. Data from these efforts, along with a newly synthesized set of more than 11,000 regional surface TA observations, are analyzed to re-examine distributions of TA and S along the United States East Coast. Overall, regional distributions of S and TA generally agreed with prior findings, but linear TA:S regressions varied markedly over time and deviated from previously developed models. This variability is likely due to a combination of biological, seasonal, and episodic influences and indicates that substantial errors of ±10–20 μmol kg−1 in TA estimation from S can be expected due to these factors. This finding has likely implications for numerical ecosystem modeling and inorganic carbon system calculations. New results presented in this paper provide refined surface TA:S relationships, present more data in space and time, and improve TA modeling uncertainty

Novel Oxygen Optode Sensor for Profiling Ocean Observation Platforms: Extensive Characterization and In-Depth Assessment of its Fast Response Time

Ocean warming severely impacts oxygen distribution, because it reduces oxygen solubility and increases stratification in the upper ocean. Quantifying changes of oxygen levels will improve the understanding of chemical, biological and physical processes, especially in Oxygen Minimum Zones characterized by intensification and spatial expansion. Despite existing optical sensors (optodes) that accurately measure ocean oxygen levels, users wish for an improved spatial and temporal measurement resolution from profiling platforms. We demonstrate the utility of a novel, commercially-available optode that shows a temperature-dependent response time (t63%) of about 4 seconds, which is significantly faster compared to other optical oxygen sensors. This optode can be used on a wide range of observation platforms such as ships, time-series stations, unmanned surface vehicles and autonomous underwater platforms such as floats and gliders. We aim to characterize this optode regarding oxygen, temperature, salinity and pressure dependence, long-term stability and drift, response time and air-calibration compatibility. Results build on data from laboratory experiments and field deployments in the Tropical and Southern Atlantic. Underway, mooring, float and CTD-cast applications promise high quality observations including fast oxygen level changes on small scales. We will conclude with a status update on our general optode technology developments

Validation of sensor and instrumentation innovations

Validated prototypes of new and enhanced biogeochemical and biological sensors and instruments. Validation will be undertaken in the laboratory, in test scenarios, and by deployment in operational condition