Oh-squared! Ocean Oxygen

New study uses high precision MIMS measurements (O2/Ar) to understand ocean metabolism

Studying oxygen in the oceans is really a study of carbon dioxide (CO2) – or so that is the most compelling reason. Understanding the role that the oceans play in regulating the CO2 balance in the atmosphere is critically important to understanding climate change, global warming, and ocean acidification. But, in order to understand CO2 in the oceans we need to understand a key factor - the balance of autotrophy and heterotrophy (i.e. the balance between the activities of the microscopic plants and animals of the sea) which remove and release CO2 in near equal proportions. Oceanographers have a number of tricks up their sleeves to make these estimates of net CO2 exchange between the atmosphere and oceans, but in every case, there is sufficient uncertainty that multiple methods are used to help corroborate the results of any one measurement.

This is where oxygen comes in since oxygen and CO2 exhibit similar, but opposing metabolic fluxes (a bit of a simplification!) and oxygen can be used as a proxy for carbon in ecological studies. For ocean studies, measuring oxygen has an important advantage relating to its lower concentration relative to CO2 plus its ionic forms (primarily bicarbonate). Oxygen is only ~1/10th as abundant in the surface oceans compared to inorganic carbon and this means that it is significantly easier to measure oxygen, compared to CO2, changes in the sea. This becomes important for studies that cover most of the surface area of the oceans where the water is very clear and the density of the planktonic organisms very low. In those oligotrophic waters, changes in daily oxygen concentration between dusk and dawn (maximum vs. minimum values) may only be a few tenths percent different. Therefore, the measurement needs to be within the hundredth percent precision level.

A new study, conducted by researchers at the Center for Microbial Oceanography Research and Education (C-MORE) at the University of Hawaii and funded by The National Science Foundation, Betty and Gordon Moore Foundation, and Simons Foundation, used our MIMS to detect the daily rhythm of oxygen concentrations in oceanic near surface waters. The study, led by Dr. Sara Ferrón, and published in Geophysical Research Letters (doi:10.1002/2015GL063555), quantified rates of change in the O2/Ar ratio during the day and night enabling the calculation of net production and consumption rates and metabolic activity over a 10 day time period. During that period, the daily excursions in O2/Ar were in the general range of 0.3-0.5%, but the frequent time series data highly resolved the intraday changes to allow the calculation of slopes and rates of change with high confidence. With those data and others, the study demonstrated that within the study site and time, the near-surface ocean was net autotrophic and therefore a net sink for CO2. Through the Hawaiian Ocean Time-Series (HOT) program and the Simons Collaboration on Ocean Processes and Ecology (SCOPE), this work will be expanded to study seasonal and inter-annual variability in metabolic processes.

By using MIMS, Ferrón and colleagues were able to exploit advantages of O2/Ar measurements compared to conventional O2 measurements. Considering the required precision, the MIMS data very adequately resolved the small fluxes associated with this oligotrophic system. Conventional measurements might have used automated Winkler titrations which can be conducted at a level of precision close to the MIMS, however, operationally, MIMS is less time consuming, allows smaller sample volumes, and is arguably easier.  More importantly, however, the measurement of the O2/Ar ratio, as opposed to strictly O2 concentrations by Winkler, provides oxygen measurements that are unaffected by ‘bubble injection’, the physical phenomenon caused by the collapse and dissolution of air bubbles formed by wave breaking. This process is variable and dependent on wave state and it can influence dissolved gas concentrations at levels at or greater than the variation due to the biological fluxes of interest. Conveniently, O2/Ar ratios are insensitive to variations in bubble injection because of the similar solubilities of the two gases – i.e. both O2 and Ar dissolve in similar proportion. Thus, O2/Ar ratio changes are not affected by bubble injection and changes in ratio directly relate to effects of the organisms. This fact greatly simplifies the measurement and analysis of biological oxygen cycling in the oceans.

The Ferrón study is the latest example of high precision MIMS measurements being conducted aboard oceanographic ships. Our precursor to the Bay Instruments MIMS first went to sea 25 years ago, well before it became commonplace to take quadrupole mass spectrometers aboard oceanographic vessels. Someday, we’ll blog about those experiences.

 

Posted in MIMS Applications, Oxygen and tagged , , , , , , .