Net community production (NCP) - O₂/Ar

Data types › Photoautotrophy › In situ measurements › Net community production (NCP) - O₂/Ar

• Page authors: PRIMO, Kate Evans
• Responsible curator: Kate Evans

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Oxygen production
Approach: tracer quantification
Context: in situ
Spatial scale: meters2
Temporal scale: days
Units: mmol O2 m-2 day-1
Community captured: all
Co-measurements: temperature, salinity, wind speed history

Method Overview

O₂/Ar compares the gas concentrations of oxygen and argon to determine the net community production (NCP) (a.k.a. net ecosystem production, or NEP) of oxygen within the depths of interest. This measurement relies on argon being biologically inert, but physically nearly identical to oxygen, such that any deviations from the standard ratio of O₂/Ar are due to biological activity .

Scale of measurement

Generally, this approach is used in in situ measurements to determine areal (m²) rates of net oxygen changes. Because this approach relies on wind speed to determine mixing rates of the mixed layer, it yields units of d^-1, but these are based on the average length of time water within the mixed layer has been away from contact with the atmosphere, so they are more of the daily overage of the past few days to few weeks.

It has also been used at higher sampling resolution to track diel level changes in the O₂/Ar ratio . This approach still requires knowledge of wind speed for mixing rates, but allows for dial-level estimates of both O₂ production and consumption.

Data generated

Direct yield is the degree of biologically-mediated oxygen over or undersaturation in (typically) the mixed layer of a body of water. This saturation, when coupled with wind speed and mixing rate estimates, thus yields the average daily change to the oxygen concentration, which is converted into the daily consumption or production of total O₂. Typically, mixed layer NCP estimates in units of oxygen.

Units & currency

Units are in days^-1, but they represent the average NCP of the days to weeks, depending on the wind speed and mixing rate, before sampling.

Sample size

Small volume water samples, often order of 5 mL.

Limitations

Requires the assumption of steady-state dynamics, and that physical transport does not influence mixed layer O₂ dynamics. Most people interested in NCP or NEP measurements want units of carbon, but this yields oxygen and must be converted via some O:C ratio.

It does, however, do a much better job of removing physical interference than most NCP/NEP estimates are capable of.

Example Applications & Protocols

Classic examples

• Craig & Hayward (1987) Oxygen supersaturation in the ocean: Biological versus physical contributions.

Recent applications

• Ferrón et al. (2021) Euphotic Zone Metabolism in the North Pacific Subtropical Gyre Based on Oxygen Dynamics. Protocol: Link to an actual protocol document if possible, can be any document type, e.g. Example

Common calculations/conversions

• E.g. conversion to standard units