Continuous dissolved oxygen (DO) optodes

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

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O2 optode method
Approach: continuous O2 measurement by luminescent optode, light and dark bottles
Context: incubation, simulated in situ
Spatial scale: point sample
Temporal scale: hours to one day
Units: µmol O2 L-1 d-1; convertible to µmol C L-1 d-1
Community captured: all
Co-measurements: temperature, PAR, salinity, pressure; photosynthetic quotient (PQ) for C conversion

Method Overview

Luminescent optical oxygen sensors (optodes) are mounted inside incubation bottles, allowing dissolved O₂ to be logged continuously throughout the incubation rather than at a single endpoint. The same light-dark bottle design as the Winkler method is used: light bottles capture NCP (net O₂ production); dark bottles capture community respiration (CR). GPP = NCP + CR. Because O₂ is measured continuously, the method can resolve short-term dynamics and diel patterns within a single incubation, and non-linear responses in the dark bottles can be detected and accounted for.

Optodes respond to the partial pressure of dissolved O₂ via a luminescence lifetime that decreases as O₂ quenches a fluorescent dye embedded in the sensor membrane. Temperature and salinity corrections are required for accurate O₂ concentration calculations.

Scale of measurement

As a bottle-based incubation, the method provides a point measurement in space. Larger bottle volumes (> 1 L) compared to Winkler bottles reduce the relative contribution of wall effects and increase the signal-to-noise ratio for low-productivity waters.

Data generated

The method yields time-resolved O₂ concentration trajectories, from which GPP, NCP, and CR are estimated as linear rates over selected time windows. Conversion to carbon requires PQ (for GPP and NCP) and RQ (for CR).

Units & currency

Units are µmol O₂ L^-1 d^-1, or µmol C L^-1 d^-1 after conversion. The currency is oxygen (carbon is derived).

Sample size

Typical samples are > 1 L in volume.

Limitations

Sensor drift over longer incubations must be monitored and corrected. Response times of optodes can lag behind rapidly changing O₂ concentrations. Gas permeability of some incubation vessel materials can introduce artefacts in the O₂ mass balance. The same biological bottle effects (grazer exclusion, altered physico-chemical environment) apply as in the Winkler method. Light-dependent O₂ consumption reactions can cause overestimation of GPP.

Example Applications & Protocols

Common calculations/conversions

• GPP (µmol O₂ L^-1 d^-1) = rate of O₂ increase in light bottle − rate of O₂ decrease in dark bottle.
• GPP (µmol C L^-1 d^-1) = GPP_O2 / PQ.