34
become widespread in coastal marine waters subject to high nutrient loads brought
about by human activity. Enclosed bays and other water bodies lacking adequate
circulation are most vulnerable, but even open waters such as those of the northern
Gulf of Mexico receiving Mississippi River outflow have for some years now been
observed to develop an anthropogenic OMZ. Similar zones are found in other areas
of the world ocean. Sustained operational DO measurement to identify OMZ onset
and evolution is thus fundamental to coastal ocean monitoring for informed
management.
The highest DO concentrations in seawater are observed in polar waters where
low temperatures favor gas dissolution, up to 350 μmole.kg
−1
(11.2 mg.kg
−1
) at 0 °C
and S P 35 and higher yet at lower salinities. At 25 °C and S P 35, conditions largely
representative of tropical surface waters, DO equilibrium concentration is close to
206 μmole.kg
−1
(6.6 mg.kg
−1
). These concentrations can be exceeded by a few percent through the release of oxygen directly into the water as a by-product of photosynthesis but is held in check by degassing to the atmosphere and biological
respiration. On the other hand, water masses removed from the surface lose DO
through microbial respiration and, in the absence of advective sources, DO concentration slowly decreases. As the photic zone (where photosynthesis exceeds respiration) only reaches down to less than 200 m in the clearest waters, the vast majority
of ocean waters only receive DO through advective transport of sinking polar
waters. Nevertheless, global scale overturning circulation causes ventilation of subsurface water masses overcoming respiration-induced hypoxia such that most of the
world ocean is oxic. Oxygen concentrations are lowest in the eastern North Pacific
basin where subsurface water masses are oldest. Coastal sediments receive
substantially greater loads of particulate organic matter than deep offshore sediments. Bacteria drawing upon oxygen dissolved in the pore waters of sediments
bearing high organic loads fuel aerobic metabolism to depletion. As a result, nearshore sediment porewaters are for the most part anoxic. However, burrowing animals can produce channels that increase the oxygen supply to porewaters.
Prior to the advent of modern electronics, oxygen was measured almost exclusively by the Winkler titration technique, a complex aqueous procedure involving a
redox cascade converting mole for mole, the oxygen fixed in a pickling process to
molecular iodine, the final oxidant in the cascade which is titrated with a thiosulfate
solution of precisely calibrated concentration. The Winkler method is of great precision and is still used today for the calibration of electronic field deployable sensors.
In current practice, automatic titrators equipped with optical or electrochemical
end-point detectors are used for the final step, but the method remains slow and
laborious and is unsuited for continuous flow-through measurement.
Compact field deployable electronic instruments capable of DO measurement
within the environmental range based on electrochemical and optical sensors now
allow extended autonomous deployment. Modular units are built for integration into
multiprobe instruments in wide configurations. Polarographic and optode-based
DO measurement systems are discussed below. Since both types are sensitive to
temperature as well, appropriate sensors can be included in the instrument package
if not incorporated directly into the sensor unit.
2 Electronic Sensors and Instruments for Coastal Ocean Observing
become widespread in coastal marine waters subject to high nutrient loads brought
about by human activity. Enclosed bays and other water bodies lacking adequate
circulation are most vulnerable, but even open waters such as those of the northern
Gulf of Mexico receiving Mississippi River outflow have for some years now been
observed to develop an anthropogenic OMZ. Similar zones are found in other areas
of the world ocean. Sustained operational DO measurement to identify OMZ onset
and evolution is thus fundamental to coastal ocean monitoring for informed
management.
The highest DO concentrations in seawater are observed in polar waters where
low temperatures favor gas dissolution, up to 350 μmole.kg
−1
(11.2 mg.kg
−1
) at 0 °C
and S P 35 and higher yet at lower salinities. At 25 °C and S P 35, conditions largely
representative of tropical surface waters, DO equilibrium concentration is close to
206 μmole.kg
−1
(6.6 mg.kg
−1
). These concentrations can be exceeded by a few percent through the release of oxygen directly into the water as a by-product of photosynthesis but is held in check by degassing to the atmosphere and biological
respiration. On the other hand, water masses removed from the surface lose DO
through microbial respiration and, in the absence of advective sources, DO concentration slowly decreases. As the photic zone (where photosynthesis exceeds respiration) only reaches down to less than 200 m in the clearest waters, the vast majority
of ocean waters only receive DO through advective transport of sinking polar
waters. Nevertheless, global scale overturning circulation causes ventilation of subsurface water masses overcoming respiration-induced hypoxia such that most of the
world ocean is oxic. Oxygen concentrations are lowest in the eastern North Pacific
basin where subsurface water masses are oldest. Coastal sediments receive
substantially greater loads of particulate organic matter than deep offshore sediments. Bacteria drawing upon oxygen dissolved in the pore waters of sediments
bearing high organic loads fuel aerobic metabolism to depletion. As a result, nearshore sediment porewaters are for the most part anoxic. However, burrowing animals can produce channels that increase the oxygen supply to porewaters.
Prior to the advent of modern electronics, oxygen was measured almost exclusively by the Winkler titration technique, a complex aqueous procedure involving a
redox cascade converting mole for mole, the oxygen fixed in a pickling process to
molecular iodine, the final oxidant in the cascade which is titrated with a thiosulfate
solution of precisely calibrated concentration. The Winkler method is of great precision and is still used today for the calibration of electronic field deployable sensors.
In current practice, automatic titrators equipped with optical or electrochemical
end-point detectors are used for the final step, but the method remains slow and
laborious and is unsuited for continuous flow-through measurement.
Compact field deployable electronic instruments capable of DO measurement
within the environmental range based on electrochemical and optical sensors now
allow extended autonomous deployment. Modular units are built for integration into
multiprobe instruments in wide configurations. Polarographic and optode-based
DO measurement systems are discussed below. Since both types are sensitive to
temperature as well, appropriate sensors can be included in the instrument package
if not incorporated directly into the sensor unit.
2 Electronic Sensors and Instruments for Coastal Ocean Observing