Page 7 - HydrographyReadMe

calibration procedures. However, the scaling factor returned to 0.6 for 2023 for unknown
reasons.
The scatter in these intercomparisons are relatively large for several reasons. First, it is simply
difficult to compare measurements at the same nominal depth from profiles not taken
simultaneously. The ocean changes! Second, the sampling depths may not even be exactly the
same. Third, it is known that daytime in-situ chlorophyll fluorescence readings very near the
surface can be lowered to as little as 50% relative to nighttime values in the Strait of Georgia
by "non-photochemical fluorescence quenching", a physiological response of chloroplasts to
large quantities of light. Presumably the decrease would be less at depth where light levels
are lower, and on cloudy or winter days, and perhaps even on bright days when vertical
mixing is vigorous

Dissolved Oxygen

The CTD profilers are also equipped with another sensor (either an Aanderaa optode or an
Alec Rinko-III) to measure dissolved oxygen using a "fluorescence quenching" technique which
has become widespread in recent years. Such dissolved oxygen measurements are measured
as a factor relative to atmospheric saturation. By using a standard equation that provides the
actual concentration, when in equilibrium with the atmosphere at different temperatures,
salinities, and pressures, these measurements can be converted into molar units of O2, which
we use here. Although no in-situ calibrations are carried out, these measurements are cross-
compared with calibrated oxygen concentrations from surveys of the Strait carried out by
Fisheries and Oceans Canada. The estimated static accuracy is around ±10 µM.

However, the oxygen sensor response time is generally longer than for temperature and
salinity, so that profiles show a smoothed version of the actual changes in oxygen
concentrations.