Salinity ranged between 31 and 35 psu. Bottle salinities, plotted versus CTD-derived minus bottle salinity differences (Figure 3), provides us with the basis to reject two spurious data points (probably related to leaking sample bottles, strong salinity gradient, incorrect sampling, etc.). Plots and regressions of salinity differences versus pressure and bottle salinities suggested no apparent depth bias. The CTD salinities were an average of 0.0272 psu higher than bottle salinities. A linear regression of bottle salinity on CTD salinity yielded the corrected salinity (Scorr) equation:
Scorr = 0.9998 * Sctd + 0.0210 ,
where Sctd is the CTD-measured salinity. The standard deviation of discrepancies was 0.0059 psu. This equation was used to correct all of the SBE-911 CTD salinity data.
d. Dissolved Oxygen: An algorithm using oxygen sensor current and sensor temperature measured by a SeaBird SBE-23Y, along with CTD water temperature, salinity, and pressure, calculates dissolved oxygen concentration in milliliters / liter (SeaBird Application Note 13-1B). Published accuracy and resolution specifications are 0.10 mL/L and 0.01 mL/L, respectively.
e. Transmissivity: A Sea Tech unit measured transmission loss of a 25 cm beam of red light as it passed through the water column, and presented it as a percentage relative to the transmission loss of pure water.
f. Fluorescence: A Sea Tech unit measures flourescence emission excited by a beam of blue light on a sample of sea water. An algorithm is used to infer the sample's chlorophyll-alpha (Chl-a) concentration in micrograms / liter.
g. Irradiance (PAR): A Biospherical QSP-200PD measures omnidirectional photosynthetically available radiation (PAR) in the 400-700 nm range. An algorithm converts the instrument's photodetector current to microeinsteins / second / square meter to within 5% of available light.