THE BLANK CAN MAKE A BIG DIFFERENCE IN OCEANOGRAPHIC MEASUREMENTS

It is hard to imagine a topic that seems more boring and trivial than the measurement of nothing. In a sense, determination of an analytical blank is exactly that — measurement of the signal associated with the absence of the property being detected. Although the magnitude and variability of an analytical blank may be very small compared to the oceanographic measurement, it must be determined as part of a calibration routine. For example, the blank for samples of particulate matter collected on filters consists of a filter that has been treated identically to field samples, except that no water with particles is drawn through it. Analysis of dissolved constituents requires measurements on samples that are analytically equivalent to field samples, but with no analyte present. It is the analyst’s responsibility to determine what constitutes the appropriate blank (e.g., purified water, artificial sea water, filtered sea water treated to remove the analyte); generally, this requires a series of experiments to establish the sensitivities of the measurement system. Experience with the controversy concerning the measurement of dissolved organic carbon and nitrogen in natural waters (Benner and Strom 1993) amply demonstrates that blanks must be carefully monitored to ensure accuracy (i.e., to minimize systematic error). For a variety of reasons, it is sometimes difficult or impossible to measure appropriate blanks concurrently with the oceanographic measurements. Deployments of in situ sensors can be particularly problematic. Changes in the blank due to fouling and instrument drift can confound the interpretation of long-term records from optical instruments that have internal light sources (Davis et al. 2000). Radiometric measurements of solar radiation and ocean color should be corrected for the signal generated in the absence of light: a dark correction. Generally, the signal in the dark is sensitive to temperature, so the correction from a laboratory calibration cannot be assumed to hold in the field (Cullen and Davis 2002). Uncertainty in the dark signal can strongly influence estimates of irradiance at depths corresponding to the limit of detection, leading to large errors in the estimation of attenuation coefficients (Morrow and Booth 1997). Fortunately, it is often possible to assess the uncertainty in the blank to determine if it compromises the measurement (see Laney et al. 2001 for an optical application). If the measured signal is much larger than the blank, nitpicking about blank corrections is unwarranted. Introduced by Carl Lorenzen in 1966, in vivo fluorometry is used widely in limnology and oceanography to estimate phytoplankton biomass in terms of chlorophyll. Consistent with basic principles of analysis, measurements of light emitted by phytoplankton should be corrected for the signal from the same water, with phytoplankton removed by filtration (potential effects of scattering must also be assessed). Lorenzen did this and then showed through experimentation that purified water was an acceptable substitute for filtered sea water because the signals from both types of blanks were small, compared to his samples, and not sufficiently different from each other to influence his results. Our experience with standard fluorometers such as the Turner Designs series shows that the difference between filtered sea water and purified freshwater blanks is small compared to the fluorescence of phytoplankton in all but the clearest oceanic waters. However, much of the ocean has very clear water. We discuss here the measurement of variable chlorophyll fluorescence, used as a diagnostic of nutrient limitation in phytoplankton (Kolber et al. 1988; Parkhill et al. 2001). A simulation, guided by real measurements in the Pacific Ocean, demonstrates that for measurements of variable fluorescence in ultra-oligotrophic waters, the blank can make a big difference.