Xylem 6-Series Multiparameter - Manual (Page 265)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-27
standard that provides the best measure of accuracy for in vivo chlorophyll sensors is a portion of a
phytoplankton suspension that has been analyzed for chlorophyll by the extractive procedure. We
recommend the use of this procedure and further recommend that the phytoplankton suspension be taken
from the site being monitored so that the species producing the fluorescence in the standard are as close as
possible to the field organisms. To truly assess data reliability in a long term monitoring study, grab
samples should be taken periodically, e.g. weekly, and analyzed in the laboratory as the study progresses.
These data can then be used to “postcalibrate” the readings logged to the instrument during the study,
perhaps using a spreadsheet for the simple mathematical treatment. In any case, getting quantitative
chlorophyll data from any in vivo fluorometric sensor is much more difficult than with most other
environmental sensors. For this reason, it is difficult to provide an accuracy specification for chlorophyll
measurement made with in vivo fluorometers and therefore no accuracy specification is quoted for the YSI
6025.
EFFECT OF CELL STRUCTURE, PARTICLE SIZE, AND ORGANISM TYPE ON IN VIVO
READINGS: Even if the only fluorescent species present for in vivo measurements were chlorophyll, and
reliable calibration standards were available, its absolute quantification would probably still be difficult
because samples are not homogeneous. Differing species of algae with differing shape and size will likely
fluoresce differently even if the type and concentration of chlorophyll are identical and this significantly
limits the accuracy of in vivo measurements.
EFFECT OF TEMPERATURE ON PHYTOPLANKTON FLUORESCENCE: As noted above, YSI
experiments indicate that phytoplankton fluorescence increases as temperature decreases. Thus, readings
taken on a phytoplankton suspension at cold temperature would erroneously indicate the presence of more
phytoplankton than when the suspension is read at room temperature. Unless this effect is taken into
account, most field readings will be somewhat in error, since the field temperature will differ from the
temperature of calibration. The use of the “Chl Tempco” factor found in the Advanced|Sensor menu will
help to reduce this error, but must be used with caution since each species of phytoplankton is likely to
have a slightly different temperature dependence.
EFFECT OF LIGHT ON PHYTOPLANKTON FLUORESCENCE: It is well documented in the
literature that the fluorescence of chlorophyll resident in phytoplankton can be inhibited by light. This
“photoinhibition” is confirmed by empirical data which indicate that, at constant phytoplankton level, the
fluorescent signal can change significantly on a diurnal schedule, showing less fluorescence during the day
and more fluorescence at night. Data showing this diurnal cycle is shown in Appendix I, Chlorophyll
Measurements. It is clear that this effect would produce errors in the absolute values of chlorophyll unless
it were accounted for by the user.
The chlorophyll section of Standard Methods substantiates these limitations along with application notes
that are offered by current fluorometer manufacturers. The limitations result in the realization that any in
vivo
“chlorophyll” sensor will be much less quantitative than any of the other sensors offered for use with
our sondes.
MEASUREMENT AND CALIBRATION TIPS
1. For best results, analyze field samples to be used for “calibration” of the sensor as soon as possible
after collection.
2. If unusually high or jumpy readings are observed during calibration, it is likely that there are bubbles
on the optics. The surface should be cleaned by manually activating the wiper before confirming the
calibration.
3. The output of the YSI fluorescence sensor is susceptible not only to the overall phytoplankton
concentration in the environmental medium, but also to the size and rate of movement of the
suspended particles that pass across the optics on the probe face. Thus, although the phytoplankton