YSI ADV6600 - Manual (Page 132)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 122
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 that 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 was 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.
9-17. Rhodamine WT
Rhodamine WT is a red dye that is commonly used in stream flow studies. The amount of the
species at various points (horizontal and vertical) in the water under examination is determined by
utilizing the fact that rhodamine WT fluoresces when irradiated with the proper wavelength of light.
Thus the YSI 6130 rhodamine WT sensor works on the same principles as described for the 6025
chlorophyll sensor.