Xylem 6-Series Multiparameter - Manual (Page 333)

Chlorophyll Measurements Appendix I
YSI Incorporated Environmental Monitoring Systems Operations Manual I-6
available water sampling devices that are designed specifically to acquire a representative sample at a
particular depth.
You should also take precautions to stabilize your samples for transport back to the laboratory. The
samples should be kept cool and in the dark (e.g., in an ice chest) until the analytical procedure has begun.
Some users prefer to filter the sample in the field and keep the filter pads on ice in the dark during
transport.
My environmental sites have a lot of visible floating algae, but my YSI 6025 chlorophyll sensor shows
very low readings. What is causing this paradox?
The YSI 6025 is not designed to measure macroscopic algal or plant material that floats on the surface.
Rather it measures the fluorescence from the microscopic phytoplankton that is suspended below the
surface of the water. It is fairly common to see mats of floating algae on environmental water that has low
subsurface phytoplankton concentrations.
What can I do to obtain the best possible data from my YSI 6025 sensor in sampling studies?
Two factors are most important. The first is to make certain that the readings are stable before manually
recording them or logging them to a computer or YSI 650 MDS display/logger. You should visually
monitor the readings for at least 1-2 minutes to assure stability after the sonde is immersed. The second
factor is that you should always manually activate the mechanical wiper on the sensor after sonde
immersion but before beginning the visual monitoring of the readings. The cleaning cycle is necessary to
remove bubbles from the optical face, which are certain to cause erroneous readings. If the observed
readings appear unusual after the first cleaning cycle, activate the wiper repeatedly to assure that all
bubbles are removed. Manual activation of the wiper mechanism is easily accomplished from the Run
screen of the 650 MDS.
You should also allow adequate time for the sonde to acclimate to the temperature of the water at the site.
What sort of “noise” level is expected for unattended monitoring studies?
All environmental water is somewhat heterogeneous with regard to phytoplankton content and this factor
will cause some noise or jumpiness in long term monitoring studies for chlorophyll. The extent of the noise
will be dependent on your site. The data from a typical 36-day deployment at a YSI test site (lake in
southwestern Ohio) is shown in the figure below as a reference. Note the diurnal cycle in the data which is
likely due to the photoinhibition of chlorophyll fluorescence as discussed in Section 5, Principles of
Operation.