Total Algae - YSI ProDIGITAL User Manual

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Total Algae

TAL Sensors
YSI offers two Total Algae (TAL) sensor options. Both are dual-channel fluorescence sensors.
The channels on the TAL-PC sensor refer to two independent data sets: one results from a blue excitation beam
that excites the chlorophyll a (Chl) molecule and the second results from an orange excitation beam that excites the
phycocyanin (PC) accessory pigment. TAL-PC sensors are typically selected for monitoring freshwater cyanobacteria.
The TAL-PE sensor is similar in having a chlorophyll channel, but utilizes a slightly blueshifted beam that excites the
pigment phycoerythrin (PE). TAL-PE sensors are typically selected for monitoring marine cyanobacteria.
TAL Units
The TAL sensors report data in RFU and g/L of pigment (Chl, PC or PE) units. YSI recommends reporting in Relative
Fluorescence Units (RFU).
RFU is used to set sensor output relative to a stable secondary standard, Rhodamine WT dye. This allows users to
calibrate sensors identically so that results from sensor to sensor can be compared. Calibration with Rhodamine
WT also enables users to monitor for sensor drift and external factors such as biofouling or declining sensor optical
performance over time as the LEDs age.
The excellent linearity of RFU, once the channels are calibrated with Rhodamine WT, translates to the best accuracy
of measurements. For example, a chlorophyll reading of 100 units will represent twice the pigment detected by
the sensor than with a chlorophyll reading of 50 units. This high linearity (R2>0.9999) doesn't always hold for g/L
of pigment since that unit was derived from laboratory monocultures, and an environmental algal population can
behave quite differently. This is also why the TAL sensors and in situ monitoring should not be regarded as a perfect
replacement for other methods such as pigment extractions and cell counting.
The g/L output generates an estimate of pigment concentration that is based upon correlations built with sensor
outputs and extractions of pigments from laboratory-grown blue-green algae. Synonymous with parts per billion
(ppb), g/L is still commonly used by regulatory agencies, but has the drawback that it is very dependent upon the
composition of the algal population, the time of day, the physiological health of the algae, and a number of other
environmental factors. Thus, users are advised to do their own check of our correlation with a population of algae
relevant to their own sites, as described below.
A 2-point RFU calibration is advised to be performed first. Next, with samples collected from the site of interest,
measure both RFU and g/L with the sensor(s). Observing careful handling and preservation of the samples, as
soon as possible extract the pigments from the samples, using standardized methods to determine the g/L in each
sample. The extraction data may be used to assess how RFU and g/L delivered by the sensor compare with the g/L
of pigment that would be predicted by RFU from the sensor. The user's requirements can guide the decision as to
whether RFU or g/L is the best unit to read from the sensor for any specific application.
TAL Raw values can only be seen under Sensor info in the System menu and are unaffected by user calibrations. These
values range from 0-100, representing the percent of full scale that the sensor detects in a sample, and are used for
diagnostic purposes.
Calibration
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