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SCUFA
Helps Make Lake Profiling More Accurate
 Chlorophyll
in a lake, a simple test? There are some problems to think about
before one begins to the process of determining the amount of chlorophyll
in our natural water. Most investigators would open "Standard
Methods for Water and Wastewater" to get the professional opinion
for technique. There are several methods that might be employed
in this situation. Water across the Earth contains wide variations
in chlorophyll concentration making choice of method important.
In Southern Oregon, for instance, lakes in the Oregon High Cascade
mountains are exceptionally pure containing 1 to 2 ug/L of chlorophyll.
At Crater Lake National Park, researchers wasted time using the
spectroscopic method for determining chlorophyll before it was noticed
that the concentration was way below the detection limit for that
method. Today the fluorometric method is used with a lower detection
limit.
Once the method
is settled upon, the problem of collection is considered. How deep
in a clear blue lake does one go to collect and identify the chlorophyll
profile for a lake? At Waldo Lake in Central Oregon, the lake is
120 meters deep at our study station. A lake clarity reading for
the lake on September 21, 2003 was measured with an eight-inch diameter
Secchi disk and was 36.0 meters using a surface viewing tube (the
lake surface was disturbed by wind). The thermocline was between
14 and 30 meters.
With this information
a scheme for collection of chlorophyll grab samples might be conceived.
One might want to collect several samples in and around the thermocline
to document algal productivity in that important zone. In this pure
water lake there is concern surrounding benthic productivity and
so samples mid-lake and lake bottom may also be collected to check
this hypothesis. But this is just a guess. Where are the primary
producers, the algae, in this lake? Grab samples may miss entire
strata of species comfortably making a living in the lake. An in
situ chlorophyll probe is the answer. Turner Designs has developed
the SCUFA, a Self Contained Underwater Fluorescence Apparatus. The
SCUFA was used by the author during the summer of 2003 to document
the entire chlorophyll profile of the lake for the first time.
The Turner Designs
SCUFA was physically coupled to the working Hydrolab Datasonde III
and both units programmed to collect data every minute. Therefore
together they collected depth, temperature, oxygen concentration,
pH, conductivity, redox potential, turbidity and the in situ chlorophyll
fluorescence signal. Water samples of 500-mL were collected, filtered,
extracted, and chlorophyll concentrations determined using the in
vitro fluorometric technique (and a Turner Designs digital bench
fluorometer, Model 10-AU). The results indicated that there was
good correlation between the extracted values for chlorophyll using
the in vitro fluorometric method and the in situ fluorometric data.
The relationship between the two was calculated to be:
in vitro Chlorophyll
Concentration (mg/L) = 0.0139 * SCUFA Signal + 0.0492
This data set
had a correlation coefficient of 0.888. This relationship also suggests
that the SCUFA can measure in the range of 0.1 ug/L chlorophyll
at its detection limit (this value is twice its zero value calculation).
The SCUFA could be calibrated to show a closer relationship to the
in vitro chlorophyll values however its use in this lake was to
determine its sensitivity to the very low concentrations of chlorophyll
in this very clear High Cascade lake.
Reviewing the
Waldo Lake SCUFA profile and the grab sample chlorophyll data suggests
that a chlorophyll layer has indeed been missed. The increase in
chlorophyll below 80 meters increases much more rapidly than the
grab samples would suggest. The SCUFA shows a sharp peak at 80 meters
where there was no grab sample. Instead this peak shows up at 100
meters, the depth of the closest grab sample. Also the shape of
this deep chlorophyll maximum may appear very different using the
grab sample data, while the SCUFA data suggests that it has some
structure between 80 and 100 meters.
The Turner Designs
SCUFA has been shown to be a very useful and sensitive in situ instrument
capable of helping discover and design water collection schemes
to identify chlorophyll in our natural waters.
John Salinas
Environmental Chemist
Grants Pass, Oregon, USA
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Spring
blooms are bad news for suppliers of drinking water. The growth
of taste and odor causing algae may bring a flood of customer complaints
about their water quality, causing you to spend much more time monitoring,
and trying to figure out how much treatment chemical to add, and
when.
Jim McCormick, our Tech Support Manager, has been with Turner
Designs for over 15 years and has extensive expertise with our entire
line of instruments.
