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An in-depth
microscopic enumeration of the dozens of species of
algae
present in
a water column each time a lake is sampled is prohibitively costly and
technically impossible for most monitoring programs. Further, in many
lakes a large portion of the algal biomass may be unidentifiable by
most experts (these are appropriately called LRGTs or LRBGTs -- little
round green things and little round blue-green things). However, measuring
the concentration of chlorophyll-a is much easier and provides a reasonable
estimate of algal biomass. Chlorophyll-a is the green pigment that is
responsible for a plant's ability to convert sunlight into the chemical
energy needed to fix CO2 into carbohydrates. To measure chlorophyll-a,
a volume of water from a particular depth is filtered through a fine
glass-fiber filter to collect all of the particulate material greater
than about 1 micron (1/1000th of a millimeter) in size. The chlorophyll-a
in this material is then extracted with a solvent (acetone or alcohol)
and quantified using a spectrophotometer or a fluorometer.
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Note
regarding Water on the Web: Although the RUSS units do not currently
have chlorophyll sensors, a prototype sensor is being developed
by Apprise Technologies, Inc. and the Natural
Resources Research Institute at the University of Minnesota
Duluth. In the interim, surface water (0-2 meter composites)
values of chlorophyll-a are being determined at 2-4 week intervals
at Ice Lake and Grindstone Lake (by NRRI) and at two week intervals
at Lakes Independence and Minnetonka during the ice-free season
(by Hennepin
Parks Natural Resources). These data, along with secchi
disk depth data and ancillary nutrient data are posted on the
WOW Web site as they become available (Data/Other).
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Both chlorophyll-a
and secchi depth are long-accepted methods for estimating the amount
of algae in lakes. Secchi depth is much easier and less expensive to
determine. However, care must be used in interpreting secchi data because
of the potential influence of non-algal particulate material, such as
silt from stream discharge or re-suspended bottom sediment. Also, the
tea color of some lakes that's due to dissolved organic matter from
bogs, can have an effect on secchi depth readings as well. Even if chlorophyll-a
is measured, it may be important to also examine the algal community
microscopically on occasion, since the mix of species may influence
lake management decisions.
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