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.
Grants Pass, Oregon, USA