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Figure 5
Set-up:
The relation between algal chlorophyll and Secchi transparency (click here for more information about Secchi)is shown in this figure. Chlorophyll is presented along the bottom axis in micrograms per liter (parts per billion), while the scale for Secchi transparency is on the left axis in inches of transparency. Note that the scale for Secchi is inverted, so that the zero is at the top of the graph. A shift downward indicates an increase in water clarity. Solid circles represent median values from before and after the treatment plant upgrade, with the arrow indicating the direction of change in chlorophyll concentrations and Secchi transparency. Empty circles are data from across the lake and show the relation between these two parameters.
Interpretation:
Median chlorophyll concentration and Secchi transparency at Site V displayed no notable change after the treatment plant upgrade. Chlorophyll concentrations decreased at the other three lake sites, resulting in an increase in water clarity. The increase in water clarity depended on site location along this curved relation between chlorophyll and Secchi transparency. At Site IV where algal levels are high, water clarity was not very responsive to reductions in algae. Transparency at Site IV only increased by 5 inches, even though the median chlorophyll decreased by 7.3 micrograms per liter. In contrast, small reductions in algal levels at Site VII led to larger increases in water clarity. At this site, water clarity increased by 21 inches, even though the median chlorophyll value decreased by only 0.4 micrograms per liter.
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