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Figure 9
Set-up:
Each bar in this graph represents the annual average for each year during 1996-2004. The years are identified along the bottom of the graph, while phosphorus is scaled on the left side in micrograms per liter (parts per billion). The horizontal line within the graph represents the overall average phosphorus value for this site (an average based on all data). Bars that extend above this line indicate years with higher than average phosphorus values, while bars that extend downward represent years with lower than average phosphorus.
Interpretation:
The long-term phosphorus trend at this main lake site is very similar to trend identified in the lower James River Arm. Annual average phosphorus values were at or slightly above the overall average value for the period 1996-1999, ranging from 11 to 15 micrograms per liter. Annual average phosphorus values for 2001-2004 have been below the overall average, ranging from ~7 to 10 micrograms per liter. Because we are dealing with low values and small changes in phosphorus, there was not a “statistically significant” trend in decreasing phosphorus at this site. However, evidence shows that phosphorus concentrations are not increasing and may very well be decreasing at this main lake site.
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