Changes in the James River Basin
Ask anyone who has lived on Table Rock Lake for a few decades and they will tell you the lake has changed. This clear gem of the Ozarks has, over time, become a little more clouded. This is especially true for the James River Arm of the lake. However, efforts to halt the loss of clarity and reverse the trend of decreasing water quality may be paying off, according to a review of data made by MU.
Brief History
The declining water quality in Table Rock Lake really came to the state’s attention in the mid 1990’s. Long-term MU data had identified a trend of decreasing water clarity at the dam. This, along with algal blooms, a fish kill and an increasingly loud voice of concern from the public motivated the state to take action. One result of the public and state’s concern about Table Rock Lake was the adoption of a regulation to limit the amount of phosphorus being released from sewage treatment plants in the Missouri portion of the watershed. The Missouri Clean Water Commission passed a regulation in 1999 to limit the phosphorus concentration in treatment plant effluent to <0.5 mg/L. The City of Springfield was scheduled to meet standards by November 2003, but moved forward with plant upgrades and was able to reduce phosphorus concentrations in the effluent to regulated levels by March of 2001. Monitoring within the basin allows us to compare water quality in the James River Arm of the lake for both before and after Springfield started to meet the new standards. Results of the comparisons have been submitted for publication in the Proceedings of the International Society of Limnology 29th Congress. The information was also presented at the congress in Finland this past summer.
Figure 1. Site locations and phosphorus concentrations before and after treatment plant upgrades
Findings
This review of data focused solely on the James River Basin. Sites ranged from a stream site located just below the Southwest Treatment Plant in Wilsons Creek to a lake site at Oswald Bluff, located 2 miles above the confluence of the James River Arm with the main lake (see Figure 1 for site locations). The United States Geologic Survey monitored Sites 1 and 3 year-round. Site 2 was sampled through the University’s Table Rock Lake Long-Term Monitoring (TRM) effort and was also sampled year-round. Sites 4 and 6 were sampled during the summer by the Lakes of Missouri Volunteer Program (LMVP). Site 7 was sampled year-round through the TRM project, while Site 5 data is combined summertime information from both TRM and LMVP.
Between July 1992 and February 2001 the treatment plant released between 4,170 to 68,570 pounds of phosphorus into the James River Basin per month, with a median value (middle) of 27,340 pounds. After the plant started meeting the new regulations, the amount of phosphorus released into the basin dropped by approximately 90%, to a median of 2,683 pounds with a range of 1,757 to 5,421 pounds per month. This decrease was mimicked in both Finley Creek and the James River (Sites 1-3) where phosphorus concentrations decreased by 69%-87%.
Within the lake (Sites 4-7) the summertime phosphorus values dropped by 33-50%, with the actual decreases ranging from a high of 49 µg/L at Site 4, to a low of 5 µg/L at Site 7 (See Figure 1). Statistically, the decreases in phosphorus throughout the basin were significant.
Algal chlorophyll within the lake (Sites 4-7) responded in a predictable fashion to the decreases in phosphorus. Actual shifts in chlorophyll during the summer ranged from no-change at Site 5 to a 17% decrease at Site 6. Generally speaking, as phosphorus decreased the algae became a little more efficient at using what was available. This is why the algal chlorophyll levels did not decrease in direct proportion to the phosphorus. Currently, the chlorophyll-phosphorus relation suggests that future decreases in phosphorus at these sites will lead to a greater change in algal chlorophyll.
The decreases that did occur in algal chlorophyll results in predictable changes in Secchi water clarity readings (Figure 2). In the upper James River Arm (Site 4) the clarity improved by only 5 inches, but this was a consistent improvement that was considered to be statistically significant. Given where this site is located on the hyperbolic curve of the chlorophyll-Secchi relationship, we would not expect very much increase in clarity with decreases in algal biomass. Secchi readings at Site 6 improved by 15 inches while Site 7 improved by 21 inches. We would expect noticeable changes in water clarity with decreases in algal chlorophyll at those sites, given their position on the Chlorophyll-Secchi curve (Figure 2).
Figure 2. Before and after concentrations of chlorophyll in the James River Arm of Table Rock Lake compared to Secchi transparency
Responses within the lake to decreases in phosphorus released from the Southwest Treatment Plant may not be a large as some would think. It must be pointed out that the data we have covered so far is from the summertime. During this period of the year the James River actually plunges underneath the surface waters of the lake due to differences in density (Figure 3). This means that the surface water samples that we are looking at were not directly influenced by the nutrient rich inflows. During the winter, when the lake is mixing, the inflows from the river directly impact the surface waters of the lake. (more information on stratification)
Figure 3. Water entering Table Rock Lake from the James River plunges deeply in the summer, but is distributed evenly in the winter
Samples collected at Site 7 during the winter indicate a greater response to decreased phosphorus than the samples collected during summer. Actual decreases in phosphorus concentrations were greater in the winter (11 vs. 5 µg/L during the summer), as were decreases in algal chlorophyll (5.2 vs. 0.4 µg/L) and increases in water clarity (57 vs. 21 inches). It is reasonable to assume that the rest of the James River Arm of the lake displayed greater response to decreased phosphorus during the winter months.
Regulation of the point sources within the Missouri portion of the watershed seems to be having the desired impact on lake water quality in the James River Arm of Table Rock Lake.
Future monitoring will allow for better quantification of the actual impacts of management. Future monitoring will also be necessary to continue to gauge the impact of nonpoint source pollution from the watershed. As population, tourism, and agriculture grows in the lake’s watershed, so will nonpoint source pollution threats. Some of the impacts are coming from Arkansas, which holds about 54% of the lake’s watershed. Continuous monitoring is the best way to identify problems as they arise and protect the gains that we have made.