by: Angela Danovi, Arkansas Projects Manager
Since 2012, volunteer citizen scientists have been conducting stream monitoring on Prairie Creek below Lake Atalanta Dam in Rogers with the StreamSmart Program.
Last year when I prepared our annual StreamSmart data report, I was surprised to see that the 2018 total phosphorus concentrations in Prairie Creek had decreased by half compared to 2016 and earlier. But, with only one year of data showing these reductions, I was reluctant to say it was a trend or even notable because with only four samples collected after the park re-opening, the lower total P values in 2018 could have been a coincidence rather than a trend.
Throughout 2019, our volunteer citizen science team continued to monitor Prairie Creek. This year, when I added the 2019 data to the existing dataset, I was excited to find that the lower phosphorus concentrations we had identified in 2018 had sustained throughout 2019 and even into our first monitoring of 2020.
Looking at the graph (above) and then plotting the data into two box plots, I realized there was in fact a noticeable difference in total P concentrations at Prairie Creek below Lake Atalanta Dam since 2018, which aligned with the timeline of the park undergoing the renovation and an extended closure due to the spring 2017 flood.
I wanted to be sure the data from 2012-2017 was actually statistically different from the 2018-2020 data. So, I conducted a statistical test called a t-test and assumed unequal variances between the two datasets. The results showed the means of the two datasets to be statistically significantly different (p-value = 0.0000269). P-values that are less than .05 are generally considered significant.
Looking only at the stream data, I thought one reason Prairie Creek may have decreased total P concentrations since 2018 was because Lake Atalanta had been dredged during the construction and renovations. Therefore, I thought we may find total P concentrations in lake Atalanta to be increasing because of the increased capacity of the lake to capture and retain sediments and particulates. Surprisingly, when I looked at the volunteer lake monitoring data at Lake Atalanta, I found a decreasing total P trend which aligned with the same decreasing total P trend in Prairie Creek below Lake Atalanta Dam, during the same years.
Unfortunately, we do not have lake monitoring data prior to the construction and dredging, so we cannot give you the total P concentrations in the lake prior to 2017.
After I identified the two aligned trends, one question arose in my mind: if phosphorus concentrations are decreasing, why does Lake Atalanta seem to have more algae growth during the summer, after the park renovation and lake dredging? I contacted an associate who helped prepare both monitoring reports this year and who is familiar with lake chemistry cycles, Tony Thorpe with the Lakes of Missouri Volunteer Program. One explanation he proposed was that prior to the renovation and dredging, Lake Atalanta may have been more turbid, due to the lower amount of time and capacity for sediment to settle out. This could have prevented light from penetrating the water column and prevented algae from growing. After the park renovations, which include a forebay that can capture tons of sediment and other low impact development features that slow sediment movement during rainfall in addition to the lake dredging, it’s possible that clearer water is allowing light to penetrate deeper into the water column, stimulating algae to grow in the lake during the warm summer months.
This is just one possibility as to why Lake Atalanta may have increased algae growth since the renovation. Although the monitoring data does not provide a definite answer or solution to the algae problem at Lake Atalanta, it does show that phosphorus, a major nutrient that stimulates algae growth, has declined over the last two years both in Lake Atalanta and in Prairie Creek. This is a positive trend for the environment and for water quality in Lake Atalanta and downstream.
In addition to the reduced phosphorus concentrations, Prairie Creek has also had consistently high ratings in StreamSmart macroinvertebrate surveys, achieving good and excellent scores in 2019 and rating as the overall highest site and only site exceeding an average composite score of 20 in 2019.
The overall story for both Prairie Creek and Lake Atalanta in a positive one, with phosphorus declining in both waterways while each is supporting and enhancing the aquatic environment.