Status of Little Rock Lake

Outreach

lrl waterqualitymeeting

 

On September 7th, 2012, the Benton SWCD hosted the 1st Annual Little Rock Lake Water Quality meeting. There were approximately 40 people who attended the meeting. Katie Winkelman provided a recap of the Little Rock Lake TMDL project. Guy Spence spoke on the new Little Rock Lake Association Monitoring Program. Gerry Maciej spoke on current funding options for interested landowners. Nathan Sanoski presented on Native Buffers and Katie Winkelman wrapped the evening up by highlighting Benton SWCD and NRCS projects and program accomplishments. Display boards were available for viewing and handouts were available for interested attendants.

 

Monitoring

lrl monitoring sites

Little Rock Lake monitoring sites

 

Water quality data is being collected by volunteers in the Citizen Lake-Monitoring Program (CLMP). Three monitoring sites (indicated in red on the map) are being used, with samples being collected twice a month from May to September each year. Secchi disk depth is recorded at each site during sampling. Water samples are sent to RMB Environmental Laboratories and analyzed for the remaining two parameters:

  • Total Phosphorus
  • Chlorophyll-a

 

lrlmonitoring secchiSecchi disk depth is a measure of the water transparency. The volunteers collect this data using an 8" circular, all-white metal plate attached to a calibrated rope. This tool is called a Secchi disk. The volunteers lower the disk into the water until it is no longer visible, noting that depth from the markings on the rope. The disk is then lowered a little further and then raised back up until it is just visible. This second depth is averaged with the first, and the final number is recorded.

 

Water transparency is a quick and easy measurement that tells us a lot about a lake's water quality. First, it indicates the amount of light penetration into a lake. Second, Secchi transparency provides an indirect measure of the amount of suspended material in the water, which in many cases is an indication of the amount of algae in the water. Long-term transparency monitoring by CLMP volunteers helps scientists detect signs of degradation to a lake. Generally, the sooner water quality problems are detected, the easier and less expensive it is to restore the lake to its previous state.

 

Phosphorus is usually measured in two ways in lakes, as ortho-phosphate (soluble, reactive phosphorus) and as total phosphorus. Ortho-phosphate is the chemically-active, dissolved form of phosphorus that is taken up directly by plants. Ortho-phosphate levels fluctuate daily, and in lakes there usually isn't a lot of ortho-phosphate because it is incorporated into plants quickly. Total phosphorus (TP) is a better way to measure phosphorus in lakes because it includes both ortho-phosphate and the phosphorus in plant and animal fragments that are suspended in lake water. TP levels are more stable and annual mean values can tell you a lot about the lake's water quality and trophic state.

 

lrlmonitoring watersampleChlorophyll-a is the pigment that makes plants and algae green. It allows plants and algae to photosynthesize, which is the process of using the sun's energy to convert carbon dioxide and water into oxygen and cellular materials. We test for levels of Chlorophyll-a to determine how much algae is in the lake.

 

Phosphorus, Chlorophyll-a, and Secchi depth are related. When phosphorus increases, that means there is more food available for algae, so algal concentrations increase. When algal concentrations increase, the water becomes less transparent and the Secchi depth decreases. The resulting numbers from these water quality measurements cover different units and ranges, and thus cannot be directly compared to each other or averaged.

 

In order to standardize these three measurements to make them directly comparable, we convert them to a trophic state index (TSI) using an equation. The overall TSI of a lake is the average of the TSI for phosphorus, the TSI for Chlorphyll-a, and the TSI for Secchi depth. Therefore, it can be thought of as the lake condition taking into account phosphorus, Chlorophyll-a, and Secchi depth. The following table summarizes the different ranges in TSI values.

 

TSI
Chl-a
(ug/L)
Secchi disk
(ft)
Total
Phosphorus
(ug/L)
Attributes
Fisheries &
Recreation
<30
 
<0.95
>26.2
<6
Oligotrophy

Trout fisheries dominate

30-40
 
0.95-2.6
13.1-26.2
6-12
 
Trout fisheries in deep
lakes only
40-50
 
2.6-7.3
6.6-13.1
12-24
Mesotrophy
Walleye may predominate
50-60
 
7.3-20
3.3-6.6
24-48
Eutrophy
Warm-water fisheries only.
Bass may dominate
60-70
 
20-56
1.6-3.3
48-96
 
Dense algae and aquatic
plants. Low water clarity
may discourage
swimming and boating
70-80
 
56-155
0.8-1.6
96-192
Hypereutrophy
Water is not suitable for
recreation
>80
 
>155
<0.8
192-384
Algal scums, few
aquatic plants
Rough fish dominate;
summer fish kills possible

 

This next table displays the standard levels for all three parameters we are testing for, as well as the monitored levels from 1979 to 2003, 2006 to 2008, and 2012.

 

 
Standard
1973-2003
2006-2008
2012
Total P
(ppb)
<60
116-179
202-315
139-384
Chl-a
(ppb)
<20
69-90
114-227
109-349
Secchi
depth
(m)
>1
0.5-1.1
0.3-0.6
0.2-0.3

 

Thank you to all of our volunteers!!!