How's the water quality on the other 20 days?
Our LMVP volunteers diligently sample every 3 weeks, but they might wonder what happens on the days between sampling dates.
In this issue of the Water Line we’re featuring some of the projects coordinated by staff at the University of Missouri’s Limnology Laboratory. One such project is our “Daily Sampling”. This is a project where 3 lakes were sampled for 100 consecutive days this summer by the lab. These lakes were monitored for Secchi, phosphorus, nitrogen, chlorophyll, suspended sediments, turbidity and color. The goal of this project is to track how parameters vary on a day to day basis in relation to rainfall. When you collect a sample every 3 weeks you can get a pretty good idea of what the average conditions are for the season. However, in order to determine how long sediments remain suspended following a storm you have to sample a little more intensively.
One of the lakes sampled for the Daily project was Little Dixie. Little Dixie also happens to be sampled by a volunteer in the Lakes of Missouri Volunteer Program. This provided an excellent opportunity to compare some numbers.
One way to examine these numbers is to see how the volunteer’s values compared to that of the University collected values. We compared the volunteer Secchi values and the MU-collected Secchi values for corresponding dates. The difference was only two inches on 3 of the 4 days that both projects sampled Little Dixie. On one day, though, the difference in Secchi values was eight inches. That eight inch difference could have been caused by cloud cover moving in, increased wave activity or even the presence of sunglasses.
Figure 1: Secchi transparency values at Little Dixie Lake as measured by a volunteer and by University employees.
Another way to look at the data is to see how well the sampling interval (e.g. every 3 weeks) captures the lake’s “character”, or average condition. The table shows some statistics for each project’s observations. The Daily project features 100 Secchi values, one for each day, while the LMVP has only 5 Secchi values, or one approximately every 3 weeks. While the LMVP sampler didn’t witness the day with the greatest clarity (50”) or the day with the lowest clarity (13”), the average value recorded by the volunteer is only three inches less than the average of 100 days. That means that LMVP volunteers are doing a pretty good job of estimating summer conditions on their lakes by visiting once every 3 weeks.
| Number of Observations | MU Measured Secchi (inches) | LMVP Measured Secchi (inches) |
|---|---|---|
| Number of Observations | 100 | 5 |
| Minimum | 13 | 25 |
| Maximum | 50 | 33 |
| Average | 31 | 28 |
Table 1: Summary statistics comparing University Secchi observations with LMVP volunteer Secchi observations
If you look at Figure 1, you’ll notice how the volunteer’s measurements follow the University staff’s Secchi values rather well. It was not uncommon for the clarity to shift up to 10 inches from one day to the next. Further evaluation of the chlorophyll and suspended solids data (after analyses are completed) will hopefully help us to explain these shifts.
“By sampling at regular intervals, we ensure that we don’t “skew” the data”
Finally, it’s worth noting that by going out every 3 weeks, volunteers aren’t simply choosing the nicest, sunniest days to sample. Imagine how the data might differ if this particular volunteer only sampled when the sun was shining, or if he only sampled after rainstorms. By sampling at regular intervals, we ensure that we don’t “skew”, or bias, the data so it represents one lake condition more than another. It’s also important to get as close to 8 samples as possible over the course of the summer. When the regular sampling intervals are combined with consistent sampling, we can be quite sure we have accurately represented that lake’s summertime conditions. Plus, sampling is another excuse to get out on a lake on a summer day.