Temporal Variation The effect of time on water quality data (page 2)

DAY-TO-DAY VARIATION
Nutrient and algal chlorophyll concentrations are constantly changing within a lake. Conventional wisdom suggests that short-term shifts (day-to-day) would be fairly small, but actually these changes can be considerable (Figure 1). In 2004 the MU limnology lab collected samples from Little Dixie Lake on 108 consecutive days during summer (this lake was also monitored by LMVP). Most day-to-day fluctuations in phosphorus, nitrogen and algal chlorophyll were small, but occasionally changes were substantial. The largest 24-hour fluctuation in phosphorus was ±15 μg/L, a one day shift that equals 28% of the average phosphorus concentration (54 μg/L) during the project. The maximum daily nitrogen shift was ±200 μg/L, 22% of the overall average of 900 μg/L. Algal chlorophyll was even more variable, averaging 42 μg/L and having a few daily fluctuations larger than 30 μg/L (71% of the average). Again, these were the exceptions and not the rule. Most of these large changes occurred as a result of heavy rain fall which transported nutrients into the lake as nonpoint source pollution; quick sedimentation of the nutrients right after the rain fall (nutrients bound to soil particles settle out fairly quickly); and in the case of chlorophyll, an algal bloom that formed and dissipated quickly.

Because daily sampling is not feasible for the volunteer program, we rely on random sample collection to account for variability in water quality. That is, we do not want to collect our samples only after heavy rains when the lake looks its worst or only on days when the lake is clear. By collecting samples on a pre-set schedule of every three weeks, we hope to avoid the potential problem of selectively targeting the worst or best of conditions. While having a pre-set sampling schedule to ensure randomness seems counter-intuitive, please remember the events that impact water quality (such as rainfall) are generally random. The pre-set schedule also assures that samples are not all collected during a short period of time (e.g. eight samples collected in one month). By scheduling sample collections evenly over the spring-fall period, we ensure data quality by monitoring the lake over a variety of conditions.

Figure 1. Day-to-day variations in phosphorus concentration in Little Dixie Lake during 108 days of monitoring in 2004.
Figure 2. Comparison of volunteer and MU phosphorus data from Little Dixie Lake during 2004.

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