A stormwater pond is built to do a job. Nobody expects it to be alive.
The pond on Route 29 in Centreville, Virginia, collects runoff from roofs, roads, and parking areas and releases it into the stream below to slow stream-bed erosion. While it looks like an engineered pond with no ecosystem, it actually supports a diverse ecosystem.
It is visible from several hundred households in the local neighborhood. Yet no one seems to perceive this freshwater system’s other critical function: naturally filtering pollutants before the water flows into the stream.
Three summers ago, I first went to the pond with my cousin, and I have returned to the same spot every summer since. On each visit, I record how many fish I catch and how long I stay, curious about what this ordinary neighborhood pond is telling me. To learn more about the water, I have also measured the water's pH on 25 of the 48 sessions.
Why fish, and not only chemistry
A pH reading shows what the condition of the water was at the exact moment the meter was placed in it and is only valid for that one instant.
My data shows how important this is: in 75 measurements, the pH ranged from 7.2 to 8.6, the average variation being half a unit within one afternoon and at times varying by a complete unit. A reading taken at 11 a.m. and another taken at 5 p.m. from the same spot could be describing two completely different ponds; a single reading taken at one place once a week gives only one point on that curve and hides the rest.
pH rose between late morning and late afternoon in 25 of 25 sessions — every session on record.
Fish are different. Even though the chemistry normalizes itself within a few days after a runoff has caused the oxygen level to fall on an August night, the fish that have died stay dead, and it takes the whole community several seasons to rebuild. The water soon forgets about a bad night, but the fish don't.
I maintain the catch log in parallel with the chemistry log and record the number of hours fished, as calculating fish per hour enables comparison between short and long sessions, unlike total catch.
That is the argument for keeping a catch log. The numbers do not prove it. Feeding and weather influence fish-per-hour as much as the number of fish that are there, and with a hook, you cannot count what lives in the water anyway. Making this pond's fish community a reliable indicator would require a second pond to compare against, and unbiased sampling with nets or electrofishing, which needs permits and training I do not have. What I have is four years of catch log data. That did not exist before.
The record
The sessions are listed in order of date, and the months that are shaded contain no data since they were never sampled. Because fishing occurs from May to August, these results reflect only late spring and summer at the pond.
- Sessions
- 48
- Hours
- 269
- Fish
- 525
- Species
- 7
- pH readings
- 7525 sessions
What the neighborhood sees
Most people call it "the drainage ditch" or "the place where the mosquitoes come from," but the people who give that answer haven't counted anything. I caught 525 fish across seven different species in 269 hours. Bluegill were present in all 48 sessions, and largemouth bass in 19; the ratio of bluegill to largemouth bass was 6.8 to 1.
There is a large number of prey, only a small number of predators, and the quantity caught has stayed roughly level over the four-year period; this indicates that the pond is a stable warmwater ecosystem and not simply a drainage ditch. Even though these basins were not meant to serve as habitat, they do so, and it therefore makes sense to take this into account when deciding how to manage them.
Where the water goes
The water from the stormwater management pond does not stay in the pond; it runs into Cub Run, then Bull Run, and finally the Occoquan Reservoir, the neighboring county's drinking-water source. That is why it is important to measure the pond.
A year after I started fishing at the pond, I read that basins of this type retain sediment and the pollutants attached to it, but not all of them; dissolved road salt passes straight through. It is not a complete filter. Some pollutants that accumulate on the roads and lawns around the pond still enter the reservoir.
Objectives
Keep a comparable catch record
Identify and count every fish, and record how long the session ran, so a good day and a long day can be told apart.
Measure the water by hand
75 pH readings, three per session from late morning to late afternoon, to capture change across a day rather than a single point in it.
Place the pond in its weather
Pair every session with daily observations from the weather station at Dulles, and estimate the water temperature the pond was actually holding.
Report what the data cannot show
Publish results that came back empty, mark every estimate as an estimate, and document the findings that had to be withdrawn.
How to read this site
Solid — measured
Counted, timed or read off an instrument at the pond. Catch numbers, session hours and every pH value are measured.
Dashed — modelled
Calculated, not observed. Water temperature is modelled from air temperature and has never been checked against a thermometer in this pond.
Recorded elsewhere
Air temperature and rainfall come from Dulles airport, about seven miles north. Real measurements — but not measurements of this site.