Today’s Photography 101 topic is mystery, and here’s my photo:
I post this photo because I’ve been working on this stream for a while to solve a mystery. The water quality is good here – surprisingly good for an urban stream – and the insects you find here frequently back that up. I’ve found riffle beetles and stoneflies here on occasion, insects that are only found in very clean waters. However, you don’t find them every time. In fact, you usually find almost no insects at all!
For the past three years, I’ve been working with some high school students to study this stream to try to explain the lack of insects. What we’ve found so far suggests that flooding is the primary factor influencing the insect population in this stream. While we still need to collect more data, particularly after a flood event, to be sure that it’s the flow that drives the lack of biodiversity in this stream, but I’m feeling more and more confident that this is the case and that we’re close to finally solving this mystery.
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Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth.
The Dragonfly Woman 
Interesting story, I hope you are able to solve your mystery soon. Good luck to you and your students.
Thanks!
I’d bet you’re on the right track. I’ve looked at a small urban stream about the same size that’s extremely flashy as it has the storm sewers from various streets and neighborhoods directed into it, and in the occasions I’ve looked I’ve seldom found more than water striders present, even though the water quality from a chemistry perspective has always been very good over the years.
A second possibility that would be very hard to track barring very frequent monitoring and a bit of luck is that there is something upstream that is periodically discharging something that’s wiping out the populations. One suggestion would be to talk to the local city planning department and see if you can get a storm sewer map of the area to see what the real watershed is, and do a windshield survey to see if you come across anything that sends up a red flag.
I once field tested a park pond near a storm sewer outlet and found phosphorus levels through the roof (not that phosphorus is toxic in most cases, but as an example). Driving around, I found that someone about a block away had just washed their car and all the soapy water from that was flowing into the storm sewers…which in that neighborhood ultimately discharged to the pond. But by the time it got there, the suds were largely gone, so I didn’t have that as an indicator. The pond in question had been having huge chronic issues with algae and duckweed overgrowth, so that right there helped answer the question of why.
So many things go into problems in aquatic systems! It’s hard to pinpoint the exact causes, but that’s part of the fun too. :)