I am fortunate to live in Pawling, NY within easy driving distance of some of the finest trout fishing in the Northeast. The closest spot happens to be in the NYC reservoir system: the Croton Watershed. My stream there is the tailwater of the East Branch Reservoir. The NYC DEP regulations allow this stream to be fished year-round and it is chock full of browns and rainbows. I am lucky enough to be able to drift flies there about twice a week all year long.

To my purpose for writing to you: Each August the tailwater emits a strong sulphur odor. Simultaneously, the blue green algae strings attached to benthic rocks die (this is where I need to have your book, because I believe you wrote that it isn't really algea, but a bacteria). The trout seem to survive, however the water column is loaded with drifting matts of the slimy whitish dead cyanobacteria strands. The strands turn into a slime that is no longer stranded and become detached from the rocks.

This time of year isn't particularly good for trout fishing. Because its a tailwater, the temps hover near 60F, but there are very few hatches of mayflies or caddis at this time on the river. Other tailwaters in the area are fishing well with hatches of sulphurs, olives, Isonychia, Potamanthus, various caddis etc, but the East Branch tailwater gets very few, if any of these insects this time of year. Only midges.

The locals don't seem to think there is any connection to the sulphur odor, dying algae, and lack of bugs, but I suspect otherwise. They just say the rotting sediment makes the water stink and fishing isn't good in August.

What would you make of this situation? Do you think the cyanobacteria could be dying causing the sulphur smell? Or is the sediment rotting, producing sulphur, and that is killing the cyanobacteria and in turn aquatic macroinvereibrates.

Thanks in advance for your reply. Don Jiskra

Response: First, thanks for the kind words about my column and book; I appreciate it.

Your input is like a cross between a research proposal and a question on an oral defense of a thesis. Seriously, you pose some interesting questions and observations, and a definitive answer would require a lot more information and familiarity with the site than could be conveyed in a brief email. Therefore, my response will essentially be some observations that may – or may not – pertain to your situation. Here we go.

Organism: I have no reason to doubt your identification of the offending mat/slime as a cyanobacteria; you're closer to the subject and I'm not sure it would really make a difference in terms of your question. But remember, filamentous green algae can also experience die-offs, detachment, and decomposition.

Two other mat/slime forming organisms that you may encounter are the diatom Didymospheniaand the bacteria Sphaerotilus. Didymo is an invasive that is causing problems in several streams and rivers, including places in New York. The latter can occur in unexpected places but usually requires a high carbon content in the water.

Sulfur: I do not think that the sulfur odor has anything to do with the apparent die-off of the cyanobacteria for several reasons. The smell is most likely due to hydrogen sulfide (rotten egg gas), and this only occurs under anaerobic conditions. It is highly unlikely that anaerobic conditions would be present in a turbulent steam where the water is rapidly oxygenated. Rather, it commonly occurs in lakes or reservoirs where the water stratifies in summer, the lower regions (hypolimnion) can become anaerobic due to decomposition of organic matter, and hydrogen sulfide can be produced. If hydrogen sulfide builds up and the discharge of water from the reservoir is from the hypolimnion, then the water could release hydrogen sulfide as it exits, producing the odor you describe. The hydrogen sulfide remaining dissolved in the water would rapidly be oxygenated into other sulfur compounds, none of which would produce an odor or be detrimental to organisms as far as I know.

Also, I've experienced many situations where there were dense mats of filamentous algae (cyanobacteria and filamentous greens) that were decomposing and I've never noticed the odor of hydrogen sulfide. I talked to a couple of colleagues and asked them if they had ever experienced such an odor in similar situations ; neither had, but all three of us have experienced the odor of hydrogen sulfide when anaerobic sediments were disturbed.

Macroinvertebrates: I have no explanation for the lack of diversity and numbers of the insect community, but would expect it to be related to other environmental factors rather than the bacterial die-off. Again, this is something I simply can't address without knowing more about the ecological characteristics of the stream and its environs.

Well, this is a lot of rambling around your direct questions, but it is the best I can do without a lot more information. In summary to your direct questions, No, I don't think the sulfur odor is coming from the dying cyanobacteria mats, and Yes, the sulfur and odor is coming from decaying organic matter in the sediments of the reservoir, but it is probably not killing either the cyanobacteria or the macroinvertebrates.

I hope it helps; let me know if you have further questions. Take care,
Bert

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584
Email: streamdoctor@aol.com

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at streamdoctor@aol.com.