Question:
I have enjoyed reading your articles on FAOL. I also purchased and
read your Streams book this winter. Unfortunately, it is at present
in the hands of another local TU chapter member so I can’t use it
for reference.
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.
Originally published September 3rd, 2007 on Fly Anglers Online by Robert Younghanz.