Thursday, February 24, 2011

Rapidan River off Graves Mill Rd., Pt. II.

Well, this is very interesting.  I made two stops on my trip to the Rapidan yesterday.  In my last entry,  I reported my findings at the first stop -- about 1 mile below the entrance to the National Park.  My second stop was about 5 miles downstream from that location, not far from the village of Wolftown.  There, I didn't take any live photos -- I just collected some insects for preservation, and this morning I took a look at them with my scope.

The insects I found at this second stop were entirely different than those I found at the first.  First of all, I did find large winter stoneflies (only a couple) -- both genera (Strophopteryx and Taeniopteryx).  And I found one tiny Perlodid stonefly -- genus Isoperla.  But I found lots and lots of Nemourids, all genus Nemoura (photo above), and note that their wingpads are already fully developed (we've not yet started to see "baby" Amphinemura Nemourids).  I regret keeping so many: I thought I was picking up something else.   Could it be that the reason we've seen this genus so rarely in our watershed streams is that it's less tolerant of stream impairment than the genus we do find -- Amphinemura (I can't find "tolerance value" information on the genus Nemoura; the general tolerance value for Amphinemura is 3.4)?   It's certainly true that I've only found this genus in pretty good streams (Long Island Creek, the Lynch River, Elk Run, the Moormans River, and now the Rapidan).  Nemoura Nemourids may be "rare" in the streams that I've commonly sampled; I cannot say they're rare in this region.

And secondly, the mayflies I found were not those I had found just 5 miles upstream.  There, you'll recall, I found a whole lot of Pronggilled mayflies, and a lot of Epeorus flatheads.  I also found two spiny crawlers, genus Ephemerella, but these were the beautifully colored species that I've seen nowhere else.  This one.

Here (the second stop), there were no Pronggilled mayflies at all, only a few flatheaded mayflies (still Epeorus), one small minnow mayfly (genus Baetis -- the same that I've found in the Doyles), and -- once I had looked at them with my scope -- four decent sized spiny crawlers, genus Ephemerella, but all a drab brownish green (note the photo below).  This is the spiny crawler I'm used to seeing in our watershed in large numbers throughout the spring.  So, they've arrived!

What accounts for the difference in the nymphs found in the two different sites on the very same stream?
That's something I'd like to study -- but I lack the training and knowledge I need.  Still, a couple of comments and thoughts: the first being that we need to be clear that the mayflies we found yesterday at the upstream location -- Pronggilled mayflies and Epeorus flatheads (lots of them!) --  indicate very good water; those downstream, not necessarily so.  (I wonder about the two different species of spiny crawler mayflies...)  So, it looks like there has been a decline in the quality of the stream even though we've only moved five miles downstream.   Moving from that observation, we have to ask, has the habitat changed?  And the answer is "Yes".  The upstream location is coming out of the national forest, with no human habitation above it; but between it and the downstream location is farm after farm after farm -- less trees by the water, more open land.  And I guess the last point I'd make is that the stream character has radically changed.  The upstream site is riffles and cobble with very few pools -- fast flowing water; downstream, the Rapidan has assumed the more common form we find as streams move out of the woods -- a constant succession of riffles and pools, with the pools being long and the riffles fewer and farther between.

Further comments on these changes are welcomed from readers.   One thing I've always wanted to do is take a good stream and have 5 - 10 teams to sample that stream at different locations, maybe 5 to 10 miles apart on the same day.  What would we find in terms of the types of insects found?  And what would we find in terms of the "scores"?  Ah...  so many fun things to do, so little time (and so few helpers)!

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