Tolerance Values and Monitoring the Health of Our Streams

                              (The Rivanna River below Palmyra, VA in Autumn)

Let me begin by showing photos of a few insects, most of which I can identify to the level of species, then note, 1) the current "tolerance value" used for that taxon by VA DEQ (the Virginia Department of Environmental Quality), and 2) the value assigned to that taxon in the new list of tolerance values posted by the NC DWQ (North Carolina Division of Water Quality).  After that, I'll discuss the possible significance of these discrepancies.  Please note that, at the moment, VA DEQ bases tolerance values on "family level" ID; NC DWQ goes to the level of species.

I. Baetidae (small minnow mayflies)

1. "Probably" Acentrella nadineae; Buck Mt. Creek, 3/1/11
VA DEQ tolerance value, "4.0"
NC DWQ tolerance value, "1.9"

2. Heterocloeon curiosum, Rivanna River, 7/11/11
VA DEQ tolerance value, "4.0"
NC DWQ tolerance value, "2.1"

3. Baetis pluto, Buck Mt. Creek, 6/10/11
VA DEQ tolerance value, "4.0"
NC DWQ tolerance value, "3.4"

4. Acentrella turbida, Buck Mt. Creek, 10/25/10
VA DEQ tolerance value, "4.0"
NC  DWQ tolerance value, "2.0"




II. Odonata, Gomphidae (Clubtail dragonflies)

1. Hagenius brevistylus, Rivanna River, 7/11/11
VA DEQ tolerance value, "1.0"
NC DWQ tolerance value, "4.4"

2. Ophiogomphus, Buffalo/Roach River, 7/6/11
VA DEQ,tolerance value, "1.0"
NC DWQ tolerance value, either "undetermined" or "5.9"


III. Common Netspinners (Hydropsychidae)

1. Cheumatopsyche, Buck Mt. Creek, 3/18/11

VA DEQ tolerance value, "6.0"

NC DWQ tolerance value, "6.6"

2. Diplectrona, Moormans River tributary, 3/4/11
VA DEQ tolerance value, "6.0"
NC DWQ tolerance value: Diplectrona metaqui, undetermined; Diplectrona modesta, "2.3"

3. Macrostemum, North Fork of the Rivanna, 7/17/11
VA DEQ tolerance value, "6.0"
NC DWQ tolerance value, "3.4"


_________________________________________


Let me note a few things before I comment on the possible significance of the data cited above.  1) Virginia DEQ is in the process of developing a new list of tolerance values for benthic macroinvertebrates based on genus level ID.  My understanding is that those values will be put into place in determining "scores" for the health of our streams sometime within the next 5 years.  2) Tolerance values are only one of the things that are used in determining scores for streams.  The "metrics" used for these calculations are complicated, and you can see how complicated by viewing the slide show put together by StreamWatch (http://streamwatch.org/volunteers/slideshows) indicating how they go about it.  I urge all volunteer monitors to review that information.  3) The genus tolerance values used by the state of Virginia might not be the same as those used in North Carolina.  Tolerance values for stream taxa, apparently, vary with locale.  I don't quite understand why that it so -- but it is.  And 4) the data presented above is for -- what? -- 8 species.  Any differences these figures might make in scores could well be offset by different values for other taxa.  I.e. "new" scores developed by using new tolerance values might not vary that much from the current scores.

All of that being said, it seems clear that more exact tolerance values for taxa will lead to more accurate stream assessments -- something all of us want to see.  At the same time, going to "genus" or even "genus and species" tolerance values could certainly affect the role played by amateur volunteers in monitoring the health of our streams.   Family ID of macroinvertebrates isn't easy; genus ID often -- perhaps most of the time -- requires microscope work in a lab; and species ID is a job for the pros.
Those of us who love to sample in streams may well have to find a new way to participate in keeping an eye on the health of our streams.  But, all of this remains to be seen.