Biological Data  
 

	9/16/98	9/15/99	10/11/00	10/10/01	10/30/02	10/20/04	12/01/04	11/14/07
Diptera	1	7	2	7	1	2	1	2
Ephemeroptera	0	1	0	3	0	0	0	0
Megaloptera	1	0	4	6	1	0	0	7
Plecoptera	1	1	0	31	0	0	0	0
Trichoptera	8	16	7	40	62	41	37	24
Coleoptera	3	0	0	7	1	0	0	0
Decapoda	1	2	0	0	0	0	0	0
Amphipoda	0	4	0	3	47	2	11	95
# Individuals	15	31	13	97	112	45	49	128
H=-Σpilnpi	1.37	1.33	0.99	1.49	0.85	0.36	1.33	.75908

    The majority of the organisms found are classified as benthic macroinvertebrates. We collected them from the stream substrate using a Surber sampler. We then brought a sample of each organism found back to the lab for identification.  Macroinvertebrates serve as very useful indicator organisms for water quality because different species are associated with particular water conditions and, therefore, we can use the species distribution collected to make a statement about the environment at Holmes Run. While we consistently found trichoptera and megaloptera, two species that prefer very good water conditions, we also found diptera, amphipods, and simulidae, species typically associated with a decline in water quality. When used to construct a pollution tolerance index, the distribution of all species collectively gave Holmes Run a "Fair" rating in terms of water quality.

    For comparison the class occasionally visits the headwater streams of the Thorton River outside of Sperryville, VA.  The watershed of these streams is mostly Shenandoah National Park, and there are few sources of pollution input outside of roads (Rt. 211 and Skyline Drive).  Below are a picture and some macroinvertebrate data taken from this stream.

    The diversity index for both days that we visited Thorton River is far higher than the diversity of Holmes Run.  These numbers are suggestive of the higher degree of human impact in Holmes Run which typically reduces diversity.

    Another group of stream organisms studied are the diatoms.  Diatoms are unicellular algae that colonized rocks and other stream substrate material.  They are characterized by cell walls made of silica compounds, and these cell walls (frustules) make identification easier.  The diatoms were collected from rock substrate using a stiff brush.

Video Clips of Organisms Found in Holmes Run (open in a new window with WinDVD 4 for best results and use the back button to return to this page):

Order Megaloptera - Also called Hellgramites, these insects are the larvae of dobsonflies.  They are found under stones in running water and are predacious on the naiads of dragonflies, stoneflies, and mayflies.  Close observation of this specimen shows the abdominal gills.

Order Amphipoda - Amphipods are crustaceans that inhabit a wide variety of unpolluted lakes, ponds, streams, and springs.  They are usually 5 - 20 mm long.  They are voracious feeders preferring freshly killed animals.  High dissolved oxygen levels in the water are necessary.

Order Diptera - Family Chironomidae & Family Simulidae - Chironomids are dipterans commonly referred to a midges.  They inhabit all types of aquatic environments where they feed on the surface or remain attached to rocks at the bottom.  Simulids or blackflies inhabit swift water where they remain attached to the bottom and filter-feed on water particles as they drift by.

Order Trichoptera - These insects are the larvae of caddisflies.  They build characteristic cases of sand or leaf material, and feed mostly on attached algae of periphyton in running water.

Assorted Diatoms - These algae are included in the periphyton or attached algae found on rocks and other substrates on the stream bottom.  They contribute to the autochthonous (from within) production of the stream through their photosynthesis.  Other energy input, especially in smaller streams is from allochthonous (from outside) sources such as leaffall carried downstream.  The diatoms here were collected from stream rocks.  The sample was then placed on a glass coverslip left to dry.  Heating the coverslip on a hot plate then incinerates all organic matter leaving behind the characteristic glass frustule, or outer case, of the diatom.  A frustule consists of two halves and is arrange much like a shoe box.  Some diatoms pictured are in valve view, or from the top/bottom.  With this view characteristic etchings, dots, and canals in the frustule can be seen.  Other diatoms are in girdle view, or from the side.  Identification is much more difficult with girdle view as the frustule etchings cannot be viewed.

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