Pollution-sensitive mayflies and caddis flies re-colonize Big Spring
Dr. Todd M. Hurd, Department of Biology, Shippensburg University
The aquatic insect community of a stream can be the best indicator of water quality. On spring creeks of Pennsylvania like Big Spring, impairment from organic pollution is indicated by overabundance of pollution-tolerant crustaceans, commonly called cress bugs or sow bugs. Studies by William Botts, an entomologist with PA-Department of Environmental Protection, notes that when quality of spring creek water is good, these cress bugs are outnumbered by other crustaceans, commonly called scuds or freshwater shrimp, and there is an abundance of pollution-sensitive mayflies, especially of the family Ephemerellidae 1, which include the group well known to fly-fishers as the "sulphurs", due to their yellow coloration, and consistent hatches that attract trouts' attention to the water surface.
Some impressive changes have occurred in the invertebrate community on Big Spring since the closure of the PAFB hatchery in November 2001. Pollution-tolerant cress bugs have decreased in relative abundance, and scuds or freshwater shrimp have increased 2. Concomitant with increased spawning activity by trout, Big Spring was recolonized within two years by Glossosomatidae caddis flies, called "the clean water caddis fly", due to its strong intolerance of polluted conditions. By spring 2004, there were great increases in Baetis mayflies (known to fly-fishers as blue wing olives, or "olives"), especially in upper reaches. This spring there have been hatches and spinner falls of several other mayfly groups, including the sulphurs noted above, and members of the Heptageniidae family (flatheaded mayflies), imitated since the dawn of American fly fishing by light cahill dry flies. It was a privilege on a recent, cold week in January to watch my Ecology class at Shippensburg discover the nymphs of these mayflies in Big Spring, while they referenced past class samples and Mr. Bott’s reports that showed previous absence of these taxa from the stream.
Earlier in the spring, adult black stoneflies were abundant over Big Spring. These flies, as well as Baetis and other slightly more pollution-tolerant net-spinning caddis flies have been attracting trout to the surface, even in lower reaches of Big Spring. When I was on the stream for an evening in early June, egg-laden sulphur spinners were winging their way upstream over clean gravel from somewhere below Nealy Road. Large, cream-colored Heptageniidae were emerging in densities high enough to attract attention of fly-catching birds and wild trout, along with sporadic emergence of caddis flies. Recent high flows, and clearing of debris from the rail culvert above Newville appear to have flushed sediment downstream, improving substrate quality for invertebrates, as well as trout. Big Spring is clearly poised for continued population increases of both invertebrates and trout. All organisms’ possess the capability to increase geometrically (non-overlapping generations differ in size by a constant ratio) or exponentially (continuous increase in rate of population growth with overlapping generations). These natural features of stream recovery are in place on Big Spring, and will depend on continued discharge of high quality spring water, that remains unimpaired along its flowpath.