Effects of Small Impoundments on Hydropsychid Caddisfly Production In Valley Creek, Minnesota

Abstract

Annual production by filter—feeding caddisfly larvae (Trichoptera: Hydropsychidae) was estimated by the size—frequency method and compared in riffles immediately above and below small (<1 ha) impoundments or ponds at three sites in Valley Creek, Minnesota in 1982—1983. At all three sites, production was significantly higher below impoundments than above. At the most upstream site, annual production by Hydropsyche slossonae was 0.9 g/m2 (dry mass) in the upper riffle but was 40.0 g/m2 in the riffle located below inputs by ponds beside the main stream. At the middle site, production by H. slossonae and a small population of Cheumatopsyche pettiti was 16.2 g/m2 in the upper riffle, but was 31.7 g/m2 below two mainstream impoundments and a side pond. At the lowermost site, production by co—dominants H. slossonae and C. pettiti together with Hydropsyche riola totalled 5.8 g/m2 in the upper riffle but 34.9 g/m2 below a mainstream impoundment. Although the number of hydropsychid species increased at successive downstream sites, the proportionate representation of species in the upper riffle at a given site was almost identical to that in the lower riffle. The higher production in lower riffles was attributed to factors in pond or impoundment outflows that provided favorable conditions for species already established in the reach of creek containing the impoundment. The most likely factor was postulated to be an increase in abundance or quality of seston. Other factors were better growths of moss and filamentous algae, which provided attachment sites for retreat construction, and the absence of abrasive sand after deposition in impoundments.