Summer Ephemeroptera, Plecoptera, and Trichoptera (EPT) species richness and community structure in the lower Illinois River basin of Illinois

Ephemeroptera, Plecoptera, and Trichoptera (EPT) species richness is useful for monitoring stream health, but no published studies in Illinois quantit atively document EPT richness or assemblage structure. The objectives of th is study were to characterize adult EPT richness and structure and relate t hese to relative water quality at eight stream sites (160-69,300 km(3) area ) in the lower Illinois River basin. Adults were ultra-violet light trapped in June, July, and August 1997. Nutrient enrichment by nitrate and nitrite nitrogen was strongly evident, especially in smaller drainages, while crit ical loss of stable habitat was observed in larger water bodies. Seventy EP T species were identified from 17,889 specimens. Trichoptera were by far th e most speciose (41 species), followed by Ephemeroptera (26), and Plecopter a (3). Caddisflies also dominated species richness across sites, contributi ng 18.0 of the average 28.9 total EPT species collected. Site EPT richness varied. significantly (F = 5.51, p = 0.003, df = 7), with smaller drainages supporting greater richness, generally. Differences were also evident for months (F = 21.7, p = 0.0001, df = 2), with June being lower (11.8 average) than either July (20.6) or August (18.1) values. Hilsenhoff biotic index ( HBI) scores did not vary significantly across sites (F = 0.7, p = 0.7, df = 7), but were different across months (F = 5.4, p = 0.02, df = 2). June (4. 23) and July (4.53) means were not different, but both were lower (of bette r quality) than August (5.33) scores. The relationship of EPT to HBI scores was not investigated statistically due to problems of sample size and inte rdependance of monthly samples, but graphical analysis suggested no consist ent relationship. This suggested a decoupling of the HBI from the EPT and i mplied that the gain in taxonomic resolution achieved by using adults outst ripped the resolution of the HBI. Use of the HBI to characterize adult aqua tic insect communities is discouraged. New state records and range extensio ns for Ephemeroptera and Trichoptera are presented and possible loss of sen sitive Plecoptera in the drainage is discussed.