Influence of Different Densities of the Mayfly Grazer Heptagenia criddlei on Lotic Diatom Communities

Abstract

Localized high densities of the mayfly nymph Heptagenia criddlei McD. occur in Oak Creek, Arizona during the summer. We examined the effects of different grazer densities of H. criddlei on diatom cell density and community structure in a laboratory stream over selected time periods ranging from 1 d to 4 wk. Insect densities at 800-1000 nymphs/m², 2800-4100 nymphs/m², and 7100-7700 nymphs/m² of periphyton cover were compared with ungrazed control channels. Our observations show that localized differential grazer densities are important in determining the abundance, composition, and physiognomy of lotic diatom communities. At grazer densities of 800 nymphs/m² there were no notable differences in diatom abundance and diatom assemblages between grazed and ungrazed systems throughout a 4-wk period. However, major differences were noted after 10 d at 2800 nymphs/m², 3 d at 4100 nymphs/m² and within 24 hr at grazing densities ≥7100 nymphs/m². At grazer densities ≥2800 nymphs/m² the overall density of diatoms declined and the relative proportions of larger upright diatom species to smaller adnate diatom species decreased through time with increased grazing pressure. Adnately attached diatom taxa like Cocconeis placentula var. euglypta (Ehr.) Cl. and Epithemia adnata (Kütz.) Breb. showed greatest resistance to grazing, while diatoms with smaller cell dimensions like Achnanthes minutissima Kütz. were next most resistant to grazing by Heptagenia.