THE EFFECTS OF HYDROPSYCHID COLONIZATION ON ALGAL RESPONSE TO NUTRIENT ENRICHMENT IN A SMALL MICHIGAN STREAM, USA

1. We tested the hypothesis that the indirect effects of colonization by Hydropsyche spp. (Trichoptera: Hydropsychidae) may be greater than direct effects of nutrients on the benthic algal community growth. Two sets of nutrient-releasing substrates (a total of twenty-few) were deployed into a small pristine stream in northern Michigan. Each set was composed of four treatments replicated three times: (i) no nutrient enrichment (C), (ii) 0.5 M phosphate-P enrichment (P), (iii) 0.5 M nitrate-N enrichment (N) and (iv) 0.5 M phosphate-P plus 0.5 M nitrate-N enrichment (P + N). All hydropsychids colonizing on the substrate in one set (twelve substrates) were removed regularly and the other set (twelve substrates) with undisturbed hydropsychids served as the controls. 2. Algal biomass and gross primary productivity were estimated as chlorophyll a (chi a) concentration, algal biovolume, and carbon fixation rate, respectively. There was a significant interactive effect of hydropsychid colonization and P enrichment on algal biomass measured as chi a concentration. With removal of hydropsychids, chi a concentration increased Ii-fold in the P enrichment treatments relative to the controls. The effects of P on chi a was, however, not significant in the presence of hydropsychids. Such interactive effects were not observed when algal responses were measured as biovolume and carbon fixation rate (GPP). 3. It is recommended that algal responses to nutrient enrichment should be measured as biovolume or carbon fixation rate in small streams where hydropsychids are commonly present.