SHIFTS IN THE EFFECTS OF TUFT-WEAVING MIDGES ON FILAMENTOUS ALGAE

During summer low-flow periods massive growths of filamentous green algae (dominated by Cladophora glomerata) in the Eel River of Northern California become infested with chironomid larvae that weave the algae into retreats, or "tufts." Field experiments suggest that these midges (primarily Pseudochironomus richardsoni) have negative early effects, and positive later effects on the biomass of algae they inhabit. During initial colonization and tuft construction by midges, weight loss of Cladophora and associated epiphytes in stream enclosures increased with increasing midge densities. In an in situ incubation experiment, chlorophyll a content and final damp weight of Cladophora and epiphytes were higher with old, feces-filled tufts, or with nitrate fertilizer, and were lower in controls or with new midge tufts. These results suggest that tuft-weaving midges have complex effects on algae they inhabit that change with the ontogeny of midges and the age of the retreats they construct.