Disturbances from flooding are a dominant feature of the habitat template in streams. Frequent floods created by recreational releases (1–2 d between releases/floods) from Abanakee Dam in the Adirondack Mountains, USA, result in a static mosaic of scoured patches in areas of high shear stress (low diatom densities) and mats of algae in areas of low shear stress (refugia); this pattern is different from the shifting mosaic of patches more typical of streams recovering from floods. Predictable, recreational release floods and similar bed size particle distributions in this stream enabled us to use a reciprocal replacement experiment to examine macroinvertebrate community responses in high scour and flow refugia areas without the confounding variable of different substrata. Trays of substrata were embedded in both high shear stress (HSS) and low shear stress (LSS) areas. Half of the trays from each area were then reciprocally exchanged while resident trays remained in their original positions. After 2, 6, and 12 releases, 20 trays for each of 4 treatments were sampled. Lower macroinvertebrate densities in the LSS area reflected an absence or loss of filter-feeders that require higher current velocities than were present between releases, and this absence probably was responsible for the lower species diversity in the LSS area. The net-spinning caddisfly, Macrostemum, dominated the HSS area because its retreat design protects the fine-meshed net from high velocities. The filter-feeding mayfly, Isonychia, thrived in the HSS area because it could seek refuge during releases and could benefit from the potentially higher particle delivery in the higher flow velocities between releases. Ordinations showed that community composition shifted quickly (2–12 d) in trays moved from the LSS area to the HSS area because of the rapid colonization by filter-feeders in the HSS area. However, trays from the HSS area moved to the LSS area were slower (>21 d) to change to the LSS community composition. We suggest that hydrologic conditions strongly influenced macroinvertebrate composition with filter-feeders dominating where shear stresses scoured periphyton. However, in LSS areas where periphyton was abundant, the community was dominated by collector-gatherers and scrapers. Also, HSS substrata transferred to the LSS areas were slow to be colonized by gatherers and scrapers probably because periphyton had not yet developed on the HSS rocks.
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