Periphyton and macroinvertebrate assemblage responses to flow regime in spring-fed headwaters

Dry weather is common during summer in the Pacific Northwest of North America. As a result, biota of headwater streams have evolved various adaptations in response to these predictable periods of low flow (baseflow; Boulton 2003). Headwater streams may have a determinate source (spring), or flow may be dominated by shallow groundwater input generated by relatively recent precipitation. These 2 types of systems vary in the permanence of flow, with spring streams typically supporting continuous year-round flow to the channel head. In headwater streams not sourced by a spring, the upper extent of surface flow varies seasonally and clear differences have been noted between the communities utilizing areas with permanent flow and areas that dry in summer (Delucci & Peckarsky 1989). However, the extent to which summer low flow influences stream biota in headwater spring streams has not been fully evaluated. Spring-fed headwater streams usually support surface flow throughout the year. The proportion of spring water changes with season and distance from the spring. Near the spring and during extended periods of low precipitation there is a higher proportion of spring water. Even some spring streams may experience periodic dewatering during severe droughts; therefore, despite the relatively consistent flow in spring-fed headwater streams, summer low flow may still serve as an important environmental attribute dictating longterm species persistence and shaping the biotic assemblage (Lake 2003). We examined macroinvertebrate communities in springfed, headwater streams in the Oregon Coast Range in early and late summer, during the driest part of the year. We also characterized the composition and biomass of the periphyton assemblage in spring, examined invertebrate and periphyton assemblages with a focus on taxa resilience in these habitats at the head of the fluvial network, and explored how seasonal low flow and periodic droughts might influence assemblage characteristics.