Postflood Recolonization Pathways of Macroinvertebrates in a Lowland Sonoran Desert Stream

Abstract

Most macroinvertebrate taxa recolonizing after floods in Sycamore Creek did so via aerial pathways. After frequent winter flooding, most aerial colonists were aquatic adults (Coleoptera and Hemiptera), whereas ovipositing adults (Ephemeroptera and Diptera) dominated after summer flooding. Drift and upstream movements by relatively few taxa contributed most individuals after floods. Upstream movements were greater during high discharge than during low discharge periods. Few taxa and individuals were present in sediments below 10 cm. INTRODUCTION Stream macroinvertebrates are periodically decimated by various natural catastrophes, such as ice scour, floods and drought. An important aspect of recovery after a catastrophe is to be found in the palthways of recolonization. These pathways influence community composition and suggest common selective forces on -the life histories of species present. Williams and Hynes (1976) identified four principal recolonization pathways of stream benthos: aerial movements, downstream drift, upstream movements and vertical movements from deep substrates. Although all four may contribute colonists, previous studies indicate that one pathway usually dominates. In permanent streams, downstream drift is most important (Townsend and Hildrew, 1976; Williams and Hynes, 1976), while in intermittent streams, aerial movements and vertical movements from deeper substrates are the two main pathways. Harrison (1966) and Hynes (1975) found aerial sources dominant in two African streams. Organisms using this pathway were aquatic adults (Coleoptera and Hemiptera) and ovipositing adults (e.g., Ephemeroptera and Diptera). Williams (1977) found vertical movements predominant in two Canadian streams subject to drying. Organisms were present in dormant stages until water returned. Recolonization processes are integral to macroinvertebrate community dynamics in intermittent desert streams as a consequence of frequent disruption by floods. Floods occur during two distinct rainy seasons in the Sonoran Desert, winter (November to April) and summer (July to October). Winter precipitation results from large-scale frontal systems that affect large areas, producing floods and extended high flows lasting from days to weeks. Summer rains, caused by locally intense thunderstorms, affect variable portions of the watershed resulting in flash floods that typically last only a few hours (Deacon and Minckley, 1974). In both seasons, floods scour substrates and eliminate 80-100% of the benthic fauna (Gray, 1980). Recolonization studies of benthic invertebrates were conducted at Sycamore Creek, Arizona (33?45'N, 111?30'W), a lowland Sonoran Desert stream (see Fisher and Minckley, 1978, for a description of the Sycamore Creek watershed). In addition to descriptive studies of each recolonization pathway during stable-flow periods, the hypothesis that aerial pathways are used by most taxa to recolonize after flooding was tested for winter and summer floods. This hypothesis was suggested by observations on species composition and life history characteristics of Sycamore Creek macroinvertebrates (Gray, 1980). Forty of the 104 taxa present have long-lived aquatic adults capable of flight (Coleoptera and Hemiptera), and most remaining species (e.g., mayflies, caddisflies and small dipterans) reproduce throughout the year and lack dormant stages in substrates. Few taxa exhibit dormancy [e.g., Mesocapnia arizonensis (Baumann and Gaufin) and Tabanus dorsifer Walker]; thus vertical movements of these