Effects of flow regime and Cyprinid predation on a headwater stream

We used descriptive and experimental approaches on Gould Creek, a first-order tributary of the Mississippi River near Lake Itasca, Minnesota to assess the influence of (1) flow regime on the colonization dynamics and abundance of invertebrates and cyprinids, and (2) cyprinid predation on invertebrates and fishes. Stream flow varied annually during the 3-yr(1984-1986) period. One dry year(1984), with few periods of elevated flow, was followed by two wet years (1985-1986), with prolonged elevated (nonscouring) discharge. The density of benthic and drifting invertebrates increased dramatically with elevated flow. Benthic riffle invertebrates in particular increased, from a maximum of 9000 individuals/[m.sup.2] in 1984 to 91000 individuals/[m.sup.2] in 1985 and 51000 individuals/[m.sup.2] in 1986. Larval Hydropsychidae and Simuliidae were the primary groups increasing in abundance during elevated flow. To assess directly the influence of flow regime on benthic insect densities, flow was manipulated in six subsections of a riffle and colonization of natural rock substrates monitored. Total insect abundance was higher under elevated (nonscouring) vs. low flow within 6-8 d; after 24 d insect abundance was three times as high under elevated flow. Insect families responded differently to elevated flow, with the largest increase resulting from a pulse of colonization by larval Hydrospsychidae in 6-8 d. Cyprinid density in Gould Creek also increased with elevated flow from 0.1-0.3 cyprinids/[m.sup.2] in 1984 to 1-2 cyprinids/[m.sup.2] in 1985 and 1986. The increased fish density was most pronounced during spawning periods in spring (May-June) and consisted primarily of older individuals. All cyprinids, except the creek chub (Semotilus atromaculatus), decreased in abundance 4-5 wk after spring colonization, regardless of flow conditions and invertebrate abundance. All cyprinids in Gould Creek selected pool habitats. Experiments in an artificial stream on Gould Creek indicated cyprinids influenced invertebrate abundance, but the effect of cyprinid predation was variable among habitats. Invertebrate abundance decreased most in structurally complex pools but exhibited little response to cyprinid predation in shallow riffle and raceway habitats. Because predation intensity varied among habitats, pool-dwelling invertebrates such as Chrinomidae and Crustacea decreased more in the presence of cyprinid predation than riffle-dwelling Hydropsychidae and Simuliidae. However, if Simuliidae occurred in pool habitats, they were strongly selected by cyprinids, resulting in a significant depression in prey size in pools. Experiments in the artificial stream indicated creek chubs preyed on adult cyprinids, but larger species (adults 70-80 mm) were less susceptible to predation than smaller species (adults 50-60 mm). However, even taxa with small adult size were preyed on at a low rate, and all cyprinids strongly selected pools, with creek chubs having minimal effect on habitat use. These results suggest that (1) the hydrologic regime has broad and pronounced effects on the colonization dynamics and abundance of invertebrates and fishes in headwater streams, and (2) cyprinid predation was weaker but variable effects on the abundance of stream organisms. Predation intensity varies (a) over short temporal scales, because of the dynamic nature of flow regime and the rapid colonizing ability but short post-spawning persistence of cyprinids, (b) over small spatial scales, because of increased abundance of cyprinids in pool vs. riffle habitats, (c) between invertebrate and vertebrate trophic levels because the creek chub is a relatively ineffective piscivore, and (d) between small and large fish because many minnows have a size refuge from creek chubs.