Striking differences in pool—to—pool distributions of an algae—grazing minnow (Campostoma anomalum), attached algae (predominantly Spirogyra sp. and Rhizoclonium sp.), and bass (Micropterus salmoides and M. punctulatus) are known to occur in some small Oklahoma streams. This study evaluates the complementarity of bass, Campostoma, and algae at different seasons, and uses in—stream experimental manipulations of bass and Campostoma to determine if the patterns resulted from strong interactions between predators, herbivores, and algae. In a 1—km reach of Brier Creek (south—central Oklahoma), bass and Campostoma distributions in 14 consecutive pools were inversely related in six of seven censuses conducted from 8 November 1982 to 5 September 1983. Bass and Campostoma co—ocurred in more than two pools only on two occasions, following the largest floods of the year. Campostoma and algal abundances were inversely related during late summer and in both autumns of this study. This relationship did not hold during the spring, when floods strongly affected algal distributions. During autumn of 1983, we removed bass from a pool, fenced it longtidinally, and added Campostomata to one side (1.4 individuals/m2). Over the next 5 wk, standing crop of algae decreased significantly on the Campostoma side but increased on the control side. In a nearby unmanipulated Campostoma pool, standing crop of algae was consistently low. We added three free—swimming bass to a Compostoma pool to evaluate presumptive predator—prey interactions. Within 3 h, the Campostoma moved from the deepest part of the pool to shallow areas. Over the next 5 wk, numbers of grazing Campostoma declined due to behavioral changes, emigration, and (presumably) predation. The standing crop of algae increased significantly 10—13 d after bass addition. In a second bass—addition experiment in June 1984, Campostoma responses were almost identical, and algal standing crop in deeper areas increased significantly after 1 wk. Collectively, our censuses and the experiments indicate that in Brier Creek, biotic interactions strongly influence the pool—to—pool distributions of Campostoma and algae, particularly during long periods of constant low discharge.

Surveys of 262 pools in 3 small streams in eastern Tennessee demonstrated a strong positive relationship between pool depth and the size of the largest fish within a pool (P<0.001). Similarly, the largest colonizers of newly-created deep pools were larger than the colonizers of shallow pools. We explored the role of predation risk in contributing to the "bigger fish - deeper habitat" pattern, which has been noted by others, by conducting five manipulative field experiments in two streams. Three experiments used stoneroller minnows (Campostoma anomalum); one used creek chubs (Semotilus atromaculatus); and one used striped shiners (Notropis chrysocephalus). The stoneroller experiments showed that survival of fish approximately 100 mm in total length (TL) was much lower in shallow pools (10 cm deep) than in deep (40 cm maximum) pools (19% versus 80% survival over 12 d in one experiment) and added cover markedly increased stoneroller survival in shallow pools (from 49% to 96% in an 11-d experiment). The creek chub experiment showed that, as for stonerollers, pool depth markedly influenced survival: the chubs survived an average of 4.9 d in shallow pools and >10.8 d in deep pools. In the striped shiner experiment in shallow artificial streamside troughs, no individuals 75-100 mm TL survived as long as 13 d, where-as smaller (20-25 mm) fish had 100% survival over 13 d. The results of the experiments show that predation risk from wading/diving animals (e.g., herons and raccoons) is much higher for larger fishes in shallow water than for these fishes in deeper water or for smaller fish in shallow water. We discuss the role of predation risk from two sources (piscivorous fish, which are more effective in deeper habitats, and diving/wading predators, which are more effective in shallow habitats) in contributing to the bigger fish - deeper habitat pattern in streams.

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Stony substrata in streams of the southwestern Ozark Mountains are covered with cyanobacterial felts dominated by Calothrix sp., an active nitrogen fixer. To study the effect of grazers on these producer assemblages, we exposed or protected periphyton from gazers for periods ranging from 4 to 33 d. When protected from grazing fishes and invertebrates, cyanobacterial felts are overgrown by turfs of benthic diatoms within 4—10 d. Diatom turfs also develop on bare substrata that are incubated in sites in the stream inaccessible to grazers. Diatom turfs exposed to grazing minnows are stripped off in 3—5 min, and when left exposed to grazers, are replaced by cyanobactrial felts within ll d. Basal regeneration of Calothrix trichomes may contribute to their persistence under intense grazing.

Diel migrations of the golden shiner (Notemigonus crysoleucas) from the littoral to limnetic zone of a small Michigan lake were documented through visual observations and gill netting. During the day golden shiners schooled in the littoral zone. Just after sunset schools broke up and the golden shiner migrated to the open water regions of the lake. Feeding by the golden shiner was largely coincident with the evening migration. The majority of the shiner population fed on Daphnia and most prey were obtained within 1 h of sunset. Feeding was minimal through the night with some feeding evident at dawn. Daphnia underwent a strong diel vertical migration, especially larger individuals, and showed a strong horizontal gradient of decreasing size and density from the limnetic into the littoral region. Size of Daphnia eaten by the shiner increased dramatically across the evening feeding period. This increase is due to the horizontal gradient in Daphnia and availability of larger Daphnia as they migrate to the surface. On only one date when prey densities were highest did the shiners actively select large Daphnia. The golden shiner faces a dynamic prey system both in time and space which influences prey "selection." We relate adaptations of planktivorous fishes that migrate to relative abundances of fishes in small lakes. Key words: coevolution, Daphnia, diel migration, fish, foraging behavior, golden shiner, Notemigonus, planktivore, predator–prey