The seasonal change in gut contents of nymphs of Isoperla grammatica and I. di#formis from six streams in southern Sweden was analysed. Both species had ingested a variety of benthic prey and vegetable matter, predominantly diatoms. Some seasonality was evident with high percentages of diatoms in spring in I. grammatica, and in autumn in I. dzfformis. The scope of food was larger in the latter species which contained about equal proportions of vegetable matter, chironomids, mayfly, stonefly, and black fly larvae. In I. grammatica plant matter and chironomids dominated strongly, comprising > 50% of the gut contents on an annual basis, > 90% in spring. While small nymphs of I. dfformis contained a low proportion of animal matter, only gradually increasing with size, the nymphs of I. grammatica were carnivorous from very early instars. Both species switched to a temporarily strong utilization of algae in spring. This differed among sites, and appeared to reflect differences in insolation and thus the availability of algae. There was a significant negative relationship between the mean densities of Isoperla nymphs and the proportion of animal material found in the guts of I. grammatica (R2 = 0.86). Considering the density of I. grammatica alone, the relationship was weaker (R2 = 0.56). A positive correlation between predator and prey size was observed. With chironomid prey the size range increased with predator size. With simuliid prey, however, prey size increased with predator size in such a way that it suggests selection rather than just an expanding prey size range. Correlations were stronger and regression coefficients significantly higher for I. grammatica than for I. dfformis. We suggest that I. grammatica, which ingests a much wider size range of prey might choose prey of optimal sizes more readily than the more synchronously developing I. difformis. Although the life cycles of the two species are staggered, overlap in size distribution indicates that competition for food could be important in spring. However, observed differences in diet should facilitate coexistence. Gut content differences might in turn be accomplished through microhabitat segregation.

We examined the relationship between drift and foraging activity in Gammarus pulex L. by comparing collections from the benthos, drift, and small traps baited with cheese. Two sites were employed, one with both sculpins and trout, and one lacking fish. Baited traps collected large numbers of G. pulex within as little as 15 min, demonstrating the effectiveness of chemical attractants. Limnephilid larvae also were attracted rapidly, and no counteracting influence on either taxon could be detected to result from a sculpin in an adjacent cage. Trap collections indicate a highly aggregated distribution and suggest that one use of this approach is for detecting small scale spatial patterns.Individuals of small size comprised the majority of the G. pulex population. Of three size categories used ( < 4,4-8, > 8 mm total length), between 63 and 67 % of the benthic collections were < 4 mm long.Traps captured exclusively the two largest size classes of G. pulex. Drift collections consisted almost exclusively (93-1002) of < 4 mm individuals during the day, and larger G. pulex appeared only in the night drift. Based on stomach analysis, trout and sculpins selectively captured larger prey but this preference was proportional to fish size, as small sculpins captured relatively smaller prey. The rarity of larger G. pulex in the daytime drift appears to be attributable to greater risk of predation by day, but not to the absence of foraging activity in the amphipod, as baited traps and direct observation indicated that G. pulex is continuously active.