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[1] Subsurface flow within a single riffle of a low-gradient gravel bed stream was modeled in three dimensions using MODFLOW, a finite difference groundwater flow model. Model simulations showed that exchange flows can only occur in this low-gradient, gaining stream because of a zone of alluvial sediment around the stream that has much higher permeability than the surrounding catchment (K = 10 À4 m s À1 , compared with K = 10 À6 to 10 À8 m s À1 ). The key factors controlling exchange flow within the alluvial zone were identified as the hydraulic conductivity of the alluvium, the hydraulic gradient between upstream and downstream ends of the riffle, and the flux of groundwater entering the alluvium from the sides and beneath. In the study riffle each of these factors changes with season, causing a reversal of flow paths in the alluvium and a reduction in exchange flows from about 0.2-0.5 m 3 d À1 per meter stream length in summer to about 0.008-0.04 m 3 d À1 per meter stream length during fall to spring. The model also revealed that exchange flows are up to twice as strong, but more variable, at the sides of the stream than near the center, and that vertical flow paths beneath the channel are more persistent under the range of conditions modeled than lateral flow paths into the banks.

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With the purpose of obtaining the real and imaginary parts of the complex refractive index N; = n + ik, we have made quantitative measurements of spectral transmission and reflection of sulfuric acid solutions in the visible and near infrared. On the basis of the results, we have obtained values for n throughout the entire region and values of k in the near infrared together with upper limits for k in the visible region. These optical constants can be used to interpret the results of polarization studies of solar radiation that has been scattered by the clouds of Venus. We have Kramers-Kronig phase-shift analysis to obtain values of n and k from reflection measurements in the intermediate infrared region (400-4000 cm(-1)). Our measurements were made at 300 K on sulfuric acid solutions having concentrations by weight of 95.6, 84.5, 75, 50, 38, and 25%. If the particles in the Venus clouds consist of liquid droplets of sulfuric acid at a temperature of 250 K, comparison of existing Venus data with our data suggests that the acid concentration is probably higher than 70%. Various possibilities are discussed.

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We examined the effects of excluding rare species on the comparison of species richness. A river benthic data set and a randomization resampling procedure were used to show the importance of this consideration in aquatic bioassessment in particular. The data set was manipulated by deleting species at three levels of rarity as defined by occurrence frequency: once in all 24 replicates from each of three sites, no more than twice, and no more than five times. We focused on differences in species richness because many other bioassessment metrics are dependent on species richness and species composition. Species abundance patterns at the three sites were very different, with many more rare species at the least impacted site than at the more impacted sites. As sample size increased, the diffcrcnces in species richness among the three sites markedly increased in the original data set. However, the exclusion of rare species at the same level of rarity substantially reduced species richness at the least impacted site but had little effect on the most impacted site. This result led to a serious underestimation of the differences in species richness among the sites in terms of both absolute numbers and species loss percentages. Deleting rare species can damage the sensitivity of community-based methods to detect ecological changes; rare species are critical for bioassessment.

We manipulated, in accord with global—warming predictions, the thermal regime of a permanent first—order stream near Toronto, Ontario, Canada. We examined the effects of a 2—3.5°C water—temperature increase on densities, biomass, species composition, and life histories of resident stream invertebrates. The stream was divided longitudinally at the source into two channels, one control and one experimental, and a before and after (BACI) design was employed such that one pre—manipulation year was followed by 2 yr of the temperature manipulation. Changes in the experimental channel following commencement of the manipulation included: (1) decreased total animal densities, particularly Chironomidae (Diptera); (2) earlier onset of adult insect emergence; (3) increased growth rates and precocious breeding in Hyalella azteca (Amphipoda); (4) smaller size at maturity for Nemoura trispinosa (Plecoptera) and H. azteca; and (5) altered sex ratios for Lepidostoma vernale (Trichoptera). These results partially corroborate previous laboratory and field studies. However, variation in the responses of individual target species to the manipulation was unexpected and may have been influenced by the genetic structure of local populations. We conclude that levels of gene flow among habitats may be critical to the degree of impact seen as a result of large—scale thermal perturbation (e.g., global warming).