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Detecting the magnitude of human-induced disturbance events, such as forest harvest, on biological communities is often confounded by other environmental gradients and scales at which these effects are examined. In this study, benthic invertebrates were collected from 43 streams across four basins and two geographic regions to (1) determine whether invertebrate abundance and community structure are best explained by historic forest harvest, landscape variables or a combination of both, and (2) evaluate associations among harvest, landscape variables, in-stream physical habitat, and invertebrates. Nonmetric multidimensional scaling showed that invertebrate community structure was primarily explained by watershed area and elevation, and basin and region but not by measures of forest harvest. Model selection using an information-theoretic approach and Akaike's information criterion indicated that watershed area was the most important variable explaining clinger and long-lived taxa richness, while basin was the most important variable explaining total abundance, and total, Ephemeroptera, Plecoptera, and Trichoptera taxa richness. Forest harvest ranked lower than landscape variables in relative importance in all models. These results suggest that landscape characteristics were relatively more important in predicting invertebrate community structure than forest harvest, and should therefore be considered when assessing the impacts of both reach and watershed scale forest harvest on benthic communities. Perhaps, the levels of forest harvest examined in this study had only marginal effects on benthic invertebrates because these ecosystems are naturally resilient as a result of frequent disturbance from forest fires. [ABSTRACT FROM AUTHOR]

Abstract: Riparian vegetation strongly influences the structure and function of terrestrial and aquatic ecosystems; therefore, harvest of riparian trees can profoundly impact these habitats. Riparian reserves (no harvest zones) are used world-wide to reduce these impacts, including effects on organic matter dynamics. There are, however, few experimental evaluations of their efficacy. Particulate organic matter delivery from riparian vegetation to streams, a critical ecosystem process, was measured over an eight year period after logging in southwestern British Columbia, Canada to quantify the relationship between these litter fluxes and riparian reserves (unlogged controls, 30 and 10-m wide uncut reserves, and clear-cuts). We hypothesized that the timing, composition and quantity of organic matter entering these streams would vary as a function of reserve width, but that these differences would decline with time since logging. We also hypothesized that inter-annual variation in precipitation patterns would partially contribute to litter dynamics. Riparian treatment significantly affected the quantity and composition of litter flux: inputs of needles and twigs were significantly lower and proportion of deciduous litter higher in clear-cut streams compared to other treatments. ANOVA models indicated that treatment differences in litter flux persisted through year 7 for some components of litter, while regression models indicated positive trends between reserve width and litter flux through year 8. This positive slope indicates the clear-cut treatment had yet to recover from logging. Variation in snowfall and plant succession contributed to temporal variation in litter flux; in some cases, litter inputs increased with annual snowfall. Another key finding from this study was that riparian reserves provided total litter inputs to streams that were similar to fully forested controls. [ABSTRACT FROM AUTHOR]

Light, nutrients and predators can constrain primary producers and consumers; however, the ecological effects of these factors have rarely been tested simultaneously in open systems. To partly address this knowledge gap, primary producer biomass and consumer abundance and biomass were quantified on nutrient-diffusing substrata along a light gradient created by four riparian reserve treatments (forested controls, 30-m wide riparian reserve, 10-m reserve, clear-cut) each replicated twice in headwater streams of southwestern British Columbia in 2001 and 2002. Predation pressure, as estimated by trout abundance varied among streams, and was treated as a covariate, with nutrient enrichment and riparian reserve width as fixed effects. Primary producer and consumer biomass were limited by light flux as determined by reserve width, and nutrients, especially phosphorus. In 2002, nutrient effects on algal biomass were conditional upon reserve width: chlorophyll a biomass was higher on P- and N+P-amended pots compared to controls, but only in the 10-m reserve and clear-cut treatments or when light flux was greater than 75 μmol m−2 s−1. The effects of fish predators on primary consumers were also conditional upon reserve width, with strong predator effects occurring in the 10-m and clear-cut riparian treatments. The interactive effects of light flux, nutrient supply and predator abundance in constraining lower trophic levels may be common in nature. My results, therefore, suggest interactive effects were manisfested above a specific threshold condition. The interactive effects of light flux, nutrient supply and predator abundance in constraining lower trophic levels may be common in nature. Results from my study support this hypothesis and suggest that variation in light flux was an important condition dictating the strength of nutrient limitation and redation pressure in these headwater streams. [ABSTRACT FROM AUTHOR]

In headwater coastal streams of southwestern British Columbia, previous research suggested that light limited periphyton growth and abundance of grazing invertebrates. Logging along a reach of stream allowed us to further examine the importance of light, as well as other abiotic factors, in regulating stream periphyton and grazers. We placed unglazed ceramic tiles in three watershed, two of which served as controls. In the third watershed, we placed tiles in one reach that was newly harvested, as well as in an upstream, forested reach. Tiles were placed in streams in late June and removed weekly over a six-week period for determination of periphyton chlorophyll a and ash-free dry mass. We also measured discharge, dissolved nitrate and phosphate, and counted the number of invertebrate grazers on each removed tile weekly. Peak biomass, as chlorophyll a, was reached on day 29 with alga biomass at the logged site, seven to fourteen times higher than at the control sites. Stepwise, multiple linear regression suggested that light was the single best predictor explaining 64% of the variation in peak biomass of chlorophyll a. Although periphyton biomass on tileswas much higher in the clear-cut reach, so was fine sediment. Inorganic mass entrapped in the periphyton mat was two to four times higherin the clear-cut stream than at other sites. Grazer abundance was not related to periphyton biomass, but was negatively related to sediment levels. Our results are consistent with the hypothesis that grazerabundance was determined by sediment levels rather than alga biomass. [ABSTRACT FROM AUTHOR]

Metal contamination is one of the most widespread impacts on surfacewaters. Experimental flumes receiving water and aquatic invertebrates from an undisturbed, forested stream were used to determine the impact of metals in a low-conductivity stream. The experimental flumes were exposed to a gradient of doses maintaining a constant ratio of metals (1995: Cu, Zn, Mn, and Pb; 1996: Cu and Zn) for 6 d. Benthos andemigration were sampled from each of the 16 troughs. The overall densities of benthos declined, but not significantly, as the dose of metals increased. On the basis of the slopes of the concentration-response curve, Baetis, Ameletus, and Paraleptophlebia were the most sensitive taxa present. Other taxa (e.g., Nemouridae and Oligochaeta) were mildly affected by high metal concentrations. Chironomidae showed no significant decrease in densities with increasing dose. Chironomids made up >80% of the benthos and is the primary reason for no significant dose effect on overall densities. No treatment effect was observedon either algal standing crop or bacterial respiration rates. The invertebrate genera most affected by exposure to metals in this study were also absent or rare in nearby urban streams with high metal concentrations. [ABSTRACT FROM AUTHOR]