| Stable-isotope ratios of carbon and nitrogen were used to examine food-web relationships in Catamaran Brook, New Brunswick. Efforts were made to quantify trophic fractionation between macroinvertebrates and their diet, and to assess spatial and temporal isotope variability at the base of the aquatic food chain leading to juvenile Atlantic salmon production. Following this, stable-isotope data were synthesized in an attempt to investigate the utility of a two-source isotope mixing model to predict the relative importance of allochthonous and autochthonous inputs to lotic food webs.; In detail, stable-isotope ratios were used to confirm a parasitic relationship between a midge (Nanocladius [Plecopteracoluthus ] undescribed sp., nr. branchicolus) and its stonefly host (Pteronarcys biloba). Nanocladius ( P.) sp. always had more positive delta13C and delta 15N values than P. biloba, and average fractionation factors (isotope differences between symbiont and host) were +1.2‰ and +3.5‰ for carbon and nitrogen, respectively. Nanocladius (P.) sp. were also more enriched in 15N than other chironomids, and values fell within the range of other known invertebrate predators. Epilithic algae ranged from -35‰ to -19‰ and -0.8‰ to 6.5‰ for delta13C and delta 15N respectively, and values were related to the dissolved inorganic chemistry at each site. Water velocity did not affect delta13C and delta15N values at all sites, suggesting that other factors (e.g., CO2-concentrating mechanisms) may be more important in the determination of stable-isotope ratios in lotic microalgae. The grazer, Glossosoma nigrior, showed delta13C and delta15N values that correlated well with those of algae.; Hydropsychid caddisflies showed highly variable delta13C (-34.8‰ to -25.0‰) and delta15N (0.8‰ to 8.3‰) values at 31 sites along an 18-km section between the headwater lake and the mouth of Catamaran Brook. Hydropsyche slossonae and Arctopsyche sp. were isotopically more enriched than 5 other hydropsychid species, suggesting a higher trophic position for these larvae. Cheumatopsyche aphanta was isotopically more depleted than other species, possibly implying a greater reliance on algal food sources. These results suggest that, at natural abundance levels, most stable-isotope data are qualitative estimates of diet, and that only robust datasets should be used for quantitative purposes. (Abstract shortened by UMI.)... |
