Food-web relationships in Catamaran Brook, New Brunswick, as revealed by stable-isotope analysis of carbon and nitrogen

Abstract

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 613C and 615N values than P. biloba, and average fractionation factors (isotope differences between symbiont and host) were +l.2%o and +3.5%o 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 invenebrate predators. These findings highlight the usefulness of stable-isotope technology to distinguish between phoresy and parasitism in ectosymbiocic relationships. The results also demonstrate that at least 1 of the many tenets of isotope ecology <1 3C and 15N enrichment between animal and diet) holds true in field situations.

Epilithic algae ranged from -35%o to -19%o and -0.8%o to 6.5%o for o13C and 615N respectively, and values were related to the dissolved inorganic chemistry at each site. Water velocity did not affect o13C and o15N values at all sites, suggesting that other factors (e.g., CO2-concentrating mechanisms) may be more imponant in the determination of stableisotope ratios in lotic microalgae. The grazer, Glossosoma nigrior, showed o13C and 615N values that correlated well with those of algae. Isotope differences among grazer species appear to be related to feeding mode and microhabitat preferences. Grazer 613C may be a better indicator of autochthonous carbon than algal o13C because of terrestrial and heterotrophic contamination in the biofilm matrix, but care should be given to the choice of isotope surrogates of primary food sources because of feeding selectivity among primary consumers.

Hydropsychid caddisflies showed highly variable o13C (-34.8%o to -25.0%o) and 615N (0.8%o to 8.3%o) values at 31 sites along an 18-km section between the headwater lake and the mouth of Catamaran Brook. Isotope differences were related to species specific and seasonal changes in diet, as well as chemical influences from local tributaries. 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. Results showed that stable-isotope ratios provide a valuable means of obtaining dietary information for co-existing species. The data also illustrate that stable isotope ratios are site-specific and are not comparable over the entire stream ecosystem.

The ability of a quantitative two-source isotope mixing model to predict the relative imponance of allochthonous and autochthonous inputs to an aquatic food web was assessed using a statistical resampling technique. When differences between food sources were small (e.g., 2-3%o), the model performed poorly, assigning 95% confidence limits in the range of ± 20% to 70%. Performance was enhanced when the food-source differential was increased, but predictive power was invariably dependent on sample size. 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. Power curves are presented to aid ecologists in future attempts to design quantitative stable-isotope studies.