US EPA

2487574 

Journal Article 

Examining primary producer-consumer interactions in a Lake Superior tributary using N-15-tracer, grazer-reduction, and nutrient-bioassay experiments 

Rezanka, KM; Hershey, AE 

2003 

Yes 

Journal of the North American Benthological Society
ISSN: 0887-3593
EISSN: 1937-237X 

22 

3 

371-387 

10.2307/1468268 

WOS:000185212400004 

Primary producer-consumer interactions and N dynamics were studied in a 3(rd)-order reach of Amity Creek, a Lake Superior tributary in northeastern Minnesota (mean discharge = 0.08 m(3)/s). A nutrient-limitation-bioassay experiment showed that primary production was co-limited by N and P To evaluate the importance of grazers in controlling epilithic algal biomass, a grazer-reduction experiment was conducted using ceramic tile epilithon substrates (ambient grazing and low grazing treatments). Grazers significantly reduced epilithic biomass as measured by chlorophyll a, demonstrating that grazers constrain algal primary producers in this system. N-15-enriched NH4Cl was added to the stream for 6 wk, as a stable isotope tracer of N flow through producer and grazer compartments. A comparison of delta(15)N of tile epilithon, incubated for 4 wk during the N-15 addition, to that of bulk epilithon collected from rock scrapings, showed that the thinner epilithon associated with tiles was much more enriched in N-15 than the bulk material. We propose a model of layered epilithon, where the outer, presumably newer, material is more active in N cycling than the inner, presumably older material. This more active material was used more by grazers. The caddisfly Glossosoma and mayflies Paraleptophlebia and Ephemerella were notably enriched in N-15 relative to bulk epilithon, similar to tile epilithon, suggesting that they selectively grazed and/or selectively assimilated the subcomponent of the epilithon that was most active in N cycling in this stream. The mayfly Stenonema was isotopically more similar to bulk epilithon than the other primary consumers. Traditional functional-feeding group classifications of these consumers did not accurately describe their food resource assimilation, as illustrated by their N-15 signatures. Grazers may be even more important in N cycling and retention than previously believed because they assimilate much more N than would be expected based on their apparent diets. 

epilithon; grazer; nitrogen; (IN)-I-15; stable isotope tracer; mayfly; caddisfly; functional-feeding groups; nutrient limitation; Lake Superior tributary