Role of Macroinvertebrates in Nitrogen Dynamics of a Desert Stream

Organismal nitrogen budgets (nitrogen ingested, egested, excreted, and utilized in production) were constructed for collector—gatherer macroinvertebrates and grazing snails of a Sonoran Desert stream. Twenty—seven percent of ingested nitrogen was utilized in insect tissue production, 9—31% was excreted as ammonia, and the remainder (42—64%) was egested. Of nitrogen utilized in production, only 26% resulted in increased standing stock during a 20—d successional period. The remainder was lost to predation and nonpredatory mortality (70%) or as emergent adult insects (4%). Snail excretion was 9—13%, and egestion was 26—39% of ingestion. Of nitrogen ingested by snails 50—68% was used in tissue production. As a percentage of nitrogen retained by the stream ecosystem, increased storage of N in insect biomass was 10%, insect emergence was 1%, and excretion recycled up to 70% of that amount back to the dissolved nitrogen compartment. Collector—gatherer macroinvertebrate influence on nitrogen dynamics, especially via recycling of excreted ammonia, increased over successional time.