Food Web Ecology of Insect Larva in a Small Stream as Measured by Stable Isotope Analysis

Abstract

The amounts of stable carbon and nitrogen isotopes in insect tissue were used to investigate the dietary habits of insect larval stages in a small stream tributary of the North River in Massachusetts. Trichoptera and Odonata larval samples were collected in three sections of a stream starting from near its source, from a middle woodland section, and from an open meadow section, which eventually drained into a tributary of the North River and subsequently the Atlantic Ocean. Tissue samples of organisms were processed to determine the δ13C and δ15N values, which represent proportions of heavier isotopes that are used to evaluate food webs. The initial carbon source for the insect larvae, as seen by the δ13C values, changed with location from tree leaf food source to stream-based sources of plant material. The data show an increase in the base δ15N as the stream meandered from spring emergence to the meadow area, which was located below a nearby septic system. Stable isotope analysis of tissue from intermediates in the food web of fish provides a base that can be subsequently used to determine fish dietary habits and may as well indicate possible pollution inputs.