Abstract
We collected aquatic insects monthly (December 2010 to March 2011) from thalweg and edge habitats with kick-nets (250-mm mesh). We also collected insects directly from an elk carcass in May 2011. Individuals were preserved in Kahle s solution (Stehr 1987) and returned to the laboratory for gut-content analysis. Prior to gut removal, individuals were identified to genus or species, except for Chironomidae, which were identified as non-Tanypodinae (Merritt and Cummins 1996).
Most insects were identified to genus and correspond to the Standard Taxonomic Effort Level (Adams et al. 2004). We examined diets of taxa representing 4 functional feeding groups (Cummins and Klug 1979): scrapers [Rithrogena (Ephemeroptera: Heptageniidae) 4 - 11 mm] and [Bibiocephala (Diptera: Blephariceridae) 7 13 mm], collector-gatherers [Baetis (Ephemeroptera: Baetidae) 3 6 mm] and [Non-Tanypodinae (Diptera: Chironomidae) 8 11 mm], collector-filterers [Simulium (Diptera: Simulidae) 4 7 mm] and [Arctopsyche grandis (Trichoptera: Hydropsychidae) 10 24 mm], and a shredder [Pteronarcys (Plecoptera: Pteronarcyidae) 8 48 mm]. These taxa are important members of the assemblage of insects in Big Creek, representing, for instance, 53% of total benthic abundance, and 25% of the total benthic biomass of aquatic invertebrates collected during summer monitoring (Baxter, unpublished).
Diets were evaluated with standard techniques for gut-content analysis (Benke and Wallace 1980, Rosi-Marshall and Wallace 2002). Gut contents were removed from the insect body, suspended in distilled water, sonicated 60 s, and filtered onto membrane filters (0.45 mm). Filters with gut-contents were dried in an oven at 60 C, mounted on slides with immersion oil, and sealed with clear, nail polish. The number of individuals per slide varied from 1 - 4, based on the size and gut fullness of insects. Three to 7 slides per month were analyzed for each taxon.
We measured the fractional area of food particles to calculate the proportion of each particle type in the diets of aquatic insects. Food particles were identified as amorphous detritus, diatom frustule, diatom stalk, filamentous algae, fungi, leaf material, insect material, and wood with a compound microscope at 200 - 400 (Leica DMRB, Buffalo Grove, IL, USA) mounted with a digital camera (SPOT CCD, Sterling Heights, Michigan, USA). We identified diatom frustules and diatom stalks separately because stalks were large particles that were easily distinguished from the frustules and because they differ in nutritional quality (i.e., diatoms composed of lipids and fatty acids, stalks composed of muco-polysaccharides). We measured the area of 100 particles on each slide along transects (1 5) using ImageJ (National Institutes of Health, Bethesda, Maryland, USA; http://imagej.nih.gov/ij/).