Growth and Stoichiometry of a Stream Insect Detritivore Subject to Dietary Nutrient Enrichment

Consumers may respond to dietary nutrient enrichment by increasing their growth rates and releasing excess nutrients in dissolved and particulate forms. However, dietary influences on growth and waste production are still poorly understood among benthic detritivores. In a five-week laboratory experiment, we fed the detritivore caddisfly Pycnopsyche spp. oak or maple leaves conditioned under four levels of phosphorus (P) to produce a diet quality gradient (leaf carbon:phosphorus (C:P) = 4050 to 1050). Oak-fed insects exhibited strong diet-dependence of growth rate, with growth highest on a diet C:P of 1540 but lower on a diet C:P of 1050 (P<0.05). Maple-fed insects’ growth rates did not vary significantly with diet stoichiometry. Net P uptake, likely by the caddisflies’ cases, outpaced excretion on the lowest P diets, but caddisflies became net sources of dissolved P when fed higher nutrient diets (P < 0.05). Nitrogen excretion rates were net positive but not diet-dependent (P>0.05). Caddisfly feces respiration rates varied three-fold across treatments and were negatively related to caddisfly growth. Our findings illustrate that consumers may exhibit threshold responses of production to basal nutrient enrichment, as well as how consumer feedbacks on both nutrient and carbon cycling in detritus-based streams can depend on resource stoichiometry.