Resource Limitation in a Stream Community: Phosphorus Enrichment Effects on Periphyton and Grazers

A field experiment was used to test whether stream periphyton was nutrient limited, and if so, whether the grazing insects that consumed this resource were food limited. Eight replicate wooden flumes were placed in an unshaded riffle in Augusta Creek, a third—order, hardwater trout stream in southwestern Michigan (USA). Phosphorus, in the form of Ca(H2PO4)2, was added to four of these flumes for 105 d. The average concentration of soluble reactive phosphorus (SRP) maintained in P—enriched flumes (85 mg/L) was >5 times as great as in control flumes (15 mg/L). After 77 d of nutrient enrichment, periphyton biomass per unit area on substrates with reduced grazer densities was 1.8 x as great in P—enriched flumes as in control flumes. The larvae of two species of grazing caddisflies (Leucotrichia pictipes and Psychomyia flavida) in P—enriched flumes also had higher individuals mass, developmental rates, and population densities. These responses resulted in standing crops of final—instar larvae in P—enriched flumes that were 2.1x and 1.5x as great as those in control flumes for Psychomyia and Leucotrichia, respectively. This demonstration of food limitation supplements previous behavioral studies showing that both grazer species aggressively defend feeding territories, and contributes to a growing body of evidence suggesting that resource limitation is more important in some stream food webs than previously recognized. Resource limitation of these grazer populations may be due in part to the rarity of catastrophic flooding and drying in Augusta Creek. Such stable flows could permit consumer populations to reach higher densities than in frequently disturbed streams.