Stoichiometric relationship between suspension-feeding caddisfly (Trichoptera: Brachycentridae) and seston

Organisms must acquire adequate amounts of carbon (C) and nutrients [i.e., nitrogen (N) and phosphorus (P)] from their food to support growth. The growth of organisms can be constrained by consumer-resource elemental imbalances in C:nutrient ratios, especially in aquatic ecosystems. Furthermore, the elemental composition of aquatic organisms can change through ontogeny, which can impose additional challenges to growth (the growth rate hypothesis), terminal body size, and reproductive output. In streams, growth in larval aquatic insects is influenced primarily by food quality and quantity, temperature, and population density. We conducted a field study that tracked the growth of a common suspension-feeding caddisfly (Brachycentrus occidentalis) through its ontogeny by comparing the elemental composition (C:N:P) of the organism with its available food supply (suspended particulate organic matter or seston). Larvae and seston were sampled from four streams throughout 1 year. Differences in the growth of larvae among the streams were evident, even though the streams possessed similar thermal regimes. Spatial and temporal differences in the nutrient contents of B. occidentalis and seston were observed, suggesting a consumer-resource elemental imbalance. Lower C:P and N:P ratios in food were positively correlated to larval growth rate, suggesting growth was limited by P. The C, N, and P contents in B. occidentalis’ body tissue did change throughout ontogeny. C:nutrient ratios varied across sites during larval development; however, inter-site variation decreased substantially as the populations approached pupation. Ultimately, consumer-resource elemental imbalances during the larval stage did not lead to differences in pre-emergent standing stocks across sites.