Trophic Basis of Production Among Riverine Caddisflies: Implications for Food Web Analysis

We determined the trophic basis of production and quantified the food web of caddisfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Production was dominated by three net-spinning taxa (Cheumatopsyche spp., Hydropsyche rossi, and Chimarra moselyi), comprising 97–98% of the total among at least 14 trichopteran species. Annual production (as dry mass) was among the highest estimates reported for caddisflies, ranging from 43.5 to 63.9 g/m2 of snag surface (or from 14.2 to 25.5 g/m2 of channel bottom) in two consecutive years. Although all taxa were present throughout the year, C. moselyi had much higher production in summer, H. rossi was highest from late summer to winter, and Cheumatopsyche spp. showed no pattern. Ingestion was determined from production estimates, bioenergetic efficiencies, and quantitative gut analyses. The two macrofiltering taxa (Cheumatopsyche spp. and H. rossi) were omnivorous, with 50.6 and 64.4% of their production, respectively, due to eating animals. Somewhat lower amounts of their production (40.7 and 23.5%) were due to ingestion of amorphous detritus. Production of the microfiltering species Chimarra moselyi was primarily due to eating amorphous detritus (91%). Removal of amorphous detritus from the system by the major taxa was highest in summer, and diatom removal was highest in fall, whereas animals were eaten consistently throughout the year. A quantitative food web showed that while the linkages among nine taxa were complex, the ingestion pathways were dominated by amorphous detritus (total consumption = 62.5 g·m−2·yr−1) and animal prey (22.3 g·m−2·yr−1), with >99% of food resources being consumed by the three dominant filter-feeding taxa. A connectivity food web was misleading because it implied equivalence of all food resources and consumers when great variation in strength of linkages actually existed. These comparisons provide a strong argument for considering energy flow as a measure of linkage strength when evaluating food webs, and they cast doubt on the usefulness of oversimplified connectivity webs as a basis for food web theory.