A model of seston capture by net-spinning caddisflies

Six species of net-spinning caddisflies (Trichoptera) coexist in the headwater region of the Tallulah River, a rocky, high-gradient tributary of the Savannah River. These caddisflies feed on the suspended organic matter (seston) captured by their nets. We analyzed these species' nets, microhabitat preferences, and monthly abundances. These data, along with the particle size distributions and monthly abundances of seston, were incorporated into a model of seston capture by these net-spinning caddisflies. The resulting model permitted us to test the hypothesis that differences in capture net mesh sizes serve to partition, by size, the food available to coexisting net-spinning caddisflies, and thus reduce competition between them. The model predicted annual seston capture by the six species of 1300 g AFDW (ash free dry weight), over 1000 times their annual production. The seston captured per longitudinal meter of stream represents 0.005% of the total seston in transport. Resource overlap coefficients indicated that the instars of the five hydropsychid species do not partition the seston by particle size or food type in a manner that reflects competitive interactions. We hypothesize that these filter feeders are limited by the availability of high-quality food items (primarily drifting animals) rather than by the overall seston supply. Species with highly selective feeding habits must filter large volumes of water to permit them to specialize on rare items. High filtration rates require rapid current velocities, large nets, and coarse net meshes. Instars with coarse-meshed nets showed the largest proportionate declines in seston capture rates when the model was simulated using the lower current velocity and smaller seston particles sizes typical for the larger Savannah River.

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