OIKOS 36: 147-157. Copenhagen 1981 A model of seston capture by net-spinning caddisflies Theodore J. Georgian, Jr. and J. Bruce Wallace Georgian, T. J. Jr. and Wallace, J. B. 1981. A model of seston capture by net-spin- ning caddisflies. - Oikos 36: 147-157. 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 cad- disflies. 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 simu- lated using the lower current velocity and smaller seston panicles sizes typical for the larger Savannah River. T. J. Georgian, Jr. andJ. B. Wallace, Inst. of Ecology and Dept of Entomology, Univ. of Georgia, Athens, GA 30602, USA. 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Introduction '^"/c-*-,?!:^"'"5-'v7v~'^ -'SKssg The role and importance of filter-feeding organisms in the processing of organic matter in stream ecosystems has attracted increasing interest in the past several years. Most research has involved streams which receive their primary energy inputs as allochthonous paniculate detritus (Minshall 1978). These inputs pass from one functional group to another, often as suspended par- ticulate organic matter (seston). Changes in particle size and food quality occur as the detritus is processed (Boling et al. 1975, Anderson and Sedell 1979). Al- though seston customarily refers to all suspended par- ticulates (Runner 1963), we use the term here in a more restricted sense to refer to the organic fraction only. Webster (Webster 1975, Webster and Patten 1979) introduced the term "spiralling" to describe the role of benthic organisms in retaining organic matter and as- sociated nutrients that would otherwise be carried downstream and lost to that segment of the stream sys- Fig. 1. A capture net of IVth instar P. cardis. Note the small tem. The material consumed by filter feeders is either meshes in the central "seam" and the large irregular meshes assimilated (and a fraction retained as secondary pro- around the upper periphery. Scale line = 550 um. duction) or egested and is available for repeated capture and reingestion by other organisms downstream (Wal- lace et al. 1977). The degree to which filter feeders size. These differences in net structure have been inter- retard the downstream transport of fine paniculate preted as a means by which coexisting net-spinning cad- organic matter is postulated to be an important deter- disflies partition the available food resources (Wallace minant of the efficiency with which a stream ecosystem 1975, Malas and Wallace 1977, Wallace et al. 1977), processes its energy and nutrient inputs. and that any competition based on particle size would Net-spinning caddisfly larvae, members of the Hyd- involve instars as well as species. If the hypothesis of ropsychoidea, comprise a major component of the food partitioning by size is correct, competition between filter-feeding fauna of most streams. The larvae spin instars which are adjacent when ranked according to silken capture nets, with mesh opening size and overall mesh opening sizes, regardless of species, should be of a net structure differing from family to family (Wiggins level that would permit coexistence. A test of the 1977, Wallace and Merritt 1980). Hydropsychid larvae hypothesis requires that the capture of various size food spin a net with roughly rectangular meshes, oriented particles by each instar of a group of coexisting net- perpendicular to the current. The size of individual spinning species be known (cf. Pianka 1973). We have mesh openings increases from the base of the net to- analyzed the net structure, microhabitat distribution, ward the periphery (Fig. 1). In all species studied to and monthly abundances of six net-spinning caddisfly date, a new net, with larger mesh openings, is spun by species in a 4th-order stream in the southern Ap- each subsequent larval instar (Sattler 1963, Kaiser palachian Mountains, in conjunction with data on the 1965. Williams and Hynes 1973, Malas and Wallace seasonal availability of seston in the water column. 1977). Each successive net is also larger in total area. These data were combined to provide estimates of par- Mesh opening sizes, then, differ between instars within ticle capture functions for this group of filter feeders. a given hydropsychid species and also between equival- ent instars of different species. Members of the Philopotamidae construct long sac- 2. Methods like nets with an anterior opening facing into the current and extremely fine mesh openings (Wallace and Malas We studied five hydropsychid species and one 1976). Differences in net structure and microhabitat phiiopotamid species which coexist in the headwaters of result in distinct differences in the size and quality of the Tallulah River, a tributary of the Savannah River panicles captured by hydropsychid and phiiopotamid arising in the southern Appalachian Mountains. At the larvae (Williams and Hynes 1973, Malas and Wallace study site, on the Georgia-North Carolina border, the 1977). Phiiopotamid species also spin a new capture net Tallulah River is a 4th-order stream, with an elevation in each larval instar (Wallace and Malas 1976). of 835 m a.s.l., a gradient of 34 m km"1, and a drainage Differences among capture nets suggests that species, basin of 16 km2. The stream flows over a rocky sub- and instars within species, capture and feed upon differ- strate with shallow riffles predominating and small pool ent size particles, based on differences in mesh opening areas interspersed among the riffles. Width ranges from 148 OIKOS36:: (1981) 4 to 7 m, and maximum depth is about 0.6 m. Wallace et Vth-instar larvae, with respect to current velocity and al. (1977) gave a more detailed description of the site. position on rocks, were recorded using the methods of The species studied were: Arctopsyche irrorata Ross, Malas and Wallace (1977). Current velocities were Parapsyche cardis Ross, Symphitopsyche sparna (Ross), measured adjacent to capture nets with a collapsible bag 5. madeodi (Flint), Diplectrona modesta Banks (Hyd- current meter (Gessner 1950). Seston concentrations ropsychidae) and Dolophilodes distinctus (Walker) were