Bioenergetics of Lotic Filter‐Feeding Insects Simulium SPP. (Diptera) and Hydropsyche Occidentalis (Trichoptera) and their Function in Controlling Organic Transport in Streams

This study derives a set of bioenergetic parameters for the blackfly Simulium spp. (Diptera: Simuliidae) and the caddisfly Hydropsyche occidentalis (Trichoptera: Hydropsychidae) and evaluates the impact of these filter feeders on the lotic environment. In addition, comparison is made between field— and laboratory—derived estimates of ingestion and between 2 methods of calculating assimilation rate. Digestion (gut—clearance) time for Simulium ranged between 0.33 and 1.25 h compared to 1.54 to 2.63 h for Hydropsyche. Mean ingestion rate for Simulium in a series of experiments ranged from 1.36 to 54.74 mg DW/mg DW/h. The mean assimilation efficiency for diatoms was about 55% as measured by the dual—label technique. Mean ingestion rates of diatoms for Hydropsyche in 3 experiments were 31.3, 31.0 and 29.2 mg DW/mg DW/h, respectively; low values (3 mg DW/mg DW/h) from a 4th experiment were associated with high concentrations of green algae. An assimilation rate for diatoms of 7.5 mg AFDW/mg DW/h was derived for Hydropsyche. Measurements of growth rates of Hydropsyche under natural conditions were 3.6 to 4.4 mg DW/mg DW/h. Respiration rate of Hydropsyche for water >62% O₂ saturation was about 2.3 μl O₂/mg DW/h at 20°C. The results of this research in combination and literature derived values, yielded the following mean energy budgets for Simulium and Hydropsyche, respectively (J/mg/h): ingestion (C) 0.318, 0.393; respiration (R) 0.092; 0.042; growth (G) 0.092, 0.100; assimilation (A) (as C x AE) 0.182, 0.192; (as G / R) 0.184, 0.142 (values times 0.239 = cal/mg/h). The role of these filter feeders is discussed in terms of their processing of suspended organic material and in directing energy to higher trophic levels. Simulium and Hydropsyche (with net growth efficiencies of 49.5 and 69.6%, respectively) are shown to be comparable to bacteria in terms of metabolic removal of organic C from food sources. During the months in which they were present, the mean biomass and density of Hydropsyche occidentalis in Deep Creek (station 2), Idaho, were 2.45 g DW/m² and 1786/m² and of Simulium were 0.22 g DW/m² and 1699/m². Based on all 12 monthly samples, Hydropsyche comprised 44% of the total invertebrate standing crop biomass of 4.84 g DW/m². From mean biomasses and numbers of invertebrates in riffles of Deep Creek and mean ingestion rates for Simulium and Hydropsyche (30.5 and 80 mg/mg/h, respectively) it was calculated that the filter—feeders remove °1% of the seston flowing over them each day. The filter—feeders also are an important energy based for feeding by carnivorous insects and fish in Deep Creek.