Stream Periphyton and Insect Herbivores: An Experimental Study of Grazing by a Caddisfly Population
1983
Lamberti, Gary A. | Resh, Vincent H.
The effects of grazing by the herbivorous caddisfly Helicopsyche borealis on benthic algae and bacteria were experimentally studied in a northern California stream, Big Sulphur Creek. By elevating artificial substrate tiles above the stream bottom, larvae of Helicopsyche, but not other herbivorous insects, were effectively excluded. In three replicate experiments, grazing by Helicopsyche resulted in low amounts of algae (as chlorophyll a, 0.3—1.4 mg/cm²) and bacteria (0.3—0.7 ° 10⁸ cells/cm²) but a high algal turnover rate (O² evolved per unit chlorophyll a = 34 mg°mg— ¹ h— ¹). When larvae were excluded, higher amounts of algae (as chlorophyll a, 8.1—15.1 mg/cm²) and bacteria (1.9—4.8 ° 10⁸ cells/cm²) were present, but the algal turnover rate (O₂ evolved per unit chlorophyll a ° 7 μ°μ—1 h—1) was lower. In a second set of field experiments, the effects of periphyton on the density and spatial distribution of grazers were assessed by varying the standing crop of periphyton. Nearest—neighbor analysis indicated that Helicopsyche larvae were aggregated on periphyton with higher standing crop than that which was present as background levels on surrounding substrate. Such larvae became randomly distributed after their grazing reduced the experimentally enhanced periphyton to the background levels. Helicopsyche's grazing resulted in a diatom—dominated algal community with a low standing crop that, because of a high turnover rate, was able to support a high biomass of consumers. This high rate of production probably occurred because competition for light and nutrients was less severe in the grazed than in the ungrazed periphyton.
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