Cow manure in headwater streams: tracing aquatic insect responses to organic enrichment

Abstract

Cattle grazing in riparian areas can affect the structure of aquatic insect assemblages by adding nutrients (manure) to the stream, or by physically altering the habitat through trampling or foraging. Although cattle grazing is a well-described source of disturbance in stream habitats, the effects of manure inputs have not been previously isolated from effects of the physical disturbance. We traced the responses of aquatic insects representing 5 different functional-feeding groups to this introduced food source. We simulated manure enrichment from light grazing intensity and hypothesized that insects that feed directly on imported organic matter (shredders, filterers, gatherers) would have higher assimilation rates and densities than predators or periphyton-grazers in response to manure enrichment. We expected insect responses to increase with incremental enrichment over time, and decrease with increasing distance from the manure input. We introduced manure (composed of undigested C4 plant particles) from corn-fed cows into 7 streams that drain forests of C3 plants in Coastal and Sierra Nevada regions in California. Stable C isotope ratios indicated all feeding groups assimilated the isotopically distinct manure. In the Coastal streams, groups assimilating the most manure were gatherers (net increase of 21% towards C4 plant signal) and filterers (20%), whereas shredders (9%) assimilated the least. In the faster-flowing Sierran streams, assimilation by each group was ≤9%. Temporal increases in manure uptake were detected in the Coastal mayfly gatherer Paraleptophlebia pallipes, suggesting increased manure assimilation over time. Manure uptake by insects was spatially localized within 3 m downstream of the site of manure input. Densities of all 6 genera representing 5 functional-feeding groups were not significantly altered in response to manure enrichment. However, chironomids increased >5-fold in densities after 4 wk of enrichment. The composition of chironomid genera did not shift, and insect taxa richness in the enriched treatments did not change in response to manure enrichment. In our simulation of enrichment effects, which approximated low-density grazing of 6 cows visiting each stream reach weekly for 8 wk, we found that in the absence of physical disturbances from cattle grazing, manure is an important food source for gatherers in particular, and elicited responses from chironomids that are characteristic of organic enrichment.