Irrigation ditches as novel intermittent stream networks that provide energetic subsidies to terrestrial ecosystems via aquatic insect emergence

Abstract

Non-perennial streams dominate fluvial networks, comprising over half of lotic ecosystems globally. Although little attention has been paid to these systems relative to those that are perennial, perhaps even less is known about the ecology of artificial intermittent streams. In irrigated river valleys, ditches comprise a substantial proportion of surface water networks, but little is known about their relative contributions to lotic habitat, freshwater biodiversity, and ecosystem processes. Because ditches are abundant and permeate arid floodplains, they create new opportunities for cross-ecosystem subsidies through emergence of adult aquatic insects. Here, we mapped the extent of an irrigation ditch network relative to natural surface waters in the Gallatin River Valley, Montana, USA. We also quantified the magnitude, composition, and phenology of aquatic insect emergence in ditches throughout a full irrigation season and compare emergence to nearby natural streams in the valley. We found that non-perennial streams, both natural and artificial, dominated the surface-water network, representing over 70% of total length. Irrigation ditches also constituted 37% and 23% of total length and surface area of all waterbodies in the valley, respectively. Insect emergence production from ditches averaged 32.0 mg m -2 d -1 and exceeded fluxes from nearby natural streams, which contained more classically sensitive taxa (i.e., Ephemeroptera, Trichoptera, and Plecoptera). Ditches varied in water temperature, substrate size, and flow permanence, leading to distinct emergence timing, species composition, and magnitudes of biomass flux. One of the study ditches dried early because of more 'junior' water rights, but this ditch provided the largest emergence subsidy, including a pulse of dipteran emergence at the onset of flow cessation. Annual production from intermittent ditches was ~6 g m -2 y -1 and was comparable to estimates from natural perennial streams. Although our study focused on one river valley of the western US, results suggest that these ecosystems contribute broadly to lotic habitat, heterogeneity, and cross-ecosystem subsidies via aquatic insect emergence. Given increasing demand for water and changes to flow regimes and water management driven by climate change, additional study is needed on these novel and underappreciated artificial ecosystems.