Pulsed flows, tributary inputs and food‐web structure in a highly regulated river

Abstract

Dams disrupt the river continuum, altering hydrology, biodiversity and energy flow. Although research indicates that tributary inputs have the potential to dilute these effects, knowledge at the food‐web level is still scarce. Here, we examined the riverine food‐web structure of the Colorado River below Glen Canyon Dam, focusing on organic matter sources, trophic diversity and food chain length. We asked how these components respond to pulsed flows from tributaries following monsoon thunderstorms that seasonally increase streamflow in the American Southwest. Tributaries increased the relative importance of terrestrial organic matter, particularly during the wet season below junctures of key tributaries. This contrasted with the algal‐based food‐web present immediately below Glen Canyon Dam. Tributary inputs during the monsoon also increased trophic diversity and food chain length: food chain length peaked below the confluence with the largest tributary (by discharge) in Grand Canyon, increasing by >1 trophic level over a 4–5 km reach possibly due to aquatic prey being flushed into the mainstem during heavy rain events. Our results illustrate that large tributaries can create seasonal discontinuities, influencing riverine food‐web structure in terms of allochthony, food‐web diversity and food chain length. Synthesis and applications. Pulsed flows from unregulated tributaries following seasonal monsoon rains increase the importance of terrestrially derived organic matter in large, regulated river food webs, increasing food chain length and trophic diversity downstream of tributary inputs. Protecting unregulated tributaries within hydropower cascades may be important if we are to mitigate food‐web structure alteration due to flow regulation by large dams. This is critical in the light of global hydropower development, especially in megadiverse, developing countries where dam placement (including completed and planned structures) is in tributaries.

The trophic ecology of a desert river fish assemblage: influence of season and hydrologic variability

“…The large percentage of allochthonous detritus we observed in the diets of native fishes throughout the year was most apparent after periods of extended low to moderate flooding (i.e., September and March). Recent work utilizing stable isotope techniques has indicated terrestrial resources can be important contributions to food webs in some arid streams and rivers and that this integration may be mediated by seasonal and hydrologic variability (Zeug and Winemiller 2008, Blanchette et al 2014, Sabo et al 2018. Our results support this generalization, indicating seasonally and/or hydrologically mediated linkages exist between fishes native to desert rivers and terrestrial production, a trend common to both plants and animals in arid ecosystems (Brown and Ernest 2002, Chesson et al 2004, Schwinning and Sala 2004.…”

“…, Sabo et al. ). Our results support this generalization, indicating seasonally and/or hydrologically mediated linkages exist between fishes native to desert rivers and terrestrial production, a trend common to both plants and animals in arid ecosystems (Brown and Ernest , Chesson et al.…”

Dissolved organic matter variations in coastal plain wetland watersheds: The integrated role of hydrological connectivity, land use, and seasonality

“…Beyond altering the bioavailability of DOM in downstream waters, the ecosystem‐level influences of seasonal pulses of water from forested Delmarva wetlands connected to stream networks are largely unknown. The regular seasonality of DOM quantity and composition in Delmarva wetlands likely broadly changes the base of stream food webs and decreases light availability, decreasing aquatic photosynthesis and making food webs more dependent on terrestrial sources of organic matter (Hensley & Cohen, ; Sabo et al, ). Ultimately, such pulses of DOM from headwaters have far reaching implications for river networks in particular and the carbon cycle in general (Raymond, Saiers, & Sobczak, ).…”

Detrimental effects of a novel flow regime on the functional trajectory of an aquatic invertebrate metacommunity

“…Here, we propose it may also play a role in human‐altered riverscapes, for example when large dams affect long river sections (like in this study), or when cascades of smaller dams operate simultaneously. Taxa drifting and flying from tributaries may be able to rescue declining functional guilds in regulated main stems (Sabo et al., ). However, when no nearby propagule sources exist (e.g., in headwater dams, or in highly dammed river basins) the Moran effect may increase the effective risk of functional collapse.…”