Fishpond dams affect leaf‐litter processing and associated detritivore communities along intermittent low‐order streams

Abstract

Intermittent streams are common and widespread freshwater ecosystems globally. While dams greatly affect the ecology of permanent streams, the extent of their effects on intermittent streams remains largely unknown. The physical and chemical alterations induced by dams could have particularly strong impacts on the functioning of intermittent low‐order streams, especially due to the limitations they place on colonisation of aquatic biota from downstream permanent waters. Leaf‐litter breakdown (LLB) and associated communities (microbes and shredders) were studied to investigate the potential ecological continuum rupture caused by fishpond dams along the longitudinal gradient of intermittent streams. Three to four sites were investigated along three reference (no dam) and three impacted (with a fishpond dam) first‐order intermittent low‐order streams. LLB increased along the longitudinal gradient in all six streams regardless of the presence of dams (from 1.5 to 8.4 fold over <1.5 km distance). This underscores the great variability of low‐order stream functioning even at fine scales. Such upstream to downstream gradients need to be taken into account before investigating the effect of any perturbation using LLB as a functional indicator. Fishponds dams tended to increase LLB downstream, and altered the downstream microbial communities (increase in fungal densities) and shredder assemblages (increase in Gammaridae). The effects of fishpond dams on intermittent stream functioning appeared to be most significant when they were associated with changes in hydrology, that is, when fishponds resulted in permanent flows downstream of dams. Our findings suggest that better management of fishpond dams in order to limit hydrological modifications downstream (for instance, by preventing leaks from the dam) could reduce their impact on ecosystem functioning in intermittent streams.

Using stable isotope approach to quantify pond dam impacts on isotopic niches and assimilation of resources by invertebrates in temporary streams: a case study

“…This hypothesis is supported by our observations since, at this site, the community was dominated by Gammarus pulex, a taxon characterised by an essentially aquatic life cycle, while the other three sites were dominated by insects known to exacerbate terrestrial phases during dry periods (Tachet et al, 2010). Nevertheless, this previous study conducted on these streams and other similar ones in this area showed that Gammarus pulex was not dominant in permanent streams without fishpond, suggesting that flow discontinuity was not the only parameter that influenced the abundance of this taxon (Four et al, 2017b).…”

“…At downstream fishpond site the temperature was significantly higher of 1.7°C on average during the period under investigation i.e. January to April 2015 (Four et al, 2017b). However, all sampling sites were at good ecological status according to the French "water guide"SEQ-eau (MEED-French Water Agency, 2003).…”

“…However, even if their abundance was not specifically followed in this study, their presence is probably very unlikely due to the environmental conditions in these streams. Furthermore, dam-induced hydrological alterations (shift from temporary to permanent flow downstream from the dam) due to leaks from the dam could be directly involved in the observed macroinvertebrate community change (Four et al, 2017b). This hypothesis is supported by our observations since, at this site, the community was dominated by Gammarus pulex, a taxon characterised by an essentially aquatic life cycle, while the other three sites were dominated by insects known to exacerbate terrestrial phases during dry periods (Tachet et al, 2010).…”

“…the density of fish and/or the use of fertiliser/food) and/or the management of the dam are practices that can deeply influence the effect of the fishponds on the receiving streams (e.g. Banas et al, 2002;Gaillard et al, 2016a;Four et al, 2017b). These systems are known to provide multiple ecosystem services (Millenium Ecosystem Assessment (MEA), 2005;Aubin et al, 2014; Mathe´ & Rey-Valette, 2 2015), such as ensuring fish production and high levels of alpha diversity in and around the aquatic ecosystem (i.e.…”

L’empreinte stagnustre. Un indicateur spatial au service de la compréhension limnique – le cas de la France

“…Si de nombreux travaux universitaires (Aldomany et al, 2019, Banas et al, 2015Four et al, 2017aFour et al, et 2017bGaillard et al, 2016 ;Millot, 2015 ;Passy et al, 2012 ;Touchart & Bartout, 2020) ont permis ces dernières années de combler nombre de lacunes à l'échelle du limnosystème étang et d'introduire de nouveaux indicateurs comme le « centre thermique » (Choffel, 2019), la démarche par emboîtement d'échelles sortant du cas particulier d'un étang nécessite une approche différente et complémentaire. En effet, pour répondre aux approches emboîtées des « masses d'eau » à l'intérieur d'un bassin dit versant dans une vision fluvio-centrée, il est nécessaire de produire un équivalent limno-centré, qui ne soit pas basé sur la seule approche par « impact cumulé des retenues » (Carluer et al, 2017, Habets et al, 2018 qui est par essence même fluvio-centrée, mais appliquant une démarche hydrosystémique par « territoires de l'eau » (Bravard & Pourtier, 2003) où les dimensions longitudinales, verticales, latérales et temporelles sont étudiées à égale proportion et en ne négligeant pas l'anthroposystème (Lévêque et al, 2003) dans lequel ce territoire s'inscrit.…”