title: Sediments and flow have mainly independent effects on multitrophic stream communities and ecosystem functions.
abstract: Stream ecosystems are affected by multiple abiotic stressors, and species responses to simultaneous stressors may differ from those predicted based on single-stressor responses. Using 12 semi-natural stream channels, we examined the individual and interactive effects of flow level (low or high flow) and addition of fine sediments (grain size <2 mm) on key ecosystem processes (leaf breakdown, algal biomass accrual) and benthic macroinvertebrate and fungal communities. Both stressors had mostly independent effects on biological responses, with sand addition being the more influential of the two. Sand addition decreased algal biomass and microbe-mediated leaf breakdown significantly, whereas invertebrate shredder-mediated breakdown only responded to flow level. Macroinvertebrate community composition responded significantly to both stressors. Fungal biomass decreased and shredder abundance increased when sand was added; thus, organisms at different trophic levels can exhibit highly variable responses to the same stressor. Terrestrial endophytic fungi were abundant in low-flow flumes where leaf mass loss was also highest, indicating that terrestrial endophytes may contribute importantly to leaf decomposition in the aquatic environment. Leaf breakdown rates depended on the identity and abundance of the dominant decomposer species, suggesting that the effects of anthropogenic activities on ecosystem processes may be driven by changes in the abundance of a few key species. The few observed interactive effects were all antagonistic (i.e., less than the sum of the individual effects); for example, increased flow stimulated algal biomass accumulation but this effect was largely cancelled by sand. While our finding that sand and stream flow did not have strong synergistic effects can be considered reassuring for management, future experiments should manipulate these and other human stressors in experiments that run for much longer periods, thus focusing on the long-term impacts of multiple simultaneously operating stressors.
authors.affiliation: Thule Institute, University of Oulu, P.O. Box 7300, FI-90014, Oulu, Finland. kaisa.mustonen@oulu.fi.|Department of Ecology, University of Oulu, P.O. Box 8000, FI-90014, Oulu, Finland. kaisa.mustonen@oulu.fi.
authors.forename: Kaisa-Riikka
authors.initials: KR
authors.lastname: Mustonen
authors.affiliation: Thule Institute, University of Oulu, P.O. Box 7300, FI-90014, Oulu, Finland.|Finnish Environment Institute (SYKE), Freshwater Centre, P.O. Box 413, FI-90014, Oulu, Finland.
authors.forename: Heikki
authors.initials: H
authors.lastname: Mykrä
authors.affiliation: Department of Ecology, University of Oulu, P.O. Box 8000, FI-90014, Oulu, Finland.
authors.forename: Pauliina
authors.initials: P
authors.lastname: Louhi
authors.affiliation: Department of Ecology, University of Oulu, P.O. Box 8000, FI-90014, Oulu, Finland.
authors.forename: Annamari
authors.initials: A
authors.lastname: Markkola
authors.affiliation: Finnish Environment Institute (SYKE), Freshwater Centre, P.O. Box 413, FI-90014, Oulu, Finland.
authors.forename: Mikko
authors.initials: M
authors.lastname: Tolkkinen
authors.affiliation: Natural Resources Institute Finland, Manamansalontie 90, FI-88300, Paltamo, Finland.
authors.forename: Ari
authors.initials: A
authors.lastname: Huusko
authors.affiliation: Department of Ecology, University of Oulu, P.O. Box 8000, FI-90014, Oulu, Finland.
authors.forename: Nico
authors.initials: N
authors.lastname: Alioravainen
authors.affiliation: Department of Ecology, University of Oulu, P.O. Box 8000, FI-90014, Oulu, Finland.
authors.forename: Sirkku
authors.initials: S
authors.lastname: Lehtinen
authors.affiliation: Department of Ecology, University of Oulu, P.O. Box 8000, FI-90014, Oulu, Finland.
authors.forename: Timo
authors.initials: T
authors.lastname: Muotka
mesh_terms: D000818:Animals; D018533:Biomass; D017753:Ecosystem; D004784:Environmental Monitoring / Q000379:methods; D005387:Finland; D005658:Fungi / Q000502:physiology; D019015:Geologic Sediments / Q000737:chemistry; D007448:Invertebrates / Q000502:physiology; D009584:Nitrogen / Q000737:chemistry; D010758:Phosphorus / Q000737:chemistry; D018515:Plant Leaves; D010944:Plants / Q000145:classification; D045483:Rivers / Q000737:chemistry; D013696:Temperature; D014872:Water Movements
chemical_list: D010758:Phosphorus; D009584:Nitrogen
keywords: aquatic fungi; benthic invertebrates; decomposition; ecosystem functions; flow variability; multiple stressors; sand; sedimentation; streams