Multiple‐stressor effects on leaf litter decomposition and fungal decomposers in agricultural streams contrast between litter species

Abstract

1. Agricultural land use commonly exerts multiple stressors on the functioning of stream ecosystems, including leaf litter decomposition and the utilization of this resource in stream food webs. If stressors interact, their cumulative effects on biotic responses cannot be predicted from knowledge of individual stressor effects, posing challenges for management and restoration of ecosystems. 2. We examined the individual and interactive effects of four common agricultural stressors and the role of litter quality on leaf litter decomposition and fungal decomposers. In 128 outdoor, flow-through mesocosms, we manipulated levels of nutrients, a nitrification inhibitor (dicyandiamide), deposited fine sediment and flow velocity. Interactions among these stressors can ensue because, for instance, they jointly affect physicochemical conditions around leaf litter colonized by fungi such as concentrations of dissolved oxygen and nutrients. 3. The two litter species used, deciduous birch and evergreen mahoe, showed contrasting decomposition dynamics, and these differences influenced their response to stressors. Fungi were important for birch litter decomposition but played a minor role for mahoe. Overall, flow velocity reduction and deposited fine sediment had the strongest, mainly negative effects on fungi and litter decomposition, probably as a consequence of reductions in dissolved oxygen available to fungi. However, fine sediment substantially increased mahoe litter mass loss, pointing at fungi-independent processes being relevant for its decomposition. 4. Although interactions among stressors were uncommon, they showed effects of the same magnitude as stressor main effects. Potential mechanisms underlying interactions included reductions in dissolved oxygen and changes in microbial community composition. 5. Knowledge of the effects of multiple agricultural stressors and of litter quality on litter decomposition and litter-associated fungi is crucial for management of forested riparian corridors, which have been shown to efficiently mitigate impacts of agricultural stressors on streams. The contrasting responses of the litter species used in our study warrant consideration of species composition of the riparian vegetation.

High-throughput amplicon sequencing and stream benthic bacteria: identifying the best taxonomic level for multiple-stressor research

“…Multi-factor ANOVAs were used to quantitatively assess stressor effects on taxon richness and evenness at each taxonomic level. The ANOVA model included the four factors, their interactions and a random block factor (accounting for the two spatial blocks) (see Bruder et al 35. for details).…”

“…These mesocosms, which have been used to investigate multiple-stressor effects on stream periphyton, invertebrates and organic matter decomposition35414243, permit studying stressor effects under tightly controlled, statistically powerful yet highly realistic ecological conditions. They allow natural immigration and emigration of stream biota and experience the same environmental conditions as the river feeding them, a key strength of our study compared to laboratory experiments.…”

Functional redundancy in leaf‐litter‐associated aquatic hyphomycetes: Fine sediment alters community composition but hardly decomposer activity

“…A common feature of all of these anthropogenic activities is the modification of terrestrial ecosystems leading to soil erosion and increased flow of fine sediments to the aquatic ecosystems (Sponseller & Benfield, ; Wagenhoff, Townsend, Phillips, & Matthaei, ; Wood, Armitage, Hill, Mathers, & Millett, ). Although fine sediments are a natural and integrant part of streams structure, where biological and chemical processes take place (Boulton, Datry, Kasahara, Mutz, & Stanford, ; Frossard, Gerull, Mutz, & Gessner, ), excessive sediment inputs represent one of the greatest threats to the stream ecosystem integrity (Benoy, Sutherland, Culp, & Brua, ; Bruder, Salis, McHugh, & Matthaei, ; Wood & Armitage, ). Sediment deposition in the stream bottom—which leads to burial, reduction of habitat heterogeneity and limitation of oxygen exchange within interstitial spaces (Kaufmann, Larsen, & Faustini, ; Owens et al, )—and/or transport in suspension, which elevates turbidity (Wood & Armitage, ), have deleterious effects on primary producers (Jones, Duerdoth, Collins, Naden, & Sear, ), heterotrophic microorganisms (Danger, Cornut, Elger, & Chauvet, ), macroinvertebrates (Descloux, Datry, & Usseglio‐Polatera, ), and fishes (Kemp, Sear, Collins, Naden, & Jones, ).…”

“…Moreover, the manipulative experiments focused on the structure and activity of microbial decomposer assemblages report dissimilar results (see Bruder et al, ; Piggott et al, ; Sanpera‐Calbet, Chauvet, & Richardson, ). Although the results for fungal biomass by Bruder et al () and microbial respiration by Piggott et al () agreed with the deleterious effect promoted by sediments, the finding of no effect on litter decomposition capacity was uniform among these studies. These findings and the lack of information about sediments in movement and the role of grain size affecting litter‐associated decomposer microorganisms suggest the importance of understanding the specific effects of fine sediments (a worldwide impact) on the structure and activity of these key assemblages in stream ecosystem functioning.…”

“…Excessive concentration of sediments in streams has been showed to affect the capacity of the system to process leaf litter (Bruder et al, 2016;Lecerf & Richardson, 2010;Matthaei, Piggott, & Townsend, 2010;Sponseller & Benfield, 2001). However, the majority of the studies are focused on sediment deposition on resource and streambed, the information about the effects of sediments in the suspension being scarce.…”