Abstract
We studied how multiple-stresssors in tributaries affect function, diversity, and physical habitat of recipient downstream ecosystems. Using a mesocosm model of a stream network, we manipulated sediment and nutrients individually and in combination in tributaries of second-order channels, to test the effect of complex stressor interactions within tributaries on recipient channels. Sedimentation in second-order channels increased with the level of disturbance of the tributaries. Moreover, Ephemeroptera, Plecoptera, and Trichoptera (EPT) density and EPT richness were higher in second-order channels fed by tributaries where the stressors were applied separately, compared to those fed by tributaries where the stressors were applied simultaneously. Our observations suggest this result was due to the combination of the two stressors within the same tributary reducing EPT drift from the tributaries further than the addition of the stressors in separate tributaries. These results support the hypothesis that cumulative upstream disturbance can influence downstream recipient ecosystems in stream networks. However, contrary to our expectations, most observed effects were due to impacts on dispersal patterns of EPT taxa, rather than downstream accumulation of disturbances throughout the network. Our results underscore the importance of metacommunity frameworks to understand how tributary disturbance may influence population dynamics in downstream ecosystems.
Highlights
River systems are dendritic networks in which contributing streams merge along the fluvial system
Sediment additions increased the absolute amount of deposited organic material by 2.6 times (Table 1, Figure 2)
Our results generally supported the hypothesis that upstream disturbance can influence ecological function and structure of downstream recipient ecosystems in a stream network
Summary
River systems are dendritic networks in which contributing streams merge along the fluvial system. The connectivity imposed by the unidirectional flow and the spatial arrangement of river networks greatly influences ecological dynamics, as subsidies of energy, nutrients, and organic materials are transported across interconnected stream reaches [2,5,6]. This interconnected set of habitats are the reason why river networks are considered meta-ecosystems [6,7]. Fine sediment inputs are reported to have strong negative effects on stream communities, reducing primary production due to increased turbidity, smothering, and abrasion, filling interstitial spaces for benthic invertebrates, and slowing down organic matter processing [15,16,17,18]. Recent experimental investigations have demonstrated that sedimentation and nutrient enrichment often interact synergistically, causing cumulative effects that are more detrimental than predicted on the basis of the stressors’ individual effects [19,20,21,22,23]
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