Shading and sediment structure effects on stream metabolism resistance and resilience to infrequent droughts
Graphical abstract
Introduction
Intermittent streams and rivers, watercourses that cease to flow at some points in time and space, represent 69% of global first-order streams below 60° latitude (Raymond et al., 2013). Most predictions agree that the combined effects of irregular climate patterns (Botter et al., 2013, Reynolds et al., 2015) and increasing water abstraction (Barceló and Sabater, 2010, Larned et al., 2010) will cause a shift in many temperate low-order streams from permanent to intermittent flow regimes, with infrequent and brief zero-flow periods and stream bed desiccation (Datry et al., 2017, Krysanova et al., 2010). Hence, unpredictable desiccation will challenge microbial communities and ecosystem functions in temperate streams (Bogan et al., 2015, Lake, 2003).
Desiccation and rewetting are increasingly recognized as major stressors of streams ecosystems (Sabater et al., 2016). Species richness and diversity decline significantly in most biofilm microbial communities during prolonged dry periods (Acuna et al., 2015, Pohlon et al., 2013, Rothrock and Garcia-Pichel, 2005). Flow resumption in previously dried streambeds causes rewetting that adds additional stress to the sediment community (Romaní and Sabater, 1997). Studies of laboratory microcosms with disturbed hyporheic sediments indicate that more intense drying results in more distinct changes and reduction in the microbial community (Marxsen et al., 2010). In streams impacted by desiccation, algal communities react quickly after flow cessation, while heterotrophs take longer to react (Acuna et al., 2015). Given the disparate reactions of communities to drying, autotrophic and heterotrophic dominated ecosystems controlled by the light availability may respond differently to drying stress affecting stream metabolism and its recovery after flow resumption (Timoner et al., 2012).
Impact of drying intensity in a natural streambed is related to sediment structure (Shokri et al., 2010). Natural streambeds are composed of textural patches (grain-size facies) (Buffington and Montgomery, 1999a, Buffington and Montgomery, 1999b) of variable sediment grain size and hydraulic conductivity (Malard et al., 2002). Such heterogeneous sediment structures support solute supply to sediment associated biofilms (Salehin et al., 2004), and provide metabolic functions (Battin et al., 2003). However, during desiccation, coarser sediment patches become sites of higher initial evaporation rates (Shokri et al., 2010), potentially increasing the impact of drying on benthic communities in sorted coarse bed sediments. In contrast, bed hydraulic conductivity is limited in hydromorphologically degraded streams. The uniformly mixed bed sediments impacted by sand from catchment erosion (Meyer et al., 2008, Wagner et al., 2015), restrict solute supply and, during drought, limit evaporation rates from deeper sediments. Opposing roles of sediment structure on microbial communities and their functions during flow and non-flow periods may have a critical impact on intermittent stream metabolism.
Although it is assumed that drought stress interacts with light availability and bed sediment structure, the response of microbial communities and functions to droughts in the complex environment of canopied temperate streams with variable bed sediment structure and shading remains unclear. We examined the functional dynamics of autotrophic and heterotrophic ecosystem metabolism during drying and subsequent rewetting in streams. We focused on the interactions of light availability (shading level) and sediment structure with stream metabolism resistance and resilience to desiccation and rewetting. While definitions vary (Allison and Martiny, 2008, Pimm, 1984, Shade et al., 2012), we define resistance as the ability of communities to withstand drying, and resilience as the capacity of communities exposed to drying and rewetting to recover rapidly, and revert to pre-drying ecosystem functions (Acuna et al., 2015, Todman et al., 2016). The hypothesis of this study was that shifts of stream ecosystem functions caused by infrequent and unpredictable droughts interact with the shading and sediment structure of temperate streams. We tested this hypothesis by analyzing 16 experimental streams, varying the light availability and bed sediment structure, and assessing microbial ratios (H:A, F:B), periphyton biomass and structure (diatoms, green algae, cyanobacteria), community respiration (CR), and net ecosystem production (NEP).
Section snippets
Experimental set-up
We designed a 2 × 2 factorial experiment comprising 16 outdoor experimental streams, using flumes (400 × 12 × 8 cm) which were filled with a 3 cm layer of sediment (Fig. 1). Two streambed structures (both with 30% porosity) were generated through different arrangements of gravel (2–10 mm grain size, d50 = 4.75 mm) and sand (0.2–2 mm grain size, d50 = 0.57 mm), rinsed with 10% hydrochloric acid and washed with groundwater prior to use. A natural streambed (sorted structure) was created by arranging eight
Community respiration (CR) and net ecosystem production (NEP)
CR and NEP showed a significant relationship to shading during all phases (Fig. 3, CR: χ2col = 12. 08, p < 0.001, χ2des = 5. 27, p = 0.02, χ2rew = 4. 92, p = 0.03; NEP: χ2col = 4. 78, p = 0.03; χ2des = 103. 82, p < 0.001; χ2rew = 25.53, p < 0.001), and were strongly affected by drying (pairwise t-test, p < 0.001) and rewetting (p < 0.001). CR decreased more (60%) in high shade streams than in low shade streams (40%) after flow cessation (p < 0.001). NEP and CR decreased to near-zero after 20 days' drying. After flow
Effect of light availability on stream metabolism resistance and resilience
We traced stream metabolism changes from periods of low flow to desiccation, subsequent rewetting, and flow resumption, and tested the relationships between the observed dynamics sediment structure, and shading. As expected, streams with low shade became net autotrophic during colonization, while those with high shade remained net heterotrophic. Shade-affected streams were not influenced further by bed sediment structure (Fig. 3), confirming the strong control of light over both NEP and CR
Author contributions
All authors contributed equally to this study. All authors have approved the final version of the manuscript.
Acknowledgments
This work was supported by the German Research Foundation (DFG, Bonn) under project (MU 1464/5-1 and PR 710/5-1). We are thankful to G. Lippert, S. Krocker, L. Baranyai, R. Saack, T. Wolburg, R. Yague, and I. Kibet for their assistance with laboratory and field work. We thank anonymous reviewers for comments that greatly improved the quality of this manuscript. There are no data sharing issues as all information related to this study is provided herein.
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