Bacterial structures and ecosystem functions in glaciated floodplains: contemporary states and potential future shifts

The ISME Journal (2013) 7, 2361–2373 & 2013 International Society for Microbial Ecology All rights reserved 1751-7362/13 www.nature.com/ismej ORIGINAL ARTICLE Bacterial structures and ecosystem functions in glaciated floodplains: contemporary states and potential future shifts Remo Freimann1,2, Helmut Bürgmann3, Stuart EG Findlay4 and Christopher T Robinson1 1 Department of Aquatic Ecology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland; 2Institute of Molecular Health Sciences, ETH-Zürich, Switzerland; 3Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters—Research and Management, Kastanienbaum, Switzerland and 4Cary Institute of Ecosystem Studies, Millbrook, NY, USA Glaciated alpine floodplains are responding quickly to climate change through shrinking ice masses. Given the expected future changes in their physicochemical environment, we anticipated variable shifts in structure and ecosystem functioning of hyporheic microbial communities in proglacial alpine streams, depending on present community characteristics and landscape structures. We examined microbial structure and functioning during different hydrologic periods in glacial (kryal) streams and, as contrasting systems, groundwater-fed (krenal) streams. Three catchments were chosen to cover an array of landscape features, including interconnected lakes, differences in local geology and degree of deglaciation. Community structure was assessed by automated ribosomal intergenic spacer analysis and microbial function by potential enzyme activities. We found each catchment to contain a distinct bacterial community structure and different degrees of separation in structure and functioning that were linked to the physicochemical properties of the waters within each catchment. Bacterial communities showed high functional plasticity, although achieved by different strategies in each system. Typical kryal communities showed a strong linkage of structure and function that indicated a major prevalence of specialists, whereas krenal sediments were dominated by generalists. With the rapid retreat of glaciers and therefore altered ecohydrological characteristics, lotic microbial structure and functioning are likely to change substantially in proglacial floodplains in the future. The trajectory of these changes will vary depending on contemporary bacterial community characteristics and landscape structures that ultimately determine the sustainability of ecosystem functioning. The ISME Journal (2013) 7, 2361–2373; doi:10.1038/ismej.2013.114; published online 11 July 2013 Subject Category: Microbial ecology and functional diversity of natural habitats Keywords: glacier; biofilm; hyporheic sediment; stream; bacterial communities Introduction Heterotrophic bacteria are crucial in the functional ecology of aquatic ecosystems, being the driving force behind metabolic processes like respiration and productivity, nutrient cycling and fluxes, trophic links with secondary consumers and numerous biogeochemical processes (Edwards et al., 1990; Kirchman, 1994; Hall and Meyer, 1998; Acuna et al., 2008). The hyporheic zone and its heterotrophic components have an important role by integrating many of these ecosystem functions (EFs) at the interface between surface waters, groundwaters and Correspondence: R Freimann, Institute of Molecular Health Sciences, Professorship of Genetics, HPL E22.1, Schafmattstrasse 22, Zurich 8093, Switzerland. E-mail: Received 18 February 2013; revised 16 May 2013; accepted 7 June 2013; published online 11 July 2013 the riparian zone (Hendricks, 1993; Stanford and Ward, 1993; Findlay, 1995; Battin, 1999). Alpine aquatic systems are undergoing rapid change in response to glacier recession, thereby providing the opportunity to examine structural and functional responses of bacterial communities (see Milner et al., 2009) to potential changes in environmental conditions, especially in high elevation lotic systems. Globally, alpine catchments are major sources of freshwater because of relatively high levels of precipitation, often stored as snow and ice in glaciers. This stored water is then released during warm periods as snow and glacial meltwaters. Groundwater-fed streams also are common in alpine catchments. Hence, the majority of running waters in glaciated alpine floodplains can be characterized as either glacier-meltwater-fed (kryal) or groundwater-fed (krenal) channels, or streams dominated by snowmelt (rhithral) during spring (Brown et al., Alpine stream bacterial assemblages R Freimann et al 2362 2003). These different types of streams have distinct annual and diel discharge patterns (flow regimens), hydrological linkages and physicochemical characteristics (Ward, 1994; Tockner et al., 1997; Brown and Fuge, 1998; Smith et al., 2001). Krenal systems, for example, are less influenced by discharge fluctuations, whereas kryal systems show high discharge during summer ablation and an increasing influence of groundwater towards winter (Brown and Fuge, 1998). Owing to these different dynamics in biogeochemical and physical characteristics, diverse habitat patches are created. Regional climate models predict an increase in mean temperature in European Alpine regions and more rapid glacial melting (Horton et al., 2006; Zemp et al., 2006; IPCC, 2007). Krenal systems will likely become more common as glaciers retreat and precipitation patterns change, for example, projections suggest that precipitation periods will shift from reduced precipitation in summer towards increased precipitation in late winter (Swiss Climate Change Scenarios CH2011, 2011). Landscape heterogeneity, as influenced by glaciers, will be reduced and a consequent shift in flow source and regimen towards more krenal-regulated systems is expected. This shift in water source will have a large effect on the physicochemical and ecological state of alpine lotic systems (Hall and Fagre, 2003). For instance, the quality, quantity and timing of resources, such as organic matter (OM) and nutrient inputs, are highly affected by shifts in environmental and hydrological conditions, and will likely influence heterotrophic bacteria assemblages and their ecological services or functioning (Boyer et al., 1997; Findlay and Sinsabaugh, 1999; Horton et al., 2006). Although the above-mentioned changes in environmental conditions will potentially affect EFs mediated by bacterial assemblages, the underlying mechanisms and future trajectories of EF are poorly understood. This is mainly because altered EF can be either linked to changes in bacterial community composition (BCC), single-cell metabolism or changes in total cell numbers (Comte and Del Giorgio, 2011). Which mechanism has the important role in potential future shifts in EF may depend on present bacterial community characteristics (i.e. apparent functional redundancy and plasticity, domination of generalists vs specia (...truncated)