ReviewReview on environmental alterations propagating from aquatic to terrestrial ecosystems
Graphical abstract
Introduction
Freshwater ecosystems and their hydrological dynamics are important “hot spots” and “hot moments” (defined as patches or short periods of time exhibiting disproportionally high reaction rates relative to their surrounding or longer periods of time, respectively, McClain et al., 2003) of biogeochemical processes and biodiversity supporting fundamental ecosystem functions at the landscape ecosystem scale (Costanza et al., 1997, Williamson et al., 2008). These functions ultimately translate into ecosystem services (e.g., the provision of clean drinking water), which refer to the benefits society receives as a result of ecosystem productivity (cp., Kumar, 2010). Thereby, running waters are, despite their relatively low share (0.1% for streams and rivers) of continental area, of particular importance but also depend substantially on their catchments through fluxes of resources including water, organic matter, nutrients and pollutants (Paetzold et al., 2008, Richardson and Sato, 2015, Schulz, 2004, Schwarzenbach et al., 2006). In this context, existing concepts mainly focus on the terrestrial input into freshwater ecosystems and the consequences for aquatic life. For example, the ecological role of terrestrial-derived organic carbon has attracted considerable attention in both lentic (e.g. Pace et al., 2004) and lotic systems (e.g., Wallace and Eggert, 1997). For lotic systems, the importance of terrestrial-derived organic material and the associated alterations in the benthic community along the flow gradient have been conceptualized in the “River Continuum Concept” (Vannote et al., 1980). This concept considers rivers as a receiving system of terrestrial resources with no or only limited recognition of their role as a resource donator. The “Flood Pulse Concept” initially developed by Junk et al. (1989) provided a first step towards the conceptual consideration of feedbacks from aquatic to terrestrial ecosystems (see also Tockner et al., 2000). Such spatial linkages between aquatic and terrestrial ecosystems may have, however, not sufficiently been assessed to fully understand their biogeochemical and ecological consequences (e.g., Baxter et al., 2005, Richardson and Sato, 2015). A complete characterization of a system such as the aquatic–terrestrial ecotone (i.e. floodplain and riparian habitats) requires a framework that covers the interaction of (I) spatially variable food webs and (II) transport and biogeochemical conversion of resources, which has been suggested by the concept of meta-ecosystems (Loreau et al., 2003). For instance, models linking finite and irregular spatial meta-ecosystem structures that are connected through spatial flows of materials and organisms indicate that high fluxes may destabilize local ecosystem dynamics (Gounand et al., 2014, Marleau et al., 2014).
In this context, the potential importance of anthropogenic alteration in aquatic ecosystems for the biogeochemical and ecological linkages to the surrounding terrestrial ecosystems was reviewed considering two pathways fundamental for the coupling of freshwaters and riparian ecosystems (Baxter et al., 2005, Bendix, 1997, Richardson and Sato, 2015): (I) via flood or drought events (water, nutrients, particles, toxicants), while floods are substantially more relevant for the water-to-land subsidy — this linkage is hereafter considered as abiotic coupling, and (II) via emergence of merolimnic aquatic insects, hereafter considered as biotic coupling. Both pathways provide resource pulses (Yang et al., 2008) from the aquatic to the terrestrial ecosystem with a strong potential for effects in the recipient system (Leroux and Loreau, 2012). In this context, we first link the current knowledge of the abiotic and the biotic aspects of the aquatic-to-terrestrial coupling in systems with negligible or without anthropogenic alterations. Afterwards we analyze the effects of anthropogenic disturbances (i.e. hydromorphological and hydraulic alterations, water pollution, invasive species) on the aquatic–terrestrial linkage, which represents an important topic given the almost ubiquitous influence of human activities on ecosystems (Nilsson et al., 2005, Vörösmarty et al., 2010). Finally, we address the food web-mediated responses in terrestrial communities including soil systems as triggered by stimuli from aquatic systems. Thereby, research gaps and future perspectives are identified for the aquatic-to-terrestrial coupling of abiotic as well as biotic processes.
Therefore, we performed separate literature searches for each topic in January 2015 using the search engine ISI Web of KnowledgeSM (i.e., the search field “Topic”, which includes the title abstract and keywords of each database entry) supplemented by a footnote chasing (White, 2009) of the respective reference lists and exclusion of articles which according to their title or abstract did not belong to the search topic. The keywords for the abiotic and biotic coupling were detailed in Table 1.
Section snippets
General overview of literature search results
Both searches indicated a relatively constant increase in the annual number of publications since 1990 with approximately 67% of all studies published within the 10-year period between 2005 and 2014. A total of 502 (Fig. 1A) and 159 (Fig. 1B) publications on abiotic and biotic linkages were found, respectively (see the Supplementary material for the full list of papers). Overall, the increase in number of articles was stronger than the average value of 5% annual increase in scientific
Biogeochemical responses of soils to flooding
Soils of the floodplains of rivers form the main aquatic–terrestrial interface in landscapes by providing hydrologic connectivity and mutual resource exchange between both systems (Aufdenkampe et al., 2011, Bartels et al., 2012b, Richardson and Sato, 2015, Tockner et al., 2008). This mutual exchange involves water, sediments (Thonon et al., 2007), nutrients (Jacobs et al., 2007), contaminants (Malisauskas and Sileika, 2001) and organic carbon (Robertson et al., 1999) inducing fluxes of
General approach
We analyzed all articles regarding anthropogenic disturbances in the aquatic system, such as hydromorphological and hydraulic alterations, water pollution or invasive species that may propagate to the recipient terrestrial system. Overall, approximately 34% of the studies considered at least one of the three anthropogenic disturbances (Fig. 2B), which are summarized in the following subsections.
Hydromorphological and hydraulic alterations
Fluxes of water, nutrients and water pollutants from aquatic to terrestrial habitats, can be
Food web mediated responses in terrestrial communities
In the preceding sections we highlighted the role of aquatic subsidies in terrestrial ecosystems (Section 3) and the relevance of anthropogenic disturbances in the aquatic ecosystem as a modulator of this subsidy (Section 4). Within recipient food webs, resource pulses can induce profound changes that depend on their duration and frequency (Holt, 2008). A recent review by Bartels et al. (2012b) suggested that the contribution of aquatic subsidies to terrestrial systems in terms of energy was
Modeling of ecosystem services
Since running water ecosystems provide a number of important ecosystem services (MEA, 2005, Ojeda et al., 2008) it is necessary to understand the linkages of processes and systems and how these are affected by anthropogenic alterations of freshwater systems. Considering ecosystem services provided by freshwaters, the connection between those aquatic systems and human well-being may be strong in some areas (e.g. agricultural fields on periodically flooded areas such as along the Nile river) or
Synthesis and outlook
In summary, our review documents increasing research activities on the aquatic-to-terrestrial linkage (sensu Soininen et al., 2015). However, research on abiotic and biotic fluxes remains strongly separated, calling for more integrated assessment of both aspects (see also Richardson and Sato, 2015). Such joint efforts would also facilitate a mechanistic understanding of the processes and mechanisms, which affect this aquatic-to-terrestrial coupling. The paired source and sink concept has been
Acknowledgments
The authors are grateful to R. Bundschuh, D. Englert, A. Feckler, A. König, S. Lüderwald, A. Paetzold, R.R. Rosenfeldt, F. Seitz and J.P. Zubrod for their support with the literature search. This study was supported by the Federal Ministry of Rhineland-Palatinate MBWWK via the research initiative ‘AufLand’ at the University of Koblenz-Landau. This manuscript benefitted greatly from the comments of three anonymous reviewers.
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