J Freshwater Ecology Essays

Abstract

For many decades, river research has been focused on perennial rivers. Intermittent river research has a shorter history, and recent studies suggest that alternating dry and wet conditions alter virtually all biotic communities and biogeochemical processes in these rivers. Intermittent rivers constitute more than half of the length of the global river network and are increasing in number and length in response to climate change, land-use alteration, and water abstraction. Our views of the roles that rivers play in maintaining biodiversity and controlling material fluxes will change substantially when intermittent rivers are fully integrated into regional and global analyses. Concepts, questions, and methodologies from lotic, lentic, and terrestrial ecology need to be integrated and applied to intermittent rivers to increase our knowledge and effective management of these rivers.

Rivers that naturally, periodically cease to flow are found on every continent, and these intermittent rivers may be even more common than perennial rivers. We use the term intermittent rivers to refer to all temporary, ephemeral, seasonal, and episodic streams and rivers in defined channels. A conservative estimate is that intermittent rivers constitute more than 30% of the total length and discharge of the global river network (Tooth 2000). For example, in Australia, roughly 70% of the 3.5 million kilometers of river channels measured at the 1:250,000 scale are considered intermittent (Sheldon et al. 2010), and more than half of the total length of rivers in the United States, Greece, and South Africa are intermittent (Larned et al. 2010). Intermittent rivers are common in Canada (Buttle et al. 2012); 25%–40% of the total length of rivers in France is intermittent (Snelder et al. 2013); and most Alpine, Arctic, and Antarctic rivers are intermittent. These estimates do not include low-order streams (i.e., headwater streams) because they are difficult to detect by traditional mapping techniques, including airborne photography and satellite imagery (Benstead and Leigh 2012). Low-order streams can make up more than 70% of river networks and are particularly prone to flow intermittence (Lowe and Likens 2005). Therefore, the proportion of intermittent rivers in the global river network is likely to be greater than 50%.

In addition to naturally intermittent rivers, water abstraction and impoundment have caused many formerly perennial rivers to become intermittent in the last 50 years, including large rivers such as the Nile, Indus, Yellow, Amu and Syr Darya, Rio Grande, and Colorado. Most of the once-perennial rivers of arid and semiarid regions are now intermittent (Gleick 2003). In the near future, the number of intermittent rivers will increase in regions in which severe climatic drying or water appropriation occurs (Larned et al. 2010, Döll and Schmied 2012).

Intermittent rivers are a recent addition to the field of freshwater ecology. Although pioneering papers were published several decades ago (e.g., Larimore et al. 1959, Clifford 1966, Williams and Hynes 1976), they were not followed by a concerted effort to develop and advance the field of intermittent river ecology. In studies of in-stream biological communities; biogeochemical fluxes; and material exchange between rivers and the atmosphere, land, ocean, and groundwater, perennial flow and permanent surface–water connections have been presumed between river reaches. For example, according to the river continuum concept—arguably the most influential concept in river science—rivers are viewed as hydrological continua with progressive, longitudinal changes in biota and organic matter dynamics (Vannote et al. 1980). The river continuum concept and its successors (e.g., the serial discontinuity concept and the flood pulse concept; Ward and Stanford 1983, Junk et al. 1989) provide little insight into intermittent rivers, in which the complete loss of hydrological continuity affects virtually all ecological processes. In the last 20 years, intermittent river ecology has reemerged as a multidisciplinary field that links community ecology, biogeochemistry, hydrology, and river management (Larned et al. 2010). In this essay, we turn our focus to the future and the benefits and challenges of incorporating intermittent rivers into modern concepts, knowledge, and methods in freshwater and terrestrial ecology.

Shifting aquatic–terrestrial habitat mosaics

Surface-flow and groundwater-level fluctuations drive the expansion–contraction cycles of intermittent river networks (Stanley et al. 1997), which, in turn, generate alternating flowing, drying, and dry reaches. The resulting mosaic of lotic (flowing), lentic (standing), and terrestrial habitats is continuously shifting at the network scale (figures 1 and 2). The diversity, spatial arrangement, turnover, and connectivity of these habitats are controlled by the magnitude, frequency, and duration of drying events (Stanley et al. 1997, Bunn et al. 2006). Like flooding (Stanford et al. 2005, Leigh et al. 2010), drying maintains habitat heterogeneity (e.g., figure 2) and controls large-scale biodiversity and biogeochemical processes in intermittent rivers.

Figure 1.

Shifting lotic (a, d), lentic (b, e), and terrestrial (c, f) habitats across intermittent rivers. Panels (a), (b), and (c) show a single reach along the Albarine River, in France, on different dates; panels (d), (e), and (f) show different reaches (along the same river) on a single date. Photographs: Thibault Datry (b–d) and Roland Corti (a, e, f).

Figure 1.

Shifting lotic (a, d), lentic (b, e), and terrestrial (c, f) habitats across intermittent rivers. Panels (a), (b), and (c) show a single reach along the Albarine River, in France, on different dates; panels (d), (e), and (f) show different reaches (along the same river) on a single date. Photographs: Thibault Datry (b–d) and Roland Corti (a, e, f).

Figure 2.

Expansion–contraction cycle of channel networks from the Thouaret River, in western France, showing the distribution of reaches with continuous flow (lotic, blue lines), isolated pools (lentic, orange lines), and without surface water (dry, red lines). These maps were based on fine-scale visual observations by fisherman federations of the Poitou–Charentes Region. Source: Adapted from www.eau-poitou-charentes.org. Abbreviation: km, kilometers.

Figure 2.

Expansion–contraction cycle of channel networks from the Thouaret River, in western France, showing the distribution of reaches with continuous flow (lotic, blue lines), isolated pools (lentic, orange lines), and without surface water (dry, red lines). These maps were based on fine-scale visual observations by fisherman federations of the Poitou–Charentes Region. Source: Adapted from www.eau-poitou-charentes.org. Abbreviation: km, kilometers.

Intermittent rivers have long been viewed as species poor, and dry channels have been viewed as biologically inactive systems (e.g., Poff and Ward 1989, Stanley et al. 1997). However, there is growing evidence that this view is inaccurate when lentic and terrestrial habitats are taken into account. Population- and community-ecology studies of intermittent rivers are still rare, and most have been focused on lotic habitats. Generally, the lotic invertebrate communities in these studies are subsets of those in adjacent rivers or in reaches of the same river with perennial flow and are characterized by lower abundance and species richness and by truncated food webs (e.g., Williams 2006, Datry et al. 2013). Although perennial and intermittent flow regimes are often seen as categorically different, flow intermittence varies continuously, and biotic communities vary along gradients of intermittence. For example, taxonomic richness in different aquatic groups decreases continuously as the severity of intermittence increases (figure 3), which suggests a lack of clear thresholds in the response of communities to flow intermittence. Consequently, defining intermittent rivers on the basis of biota alone may be difficult or inappropriate.

Figure 3.

Relationships between annual flow intermittence (as a percentage) and the taxonomic richness (in the number of taxa) of lotic communities reported in recent studies. Annual flow intermittence is defined as the proportion of the year without surface water. The letter labels mark (a) benthic invertebrates in the Albarine River, France (Datry 2012); (b) benthic invertebrates in the Selwyn River, New Zealand (Arscott et al. 2010); (c) hyporheic invertebrates in the Selwyn River (Datry et al. 2007); (d) riparian plants in the San Pedro River, Arizona (Stromberg et al. 2005); and (e) fish in the Selwyn River (Davey and Kelly 2007). The lines are based on regression models published in the original studies.

Figure 3.

Relationships between annual flow intermittence (as a percentage) and the taxonomic richness (in the number of taxa) of lotic communities reported in recent studies. Annual flow intermittence is defined as the proportion of the year without surface water. The letter labels mark (a) benthic invertebrates in the Albarine River, France (Datry 2012); (b) benthic invertebrates in the Selwyn River, New Zealand (Arscott et al. 2010); (c) hyporheic invertebrates in the Selwyn River (Datry et al. 2007); (d) riparian plants in the San Pedro River, Arizona (Stromberg et al. 2005); and (e) fish in the Selwyn River (Davey and Kelly 2007). The lines are based on regression models published in the original studies.

Although lotic biodiversity generally decreases in response to increasing flow intermittence, increased lentic and terrestrial biodiversity can compensate for decreased lotic biodiversity (Larned et al. 2010). Descriptions and quantification of intermittent river biodiversity are incomplete if lotic habitats are considered exclusively. During lentic and terrestrial phases, intermittent river channels are colonized by lentic and terrestrial plants, vertebrates, and invertebrates. In disconnected, drying pools, lotic communities are gradually replaced by pond-like (lentic) and semiaquatic communities (Boulton et al. 1992, Stanley et al. 1997, Bonada et al. 2007). Diverse terrestrial arthropod communities have been reported from dry riverbeds in Europe, Australia, New Zealand, and Africa (Wishart 2000, Corti and Datry 2012, Steward et al. 2012). In arid regions, dry riverbeds are important habitats and refuges for terrestrial organisms, including large mammals (Steward et al. 2012), and the disconnected pools that persist during dry periods act as refuges for semiaquatic and aquatic organisms (Sheldon et al. 2010). In intermittent rivers, as in floodplains (Stanford et al. 2005), habitat patches shift among lotic, lentic, and terrestrial phases, and the differences in communities in each phase increase biodiversity at a larger scale. For example, 20% more riparian plant species occurred over a 2–8-year period along intermittent reaches than along perennial reaches in three Arizona rivers (Katz et al. 2012). Similarly, the riparian zone of intermittent reaches in two French intermittent rivers contained 10%–20% more terrestrial arthropod species than did the adjacent perennial reaches over one hydrological year (the authors’ unpublished data). In both cases, higher biodiversity along intermittent reaches was associated with species turnover in response to hydrological changes. In general, species in intermittent rivers are likely to account for a high proportion of regional biodiversity when the turnover rates (beta diversity) of lotic, lentic, and terrestrial communities are simultaneously considered.

Intermittent river biodiversity in a changing world

Freshwater biodiversity has been and will continue to be severely affected by global change (Dudgeon et al. 2006). However, the degree to which the biodiversity of naturally intermittent rivers is affected may differ from that of perennial rivers. Natural drying–rewetting cycles serve as evolutionary cues that increase biological, physiological, and ecological trait diversity (Williams 2006, Warfe et al. 2011), which may, in turn, enhance the adaptability of communities to future environmental change. Intermittent river species display a variety of physiological, behavioral, morphological, and life-history adaptations for surviving or exploiting dry–wet cycles. For example, estivating fish and encysting and diapausing invertebrates can survive multiple dry–wet cycles for months or years (Williams 2006, Larned et al. 2010). Conversely, some terrestrial invertebrates that inhabit intermittent rivers survive periodic inundation or use rainfall cues to avoid flash floods (Lytle and White 2007, Corti and Datry 2012). However, this pronounced adaptability of intermittent river communities to future environmental change can be counterbalanced by the strong dispersal capacity of many lotic species. High dispersal promotes gene flow between populations, which, in turn, favors evolutionarily static species (Nosil et al. 2009). Lotic invertebrates in intermittent river reaches tend to be r selected, with small body sizes, high fecundity, and short life cycles, which confer high passive and active dispersal potential (Williams 2006, Bonada et al. 2007). Lotic communities in intermittent river reaches are generally subsets of those from the least intermittent and perennial section; such nested communities are strongly dominated by ubiquitous species with high dispersal potential (e.g., Arscott et al. 2010, Datry et al. 2013).

Anthropogenic shifts from perennial to intermittent flow regimes may lead to irreversible modifications of biotic communities and of ecosystem processes and services. Species that are poorly adapted to dry conditions (e.g., species with low tolerance to desiccation) may be eliminated from newly intermittent rivers, and habitat fragmentation due to drying may limit recolonization (Phillipsen and Lytle 2013). In contrast, large lentic species with strong dispersal abilities and high fecundity, such as some Odonata, Hemiptera, or Coleoptera species may be minimally affected by aquatic habitat reduction and fragmentation (Bohonak and Jenkins 2003). In newly intermittent rivers, the loss of top predators, such as fish, and subsequent modifications of species interactions are expected to have cascading effects on community structure, composition, and resilience (Soulé et al. 2005). Riparian plant and animal species with high moisture requirements may be replaced by xerophilic species, and terrestrial riparian predators that rely on emerging aquatic insects may be lost. Thorough analyses of the ecological consequences of anthropogenic river drying on biodiversity will require a broader aquatic–terrestrial perspective than is currently applied in river ecology. This challenge is made greater by the fact that anthropogenic river drying often occurs in areas that are simultaneously affected by water pollution, habitat degradation, and overfishing. Many artificially intermittent rivers are characterized by hypoxia, eutrophication, and high concentrations of industrial and agricultural contaminants (e.g., Qi et al. 2005, Szynkiewicz et al. 2011). These observations suggest that artificially intermittent rivers affected by multiple anthropogenic stressors will rarely match naturally intermittent rivers in terms of ecological function or biodiversity.

In contrast to the spread of artificial intermittence described above, some naturally intermittent rivers have become perennial because of controlled releases from dams and weirs; agricultural, industrial, and municipal effluent discharge; and interbasin transfers (Larned et al. 2010

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