Effects of debris piles and pools along dry riverbeds on nutrients, microbial activity, and ground-dwelling arthropods: A Namibian ephemeral river case

Highlights

• Debris piles and dry pools acted as hot spots for organic carbon, soil nutrients and microbial activity.

• There were a positive association between microbial activity and organic carbon and nutrients.

• Debris and pools differed in assemblage composition of ground-dwelling arthropods.

• Debris and pools habitats contributed to overall diversity.

Abstract

Debris piles and dry pools are common landscape elements along ephemeral river channels. However, their effects on resource availability and arthropod species diversity have been neglected so far. We investigated whether debris piles and pools along dry riverbeds generate nodes of environmental resources, influence microbial activity, and provide key habitats for ground-dwelling arthropods. We studied three different habitats (channel, debris pile and dry pool) to analyze organic matter, nutrients and microbial activity in dry sediments, and included two further habitats (riparian and upland areas) for ground-dwelling arthropod assemblages along a major ephemeral river (Ugab River, NW Namibia). We observed higher concentrations of organic matter and macro- and micro-nutrients in debris piles than in pools, and higher concentrations in both landscape elements than in the sandy channel matrix. Microbial activity showed a positive relationship with organic matter and nutrient availability in sediments. Debris piles, dry pools and channels harbored diverse and abundant ground-dwelling arthropod assemblages similar to adjacent terrestrial areas. Debris piles and pools differed in assemblage composition not only from riparian and upland areas but also from the sandy channel matrix. Overall, debris piles and dry pools acted as nodes of soil fertility, enhanced microbial activity, and shaped ground-dwelling arthropod assemblages.

Introduction

In dryland ecosystems, primary and secondary production is controlled by water availability and nutrient concentrations in soils (Abrams et al., 1997; Jacobson and Jacobson, 2013; Gombeer et al., 2015). Landscape production is mostly restricted to fluvial systems, offering higher soil moisture and consequently more abundant vegetation compared to the surrounding dry landscape (e.g., Sponseller and Fisher, 2006; Steward et al., 2012; Soykan et al., 2012). Hence, in drylands, riparian zones offer suitable climatic conditions for many organisms, such as low solar radiation, air temperature and diel temperature fluctuation (e.g., Rutherford et al., 2004). Consequently, dryland rivers exhibit lateral gradients of resource availability, habitat quality and disturbance, thereby controlling ecosystem processes and biotic assemblages along river-upland transition zones (e.g., Soykan et al., 2012).

The Namib ephemeral rivers are distinct fluvial ecosystems, characterized by one of the highest hydrological variability globally (Jacobson and Jacobson, 2013). Along these rivers, the dry phase (i.e., period with absence of surface water) accounts for most of the network length and time, while the wet phase is limited to episodic flood events after sporadic rainstorms. Therefore, dry riverbeds are key landscape elements in drylands (Steward et al., 2012), and their ecological role might exceed the relative role they play in other climatic zones.

In Namib ecosystems, with very low human pressure, dry riverbeds are characterized by high heterogeneity in geomorphic structures, such as debris piles and dry pools. Rare flood events redistribute large amounts of organic matter along river channels and in floodplains forming distinct debris piles (wood, leaf litter, fruits; Jacobson et al., 1995). Debris pile creation depends not only on the hydrological regime and the local contribution of organic matter by riparian trees but also on the channel geomorphology (Jacobson et al., 1999, 2000a, 2000b). Debris piles induce, in turn, the deposition of sediments, and eventually the formation of vegetated islands as they can act as nurseries for woody vegetation (Jacobson and Jacobson et al., 1999). On the other hand, when surface water disappears, remaining dry pools, dominated by silt and clay sediments, are common features along ephemeral rivers (Jacobson et al., 1995).

There is evidence that debris piles play a critical role in both perennial and ephemeral rivers as retention structures for nutrients and organic matter, which may increase heterotrophic activity, and in shaping biological assemblages (Hedin, 1990; Jacobson and Jacobson, 1999; Pettit and Naiman, 2005). In perennial rivers, debris piles in riparian zones create nodes of fertile soils, facilitating the recovery of terrestrial vegetation after disturbance events (Pettit and Naiman, 2005). Concurrently, they provide valuable habitats for reptiles, birds and mammals (Steel et al., 1999). Along ephemeral river channels, debris piles form organic-rich structures leading to soil enrichment (Jacobson et al., 2000a) and providing food resources and habitats for terrestrial invertebrates (e.g., Shelley and Crawford, 1996).

In contrast, the role of dry pools in nutrient cycling and biological assemblages has not yet been studied in detail. These pools retain water for longer periods, even after the channel has dried up completely. Particulate and dissolved organic matter, as well as other chemical compounds, accumulates in drying pools. Thus, a layer of clay is forming at the bottom of dry pools, increasing the retention of micro- and macro-nutrients. Thick layers of silt deposits maintain a higher degree of humidity than the surrounding environment (Jacobson et al., 2000a), thereby creating favorable habitats for bacteria and fungi, refugia for both invertebrates and vertebrates, and suitable conditions for organic matter and nutrient cycling (e.g., Jacobson et al., 1995, 1999, 2000a).

Ground-dwelling arthropods are key biological components not only in riparian and upland habitats but also in dry riverbeds (Sánchez-Montoya et al., 2016). Indeed, dry riverbeds harbor diverse and unique terrestrial invertebrate assemblages (Wishart, 2000; Steward et al., 2011; Sánchez-Montoya et al., 2016). A recent study demonstrated that surface flow cessation enables the colonization of river channels by terrestrial invertebrates from adjacent riparian and upland habitats (Sánchez-Montoya et al., 2016). The colonization may be facilitated by the stranding of aquatic organisms, providing temporary food subsidies (e.g., Sánchez-Montoya et al., 2016). However, for arid rivers with channels exhibiting long dry periods, we have a global limited knowledge about the role of dry channels for terrestrial invertebrate assemblages, as well as the structure and composition of invertebrate assemblages along lateral river-to-upland gradients. An exception is the case of Namibian's ephemeral streams where a noticeable effort of researching on invertebrate communities in desert environment have been done (Shelley and Crawford, 1996; Ward and Seely, 1996). Even less information is available on the ecological and functional effects of debris piles and dry pools. Both, piles and pools along dry riverbeds, may promote species diversity and harbor arthropod assemblages different to those in the dry channel and in fringing terrestrial habitats (i.e., riparian and upland areas).

In the present study, we investigated the potential role of debris piles and pools as nodes of environmental resources (organic matter, and macro- and micro-nutrients) along dry riverbeds (sensu Pettit and Naiman, 2005), as areas of increased microbial activity, as well as habitats for arthropod assemblages. For this purpose, we studied the Ugab river in Namibia during the dry phase. First, we analyzed physico-chemical properties, nutrient availability and microbial activity in sediments of debris piles and dry pools. Second, we determined their spatial domain of influence on sediment properties and microbial activity. Finally, we studied the effects of debris piles and dry pools on the structure and composition of terrestrial arthropod assemblages along a distinct lateral gradient (channel-riparian-upland habitats).

We hypothesized that debris piles and dry pools act as hot spots for soil nutrients and microbial activity along dry riverbeds compared to the adjacent sandy channel matrix. Moreover, we hypothesized that debris piles and pools harbor arthropod assemblages that are distinctly different to adjacent habitats (i.e. sandy channel, riparian and upland areas) due to their specific environmental properties. Specifically, we predicted that: (i) debris piles and dry pools contain higher organic matter and macro- and micro-nutrient concentrations compared to the sandy channel matrix; ii) debris piles and dry pools exhibit a distance-dependent effect as organic matter and nutrients in sediments decrease with increasing distance to these habitats; iii) microbial activity in sediments perform similar distance-dependence effects; and finally iii) debris piles and pools contain more taxa and higher densities of arthropods than the sandy channel matrix and adjacent riparian and upland habitats, harboring distinct terrestrial arthropod assemblages.