Functional traits of hyporheic and benthic invertebrates reveal importance of wood‐driven geomorphological processes in rivers

Abstract

Large wood (LW) is a natural element of river environments and an integral component of many river restoration schemes to promote biodiversity. It is an important habitat in itself, but it also induces a wide range of hydraulic, hydrological, geomorphological and chemical conditions that influence the ecological community. However, the effects of hydro‐geomorphological processes induced by LW on local benthic and hyporheic invertebrates have not been well characterized. A functional approach was applied to invertebrate data collected in a field survey at sites with LW and without LW (control) to investigate the response of hyporheic and benthic invertebrates' trait profiles in response to local LW‐induced processes. We hypothesized LW sites to be associated with different trait modalities than control sites in relation to wood‐induced processes and conditions (i.e., hyporheic exchange flow, oxygen availability, temporal stability, organic matter, denitrification, hydraulic conductivity). Multivariate analyses and partial least squares (PLS) path modelling were used to detect the differences in trait profiles between LW and control sites and to study the variation of traits as a function of hydrological, sedimentological, physical and chemical variables. Biological (i.e., aquatic stages, reproduction), physiological (i.e., dispersal, feeding habits) and behavioural (i.e., substrate preferences) trait utilization by the hyporheic meiofauna differed between LW and control sites. At LW sites, the hyporheic meiofaunal assemblage was significantly associated with aquatic active dispersal, aquatic eggs and hard substrate preferences. This trait category selection was linked to changes in physical–sedimentological processes at LW sites when compared to control sites. Macrofaunal benthic and hyporheic functional traits did not differ significantly between wood and control sites, suggesting similar functioning of these assemblages at the surface–subsurface interface. This study found that LW affects invertebrate traits by altering fluvial processes to produce, locally, a mosaic of habitats. Hyporheic meiofauna trait responses to LW processes have suggested (a) the crucial role of LW in supporting river benthic zone functioning and thus (b) a possible benefit to river restoration by enhancing functional interactions among different ecological niches. A free Plain Language Summary can be found within the Supporting Information of this article.

Is the Hyporheic Zone Relevant beyond the Scientific Community?

“…Additionally, despite extensive research demonstrating potential effects of restoration measures on river hydrodynamics, little is known about their effects on the hyporheos. Experimental studies on large wood, commonly used in restoration design, and structure-induced HEF [251] have confirmed that there is both a taxonomic and functional effect on the local benthos and hyporheos [108,252].…”

“…These results reflect the ecotonal nature of the HZ [14] and also draw attention to the importance of ecological processes and services that the hyporheos sustains. To this end, hyporheic conditions favor the occurrence in the hyporheos of specific functional traits (e.g., body size and form) that reflect adaptation to the surrounding environment and that are dissimilar to those present in the benthos [108]. The consequences of trait diversification in the HZ extend beyond adaptive species shifts to exploit productive habitats, and suggest benefits to river restoration by enhancing functional interactions among different ecological niches [108].…”

Investigating invertebrate biodiversity around large wood: taxonomic vs functional metrics

“…LW drives river hydrological, physical and ecological processes, as a result of its interactions with water, sediment and biological communities (Wohl 2013). Previous ecological studies of benthic macroinvertebrate assemblages have focused on taxonomic diversity (TD) and found that LW promotes alpha and beta diversities through its effects on habitat complexity (Thompson et al 2018), but evidence of the ecological effects of LW on the hyporheic zone (HZ) and on functional diversity (FD) have received less attention (Magliozzi et al 2019).…”

“…habitat filtering; Diaz et al 1998). Recent research has demonstrated that LW-processes favour the occurrence of specific behavioural, biological, and physiological traits within species assemblages that are dissimilar to assemblages living in sites without wood (Magliozzi et al 2019). To this end, invertebrate assemblages that co-exist in LW might be more taxonomically and functionally diverse than assemblages in places where in-channel LW is absent.…”

“…To this end, invertebrate assemblages that co-exist in LW might be more taxonomically and functionally diverse than assemblages in places where in-channel LW is absent. Despite extensive research on benthic macroinvertebrates, few studies have considered the impacts of LW on hyporheic assemblages (Smock et al 1992;Wagenhoff and Olsen 2014;Magliozzi et al 2019) or simultaneously looked at taxonomic and functional diversities of invertebrates in the benthic and hyporheic zones in relation to LW.…”