Environment shapes invertebrate assemblage structure differences between volcanic spring-fed and runoff rivers in northern California

Abstract

Flow variability plays an important role in structuring lotic communities, yet comparatively little is known about processes governing assemblage dynamics in stream ecosystems with stable environmental conditions, such as spring-fed rivers. Volcanic spring-fed rivers (hereafter spring-fed rivers) occur in geologically active landscapes of the western USA and around the globe. We sampled invertebrate assemblages and quantified primary productivity and habitat characteristics of spring-fed and runoff rivers in northern California over 4 seasons. We predicted that abiotic factors would be more stable and nutrient availability greater and that invertebrate density would be greater and diversity lower in spring-fed than in runoff rivers. Runoff rivers exhibited high variability in discharge and temperature, whereas spring-fed rivers were relatively stable with high naturally occurring nutrient levels. On average, NO 3 − and PO 4 3− concentrations were 40× greater in spring-fed than in runoff rivers. Spring-fed rivers supported nearly 7 to 16× greater densities of invertebrates than runoff systems, depending on season. However, invertebrate species richness was greater in runoff rivers in all seasons. Spring-fed river invertebrate assemblages were strongly correlated with elevated nutrient concentrations and basal C sources, whereas runoff assemblages were associated with discharge variability and median substrate size. We suggest that strong differences in abiotic variability between spring-fed and runoff rivers play an important role in determining invertebrate assemblage structure. Because spring-fed rivers exhibit more stable temperatures throughout the year and lower temperatures during the summer than runoff rivers, they may provide essential refugia for coldwater taxa in a warming climate.

Associating metrics of hydrologic variability with benthic macroinvertebrate communities in regulated and unregulated snowmelt‐dominated rivers

“…Disturbance regimes and hydrologic variability are known to be important in structuring lotic communities (Lusardi et al., ; Naiman et al., ; Resh et al., ). One widely appreciated source of disturbance is winter peak flow, which can dramatically shift channel morphology and directly impact biota (Kondolf et al., ; Ward & Stanford, ).…”

“…In California, Lusardi et al. () found that discharge variability contributed to greater invertebrate diversity in snowmelt river systems than hydraulically stable spring‐fed rivers, and others have found a positive relationship between increasing habitat heterogeneity and invertebrate diversity (Barquin & Death, ; Von Fumetti & Nagel, ). We note that, despite variable climate conditions across years in this study, unregulated rivers consistently had longer durations and slower rates of recession than those rivers with regulation.…”

“…Continued habitat disturbance resulting from sustained high flow conditions of a long spring snowmelt recession may delay or limit population recovery of invertebrates after winter peak flows. Several studies indicate that discharge variability is associated with declines in invertebrate density (Barquin & Death, , ; Lusardi et al., ), while others have found a negative relationship between invertebrate density and magnitude of discharge (Benson, Wipfli, Clapcott, & Hughes, ; Suren & Jowett, ). While the mechanisms are not entirely clear, hydraulic stress associated with the spring snowmelt recession may reduce invertebrate density, inhibit population growth rates, or encourage migration to more stable environments.…”

Stream macrophytes increase invertebrate production and fish habitat utilization in a California stream

“…While abiotic factors, such as thermal and flow regimes, are often cited as enhancing fish populations in spring‐fed rivers (NRC, ), this study shows that primary producers (aquatic macrophytes) can strongly influence food web dynamics and represent important rearing habitat for higher order consumers, such as juvenile salmonids. We suggest that macrophytes were likely a major reason why this and similar spring‐fed rivers (see Lusardi et al, ) function as high‐quality rearing habitats for salmonids.…”

“…Macrophytes are particularly extensive in spring‐fed rivers in volcanic arc terrains; such rivers can be important for rearing of juvenile salmonid fishes (Lusardi, Bogan, Moyle, & Dahlgren, ). Macrophyte abundance in these systems is largely a product of water chemistry, stable flow, constant thermal regimes, open canopy, and low gradient.…”