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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 NO3 and PO43 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

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Environment shapes invertebrate assemblage structure differences between volcanic spring-fed and runoff rivers in northe...