Aspects of flow variability and spatial context predict temporal beta diversity in river metacommunities

Abstract

1. River catchments are dynamic networks that contain multiple levels of spatial and temporal complexity. Benthic macroinvertebrates are key indicator taxa throughout catchments and beta diversity has been used as a metric to explore determinants of community composition at the catchment scale. Commonly explored drivers of beta diversity include environmental and spatial variables such as flow, temperature, and spatial distance. While factors influencing spatial beta diversity have been explored, factors explaining temporal beta diversity have been understudied. Temporal beta diversity is predicted to also be important since community assembly mechanisms are not stable over time, and more studies are needed to determine which factors most strongly determine temporal beta diversity patterns.2. We investigated the effects of local environmental variables, flow variability, and spatial context on temporal beta-diversity using a large, publicly available biomonitoring dataset from river networks in California. Data included benthic macroinvertebrate community composition and environmental data from multiple locations and years, allowing us to explore temporal changes in these communities as a function of site-specific environmental and spatial factors. Associated gage data were used to calculate hydrograph metrics and contextualize the flow regime at each location over long timescales. We then used beta regression to model the relationship between benthic macroinvertebrate temporal beta-diversity, environmental variables, flow regimes, and spatial network context.3. Flow and spatial catchment-related predictors were the strongest predictors of temporal beta diversity, while changes in environmental variables were much weaker. Channel 3 slope, drainage density, and upstream catchment area were the most significant spatial factors. Channel slope showed a negative relationship with temporal beta diversity, while drainage density and upstream catchment area showed positive ones. Temporal beta diversity was also higher when the rate and magnitude of rises and falls in flow was higher in the hydrograph as well as when the number of zero-flow days and the duration of flow rises and falls was higher.4. Overall, our results indicate that temporal beta diversity of freshwater benthic macroinvertebrates is shaped by both long-term hydrological context and spatial context, and that these factors may serve as better predictors of long term community variability than variability in point estimates of environmental measurements. Flow regimes and spatial metrics may provide more environmental context than point-estimate environmental measurements, as the latter may not accurately capture the dynamic conditions that drive variability in metacommunity responses. 5. Our study supports the need for biomonitoring efforts at long spatial and temporal timescales, and highlights the need to consider metacommunity change in the management of freshwater systems.