Groundwater supports intermittent-stream food webs

Stream food webs are changing in response to anthropogenic stressors, such as nutrient enrichment and water extraction. One way to monitor these changes is to examine shifts in the relative importance of autotrophic and detrital energetic pathways in food webs. This understanding is particularly pertinent in intermittent streams whose prevalence is projected to increase because of a changing climate and groundwater extraction. We used the stable-isotope values (delta C-13 and delta N-15) of autotrophic and detrital basal resources and the tissue of consumers to examine the relative importance of these 2 energetic pathways during wet and dry phases in 5 groundwater-influenced intermittent streams. In addition, we used surface-water biogeochemical data to estimate the use of groundwater-derived dissolved inorganic C (DIC) in food webs. Autotrophic basal resources were the dominant C source for primary consumers during wet (> 95%, on average) and dry (> 77%, on average) hydrological phases. C-13-depleted autotrophs and consumers and negative correlations between the delta C-13 values of autotrophs and consumers with aqueous CO2 concentrations, indicated that primary producers probably are assimilating microbial respired CO2 from groundwater. Overall, we suggest that groundwater discharge is probably central to foodweb productivity in these streams by extending the duration of wetted habitat for autotrophic growth during dry phases and by delivering dissolved resources that potentially contribute to the energetic base of producers and consumers. Consequently, extended periods of reduced flow (and cease-to-flow events) caused by groundwater extraction or changed climatic conditions may reduce the overall productivity of groundwater-influenced intermittent streams.