In running waters, flow regimes, primary production, and nutrient availability are documented constraints on the productivity of upper trophic levels. However, empirical evidence on how these drivers interact to shape ecosystem function is still scarce. We assessed how flow regimes and resource availability influence production of fish biomass in eight desert streams in Arizona arrayed across a hydroclimatic gradient that encompasses variation in the magnitude and timing of floods and droughts. We measured fish population size and calculated gross primary production (GPP) seasonally for two years (spring 2016 – winter 2017), and characterized hydrologic disturbance regimes via spectral methods on long-term discharge data and variation in streamflow during the sampling period. We quantified annual secondary production of fish at each site using published production to biomass ratios to evaluate the fitness of fish populations over time. In addition, we assessed variation in food chain efficiency as the ratio of GPP to fish production across streams.
Results/Conclusions
This study spans a broad range of species assemblages (1-7 species of fish), hydrologic conditions (median daily flow = 0 - 1905 LPS), and resource availability (median daily GPP = 0.11 – 2.96 g C m-2d-1). Streams with the greatest secondary production had the lowest GPP, and GPP was not correlated with secondary production. This result suggests that our streams with the highest levels of secondary production could be subsidized by allochthonous sources of carbon to a relatively greater extent than those with low secondary production. Low-flow events had the greatest influence on secondary production. Long-term variation in the magnitude of low flow events, and the duration of extreme low flows during the sampling period were both associated with reduced secondary production. Our results suggest that disturbance regime, characterized by stream drying, may influence the realized productive capacity of desert streams, while autochthonous primary production may not be a strong predictor of production at higher trophic levels.