Lithology-Mediated Differences in the Physical Disturbance Regimes and Recovery of Primary Production in Mountain Streams

Physical disturbance in streams has important effects on gross primary production (GPP), an important driver of aquatic food webs. Light and nutrients are well-established limiting factors of GPP, but sediment motion and increased flow magnitudes can also limit GPP. The underlying lithology in a catchment controls the size and strength of the sediment that reaches streams, affecting the material available for physically disturbing primary producers. We compared the effect of increased discharge and suspended sediment in two reaches draining contrasting lithology in the Oregon Coast Range. We modeled stream metabolism and measured suspended sediment concentration (SSC) in both streams for 6 months in the wet season (November-April). As discharge and SSC increased, GPP decreased in both catchments. However, results indicate that the recovery rate and response magnitude to storms (proportional loss in GPP) varied between the two reaches. The sandstone-dominated catchment showed higher magnitude decreases and steeper, faster recovery rates than the basalt-dominated catchment. The disturbance history in the stream also resulted in a legacy effect on the recovery of production; when storms occurred rapidly in succession, GPP was further decreased. The continued cycle of disturbance resulted in extended periods of low GPP in the stream. Understanding the disturbance and recovery patterns of primary producers from suspended sediment and increased discharge in different underlying lithologies could help inform management practices that might affect primary production, especially as ecosystems experience greater extremes in drought and flooding.