Long-term trends and variability in climate interact with a geomorphic template in streams to create a disturbance regime: the types, magnitudes, temporal and spatial distribution, and effects of both high- and low-flow disturbances. A highly variable disturbance regime in an Arizonan desert stream leads to extreme interannual variability in vegetative cover and governs the establishment and persistence of desert wetlands (cienegas). Disturbance variability also is associated with variation in the amount of nitrogen transported to the stream from the catchment, and thus the extent to which growth of algae and vegetation is potentially nitrogen limited. We used long-term records of nitrogen transport to evaluate differences among wet and dry years, and projected future conditions using downscaled climate models and SWAT modeling. We also compared the abundance of cienegas as well as Cyanobacteria (indicators of nitrogen limitation) along a 12-km section of the stream among three years that differed in disturbance history.

Results/Conclusions . 

Annual discharge and nitrogen transport are highly correlated; thus, expected declines in future spring rainfall will reduce the amount of nitrogen exported from the stream. However, regime is important because the time between events may dictate how much nitrogen is available to be transported to the stream. We found that wet phases (i.e., those associated with El Nino-like weather patterns) were characterized by low nitrogen availability whereas dry periods tended to have higher nitrate concentrations in floodwater. Wetland vegetation varied among years, from <10 % to 24% cover. The lowest cover was recorded in 2010, when major winter flooding removed large volumes of sediment and dramatically rearranged the channel. Future research will examine how and whether cienegas recover from such large disturbances. Finally, an intriguing interaction between cienegas and nitrogen limitation was suggested by our large-scale survey: Cyanobacteria were present at 25% of stream sites lacking wetland vegetation, but at <2% of sites where plants were present. The negative association between wetland plants and cyanobacteria may indicate higher nitrogen availability in cienegas, but the mechanism explaining this pattern is as yet uncertain.