OOS 22-2
Macroinvertebrate community assembly following flow resumption in an intermittent desert stream

Wednesday, August 13, 2014: 8:20 AM
204, Sacramento Convention Center
Michael T. Bogan, Environmental Science, Policy & Management, University of California, Berkeley, Berkeley, CA

Streams that dry for extended periods of time tend to support less diverse aquatic communities than perennial streams. However, research quantifying patterns of community assembly in streams following flow resumption is uncommon. Studies from temperate streams suggest that community recovery following drying disturbance occurs primarily via resilience mechanisms, such as drift or aerial recolonization from perennial refuges. Desert streams experience more frequent and intense drying events than temperate streams, though, and thus selection may favor drought-resistance adaptations over resilience mechanisms for desert stream taxa. To quantify community assembly following flow resumption and explore the mechanisms contributing to community recovery, I collected benthic macroinvertebrate samples biweekly from Bear Canyon, Arizona, an intermittent desert stream, for five months following flow resumption. The 100-meter study reach had been dry for several months before flow resumption in December 2012 and dried again in May 2013. Additionally, there were no perennial flow refuges in the watershed and the nearest perennial stream was located in an adjacent canyon 2 km away. I quantified changes in macroinvertebrate richness, abundance, and taxonomic and trait composition over the five month flow period at Bear Canyon.  


Taxa with drought resistance mechanisms (e.g. egg or larval diapause stages) appeared at Bear Canyon within seven days of flow resumption, and included stoneflies, midges, and dobsonflies. Macroinvertebrate richness, density and community composition stabilized after three weeks of flow and remained fairly constant for the following ten weeks, though densities were temporarily damped by a flash flood in week 5. After week 13, richness and density increased significantly, and community composition shifted from predominately drought-resistant taxa to those with life history traits conferring resilience (e.g., aerial dispersal and multivoltinism). However, this community of resilient taxa only lasted three weeks until flow ceased. Macroinvertebrate richness and density dropped in the final week of the study, as only taxa adapted to lentic conditions survived in the last remaining pool of the study reach. The stream dried completely the following week. This study highlights how the relative importance of resistance and resilience mechanisms to community assembly in streams following flow resumption may change through time.