The Santa Ana River (SAR) is an urbanized river in Southern California harboring an endangered species of bottom-dwelling algivorous fish, the Santa Ana Sucker (Catostomus santaanae). Though declines in sucker populations are documented, less is known about what changes in sucker habitat along SAR may be contributing to observed trends. Benthic macroinvertebrates (BMIs) and diatoms are potential indicators of the ecological state of the river, and also serve as evidence of food availability for the sucker, as they consume both of these resources. As an urbanized river, much of SAR is dependent on wastewater effluent to sustain surface flow. One of primary treatment plants shuts off its flow for ~6 hours 2-3 times a year for maintenance, resulting in a dry riverbed. Utilizing a before/after/control/impact design, we characterized BMIs, diatoms, and physical properties along a 2km transect of sucker habitat on the Santa Ana River during a drying event Fall-Winter 2016.
The pre-drying samples exhibit unique patterns among themselves; the effluent from one of the treatment plants results in a significant change in community diversity and abundance compared to up and downstream sites. At the outflow, midges were the only BMIs found, while diatom richness and diversity was also significantly lower compared to other sites. However, taxonomic richness and diversity is restored when reaching ~750m downstream of the outflow. The flow shutdown resulted in a complete loss of surface water and drying of substrate at temperatures of over 28°C. Mobile, late instar odonates migrated to shaded areas and ponds, while slow and immobile taxa all died. Most diatom and BMI taxa rebounded or maintained presence after the drying event, suggesting a food web that has adapted to a highly disturbed, urbanized system and exhibits resilience and resistance strategies. Most of the BMIs found in SAR are documented as being pollution and disturbance tolerant. Further research into the state of hyporheic flow and gut content analysis of sucker will continue to illuminate the impact of shutdowns in SAR.