University of California, Riverside, United States
Abstract: The Santa Ana River is the heart of the largest watershed in Southern California. Due to large metropolitan areas throughout the watershed, the river has been dammed, channelized, and the majority of flow through the extant habitat of the federally listed Santa Ana sucker (Catostomus santaanae) is maintained by effluent. These disturbances, core components of Urban Stream Syndrome, are resulting in alterations to the composition, diversity, and density of the benthic community in the Santa Ana River. To better understand how physical modifications to the Santa Ana River affect the benthic community, we completed monthly benthic macroinvertebrate (BMI) surveys from September 2021 until August of 2022 across eight Santa Ana River reaches. These eight sites consisted of wastewater channels, mainstem reaches, and a tributary used by Santa Ana sucker as of the 2021 native fish survey of the Santa Ana River. We hypothesized that the density of BMIs would increase within wastewater discharge channels and decrease in mainstem reaches because of nutrient enrichment by effluent. In contrast to density, we hypothesized that the diversity of BMIs would increase in mainstem reaches due to the addition of groundwater, decreased channelization, intensity of urban disturbances. By assessing the BMI across space and time in this urban river we can inform how these urban disturbances impact the freshwater benthic community.
We found Hydropsyche, Baetis, and Chironomidae to make up the majority of the benthic macroinvertebrate community across sites. The density of BMIs was significantly different between sampled sites (p < 0.05) but did not differ across sampling months. The richness of BMIs was significantly different across sites and months (p < 0.05). When assessing Shannon’s and Simpson’s diversity we found significant differences between months (p < 0.05) but no significant differences between sampled sites. When assessing the differences in time we found that winter communities were significantly different (p < 0.05) from the rest of the year. These differences in BMI community density, richness, and diversity were most pronounced between wastewater and mainstem sites but did not confirm with some of the a priori hypotheses. We found that the densest, richest, and most diverse communities were found within the Rialto facility outflow channel. We had predicted the most diverse communities to be in the mainstem of the Santa Ana River. Further work will model the abiotic and physical variables to assess correlations with the density and diversity of benthic macroinvertebrates in the Santa Ana River.
