The Santa Ana River runs through dense urban development in Southern California. Along its length threats to freshwater ecosystems include habitat loss due to modification and fragmentation, water quality degradation, the introduction of nonnative species, and large-scale water removal for municipal purposes. Common invasive species include largemouth bass (Micropterus salmoides), yellow bullhead (Ameiurus natalis), and mosquitofish (Gambusia affinis). These species have been more frequently found in and around the three wastewater discharge channels that maintain flow in the urban Santa Ana River. The impacts of wastewater discharge and invasive species are resulting in impacts to native species, including the federally threated Santa Ana Sucker (Catastomus santaanae) and Arroyo Chub (Gila orcuttii). The present threats are changing community structure in the Santa Ana River and implementing a new series of environmental and biotic filters on these communities. We asked how wastewater discharge, habitat, and invasive species are altering community structure using data from native fish surveys, counts of removed invasive species during a river dry down, and isotopic analysis of the Santa Ana River food web.
Results/Conclusions
Our analyses show that there are significant differences in communities along the upper urban reaches of the Santa Ana River. Bulk C and N isotope analyses demonstrate significant differences in food web structure and composition between two wastewater discharge channels. We found that the same species hold different isotopic niches based on which channel they are located in. These differences in species were seen in all three of the major invasive fish species: largemouth bass, yellow bullhead, and mosquitofish. Tests of community composition allowed us to demonstrate how density of invasive species and wastewater regime alter community structure in the Santa Ana River. We will continue more sophisticated community analysis to assess covariance of species along the Santa Ana River and to investigate the effect of habitat type on communities. Continued sampling will be carried out to more fully examine how food webs are altered along the length of the Santa Ana River.