Flow is the ultimate determinant of habitat and habitat quality in rivers and streams. As more ambient flows are supplemented by effluent, it is important to understand the ecological impacts of recycled water. The majority of flow in the urban length of the Santa Ana is provided by two wastewater facilities. This urban effluent dominated river is home to threatened native species including Santa Ana sucker (Catostomus santaanae) and Arroyo chub (Gila orcuttii). Alterations to the physical habitat and hydrology have supported the invasion and establishment of largemouth bass (Micropterus salmoides), yellow bullhead catfish (Ameiurus natalis), and mosquitofish (Gambusia alnis). In this study we investigated of the effects of wastewater discharge on trophic position of invasive species and food web structure within the RIX and Rialto channels, two wastewater-dominated channels of the Santa Ana River, using bulk d13C and d15N isotopes. We predict that enrichment levels from differential treatment processes, filter and UV at Rialto vs. percolation and extraction at RIX, and wastewater infrastructure are resulting in trophic differences at both the community and population level in these outflow channels. We expect these differences to exist despite close physical proximity and a lack of physical barriers between channels.
Results/Conclusions
d13C and d15N analyses show significant differences in the invasive populations in the two wastewater channels. We found that the same species of invasive fish held different isotopic niches based on which wastewater channel they were caught. All three invasive species in the Rialto channel held a similar isotopic niche but held distinct trophic niches in the RIX channel. In particular, the largest and smallest 30% of the largemouth bass population in Rialto held a similar trophic position with a d15N ratio of ~17%. In the RIX channel the smallest 30% of bass had a d15N ratio of ~18% while the largest 30% had a d15N ratio of ~21.5. This difference could be due to differences between the wastewater facilities treatment processes leading to different enrichment levels and stressors placed on the biological community in each channel. End community member analysis suggests that largemouth bass in RIX are undergoing an expected ontogenetic shift to piscivory while the largemouth bass in the Rialto channel are not. These results help to show how wastewater treatment, infrastructure, and invasive species are altering the biotic and abiotic filters within this urban river ecosystem.
