Benthic macroinvertebrates (BMIs)—small, bottom-living, aquatic invertebrates—have long served as bio-indicators of freshwater health and water quality due to their sensitivity to environmental factors such as temperature, nutrient availability, and contaminants. The body size distribution of the BMI community, often referred to as the size spectra (SS), is particularly useful for understanding community dynamics, food web structure, and trophic interactions, providing insights for conservation and management purposes (Gjoni et. al., 2024; Walquist, 2021; Racanelli, 2021).
Here, we examine SS responses in BMIs to urban heterogeneity. Similar to many other urban rivers, the Santa Ana River (SAR) the largest watershed, southern California, the United States, faces challenges associated with degraded physical, chemical, and biological conditions resulting from anthropogenic activities (Huntsman et. al., 2022; Brown et. al., 2005). Most of the river flow is maintained with the continual release of treated water from wastewater treatment plants into the SAR, impacting the river ecosystem. We ask whether the BMI SS differs spatially and temporally and whether this variation can be explained by observed differences in temperature and other environmental variables like substrate, discharge, pH, conductivity, and dissolved oxygen that are known to influence BMI population growth (McGarvey & Kirk, 2018; Petchey & Belgrano, 2010; Brown et. al., 2004).
BMIs were sampled monthly across eight sites throughout the year (from October 2021 to September 2022); individual BMIs were sorted, and the biomass was measured in the lab (Edwards et.al.,2017; SWAMP, 2016; Benke et. al.,1999). Preliminary analyses suggest that there is a consistent distribution of body sizes in effluent sites compared to non-effluent ones. However, the SS may appear to change among seasons and sites. These results of SS spatial and temporal variation in the urban SAR, will provide insights useful for the conservation and management of threatened freshwater species that are influenced by BMIs in the river ecosystem.