Urbanization generates novel spatial and temporal heterogeneity, altering stream ecosystem structure and function. This includes benthic macroinvertebrates (BMIs), widely used as bioindicators of freshwater health, that respond quickly to changing environmental conditions at multiple scales. However, BMI taxonomic responses are often inconsistent, making it difficult to generalize urban impacts (Gjoni et. al., 2024; Collyer et. al., 2023). A useful tool for evaluating ecosystem dynamics is the size spectrum (SS) which describes the relationship between organism body size and biomass or abundance. Urban stressors like wastewater treatment plant (WWTP) effluent can alter stream thermal regimes, along with flow stability and water chemistry, all of which influence organismal metabolic rates and size distributions. Metabolic theory predicts that warmer environments produce steeper slopes with more small-bodied organisms, whereas colder environments should produce shallower slopes and large-bodied ones (Brown et. al., 2004). However, recent studies show that SS patterns often deviate from these predictions, suggesting additional urban stressors beyond temperature influence SS slopes.
To better understand ecosystem effects in urban areas, the study investigates spatial and temporal variation in BMI size spectra in the urbanized Santa Ana River, California, where 85% baseflow is from WWTP discharge. We ask how abiotic drivers—temperature, substrate, depth, pH, conductivity, dissolved oxygen, velocity, and discharge—shape SS patterns across eight sites over one year. BMIs were identified and converted to biomass (Benke et. al., 2011). SS slopes are estimated using the MLEbin method to evaluate environmental drivers of spatial and temporal variation (Edwards et al., 2017).
Preliminary results show body-size distributions with shallower slopes at effluent-influenced sites near WWTPs than at downstream non-effluent sites. Seasonal SS variation is evident across all sites, but does not consistently distinguish effluent and non-effluent reaches. Depth is the main driver of slope variation, with weak interactions among substrate, discharge, conductivity, and velocity. By improving understanding of SS variability under complex urban pressures, this work informs urban ecosystem resilience and freshwater management.