Particulate Organic Carbon Scarcity Exacerbates Vulnerability of the Sacramento-San Joaquin Delta Ecosystem

Abstract Anthropogenic climate change on local to global scales has transformed the cycling of particulate organic carbon (POC) in river deltas. Understanding the biogeochemical processing of POC across deltas is key to developing a predictive framework for disruptions in carbon cycling within vulnerable ecosystems. This study examined the spatial and temporal variations in POC, particulate nitrogen (PN), and total suspended sediments (TSS) concentrations across the Sacramento-San Joaquin Delta (Delta) over three water years (2021–2023), covering drought-like conditions observed in spring, summer, and fall. Comprehensive Delta-wide surveys were conducted, employing a mass balance model to estimate total POC flux and constrain POC influx/efflux. Limited seasonal variability in POC and PN concentrations reflected a constrained particle supply under hydrologic conditions increasingly representative of the Delta’s future. Results revealed an 81% decrease in annual total suspended sediment concentration inputs to the Sacramento River from the 1957 water year compared to the average of 2021–2022, with levels during the study period much lower than previous decades. POC influx ranged from 39 to 79% of total efflux during spring to fall of low water years, with 8 to 37% of daily POC flux diverted to southern water pumping stations, limiting opportunities for particle deposition and internal production within the Delta. This was reflected in average Delta POC concentrations: 0.77 mg/L (2021), 0.54 mg/L (2022), and 0.42 mg/L (2023). The sustained reduction in sediment and POC levels in the Delta, driven by human activities, hinders wetland accretion, carbon sequestration, and overall ecosystem stability.