Combined Effects of Fronts, Upwelling and the Biological Pump on Organophosphate Esters in the Changjiang (Yangtze) River Estuary During Summer

Estuarine and coastal environments are important transport pathways and regional sinks for anthropogenic pollutants. In this study, the occurrence and transport of the continuously released organophosphate esters (OPEs) was investigated together with physical and biochemical parameters throughout the water column in the Changjiang (Yangtze) River estuary during the summer. Total dissolved and particulate OPEs showed great spatial heterogeneity, with mean concentrations of 550 +/- 280 ng/L in the estuary, 110 +/- 270 ng/L in the front/upwelling zone, and 410 +/- 450 ng/L in the continental shelf. OPE concentrations in the estuarine bottom waters were high due to massive terrestrial/sediment inputs. In contrast, the "surface enrichment and depth depletion" of OPEs in the continental shelf was closely related to seasonal stratification. Reduced OPE concentrations were observed in the frontal/upwelling zone due to isopycnal heaving. Frontal activity and upwelling induced phytoplankton blooms in the coastal regions, which jointly contributed to elevated OPEs beneath surface water with high phytoplankton aggregation. The OPEs mainly originated from wastewater treatment plant discharges, industrial pollution and consumer products. These OPEs generally posed a low ecological risk to aquatic lives, but their long-term effects cannot be ignored due to their continuous high production, usage and release. The environmental behavior and fate of anthropogenic contaminants, such as the continuously released organophosphate esters (OPEs), is complex in estuaries and coastal environments. In this study, the occurrence and transport of OPEs in the Changjiang River estuary was associated with different oceanographic processes, including fronts, upwelling and impacts of the biological pump. Dissolved and particulate OPEs varied greatly from the estuary to the coastal region. High OPE concentrations were found in turbid bottom waters of estuary, likely caused by massive sediment inputs. Due to summertime stratification, the OPEs showed high levels in surface seawater, but generally low concentrations in middle/bottom layers of the continental shelf. As for the transition zone between the estuary and continental shelf where fronts/upwelling coexisted, relatively low OPE concentrations were present through the water column, because of the heaving of (cleaner) bottom water masses. Relatively higher OPE concentrations were present just below the algae-rich surface water, probably caused by the settling of biogenic particles with OPEs attached. The observed OPEs were mainly from wastewater treatment plant discharges, industrial pollution and consumer products. The ecological risks posed by OPEs to aquatic life cannot be ignored due to the continuous exposure. Sediment inputs increased organophosphate ester (OPE) concentrations in turbid bottom watersThe biological pump strongly affected OPEs in frontal/upwelling and continental shelf regionsDischarge from wastewater treatment plants contributed more than 50% of OPEs in the Changjiang River estuary