Distinguishing multiple Zn sources in oysters in a complex estuarine system using Zn isotope ratio signatures

The Pearl River Estuary (PRE), the largest estuary in Southern China, historically has suffered from metal contamination as a result of inputs from different riverine discharges. Determining the sources of metals accumulation in local aquatic flora and fauna remains a great challenge for this estuarine system with complex water circulation. In this study, Zn isotope ratios were measured in local oysters (Crassostrea hongkongensis) collected at 8 locations in the estuary on four occasions from 2014 to 2018, to better understand and assess the contamination sources. The results showed no significant differences (p < 0.05) in δ⁶⁶Zn values in oysters among the four sampling dates within individual sites. However, approximately a 0.67‰ (range from -0.66‰ to 0.01‰) difference in average δ⁶⁶Zn values was consistently found in oysters collected from the east side of the estuary compared to the west side, despite their comparable Zn concentrations. A mixing model was subsequently used to estimate the relative contributions from various sources to the δ⁶⁶Zn values in these oysters. The mixing model predicts that zinc derived from the dissolved fraction (approximately 80 %) was the dominant uptake pathway for oysters collected at the east shore whereas approximately 50 % of the Zn in oysters collected at the west shore was derived from the particulate fraction. The mixing model also was used to estimate the relative impacts of fresh versus saline water on the measured δ⁶⁶Zn values. Contributions from these two sources also varied between the east and west shores. This study presents the first data for Zn isotope ratios in oysters from the PRE, providing new insight for using Zn isotope ratios in oysters as a powerful tracer of sources in a complex estuarine system.