Shell characteristics and Sr–Nd–Pb isotope signatures of Creseis acicula during its 2020 outbreak near Daya Bay Nuclear Power Station

China ranks second in the world in terms of nuclear power generation, making significant contributions to China’s green economic and social development. Chinese nuclear power plants are located in coastal areas, which are prone to frequent outbreaks of marine organisms due to inputs from terrestrial sources. These outbreaks have led to shutdowns of varying severity at nuclear power plants. However, there is currently limited research on the processes involved in marine-organism outbreaks. Daya Bay Nuclear Power Station (DBNPS) is the first large-scale commercial nuclear power station in China. In recent years, the cooling-water system at DBNPS has frequently been affected by marine-organism outbreaks, particularly during the Creseis acicula outbreak in the summer of 2020. This event in multiple unit shutdowns and substantial economic losses. C. acicula is a planktonic organism with a fragile aragonite shell that cannot be effectively retained by existing intake screens at nuclear power plants. To characterize this event, shells collected on 5 July 2020 during the previously reported high-abundance phase were examined using complementary structural, elemental, and Sr–Nd–Pb isotope analyses. The isotope signatures were compared with published regional reference data to evaluate the potential provenance of elements incorporated into the shells, rather than to reconstruct the temporal dynamics of population growth, reproduction, aggregation, or transport. The shells consisted predominantly of aragonite (93.93%), exhibited a three-layered nanostructure, and underwent a principal mass loss of 46.27% over the temperature range of 625.4–709.1 °C. O, Ca, C, and Na together accounted for approximately 99.7% of the summed concentrations of the measured elements. The shell Sr–Nd isotope compositions overlapped with those of some published geological materials and sediments from the Pearl River Basin, whereas the Pb isotope ratios plotted close to a subset of local crude-oil reference values; these comparisons were interpreted only as evidence of potential source affinity. When these shell-based findings are interpreted in the context of previously reported buoy, field, experimental, and acoustic observations, the 2020 event is more appropriately viewed as a multifactor outbreak potentially associated with rainfall-related freshening and potential material delivery, elevated seawater temperature, potential food availability, and hydrodynamic aggregation. However, the absence of concurrent measurements of phytoplankton community composition and abundance, field grazing rates, predator abundance, and water movement precludes the establishment of a sequential causal pathway. The present single-date study therefore characterizes shell properties and potential elemental provenance during the reported high-abundance phase but neither reconstructs the entire outbreak process nor demonstrates that heavy rainfall triggered the event.