Stable isotope signatures for interpreting metal accumulation in bulk zooplankton across a nearshore-offshore gradient on the southwestern coast of Taiwan.

Zooplankton integrate metal exposure at the base of coastal food webs, while stable isotopes provide ecological context for interpreting their metal concentrations. We examined bulk zooplankton from 29 river-mouth, harbor, and offshore stations along southwestern Taiwan during May 2024 and May 2025. Zooplankton metals showed element-specific changes between surveys, and Fe concentrations in 2025 were approximately twofold higher at river-mouth than offshore stations. δ15N consistently separated nearshore from offshore zooplankton, whereas δ13C showed localized and between-survey variability without a consistent zonal gradient. The nine-metal pollution index did not differ between surveys (p = 0.966), despite changes in individual metals. Redundancy analysis identified significant associations between stable isotopes and metal composition in both surveys (adjusted R2 = 0.066-0.100; p = 0.047 and 0.027). In 2025, δ15N was most clearly associated with the Fe-Zn-Cu-Co-Cr-Pb composition. This correspondence indicates that coordinated multi-metal variation coincided with the δ15N-enriched nearshore setting, without resolving metal sources or uptake pathways. Paired isotope-metal measurements therefore provide a community-level baseline for tracking ecological conditions and multi-metal patterns in coastal zooplankton.