Seasonal variation of mercury and selenium in starfish from a contaminated coastal ecosystem

Jiaozhou Bay (JZB), a hotspot for mercury (Hg) methylation and pollution in the Yellow Sea, faces rising Hg levels due to anthropogenic activities. However, seasonal Hg accumulation in benthic fauna remains understudied. This study investigated seasonal variations in Hg and selenium (Se) in Northern Pacific Seastar (NPS), and explored how carbon source (δ¹³C) and trophic position (δ¹⁵N) influence their accumulation. We observed distinct seasonal accumulation patterns for Hg and Se in NPS from JZB: MeHg and THg concentrations were significantly higher in summer and fall/winter than in spring, while Se levels were lowest in summer. Comparisons with two open sea sites showed that MeHg levels in NPS from JZB were significantly higher than those at Hailv Island (Weihai) and Changdao Archipelago (Yantai), while Se levels were lower than both sites. The MeHg:THg ratio and Se concentrations were positively correlated with δ¹³C, whereas MeHg, MeHg:THg and Se showed negative correlations with δ¹⁵N. The two Hg health risk assessment criteria, Se:Hg molar ratio and Se health benefit value (HBV_Se) showed that Se presented in molar excess of Hg in all NPS, potentially offsetting Hg toxicity risks. The relationships between the two risk criteria and body weight, δ¹³C and δ¹⁵N in NPS, suggest that starfish with diets less influenced by terrestrial discharge, lower trophic levels, and smaller body sizes were more protected by Se. Our findings highlight NPS as a potentially valuable bioindicator of Hg–Se interactions in this MeHg pollution hotspot and underscore the need for further research on seasonal Hg sources and Hg methylation dynamics in JZB.