Mercury bioaccumulation and biomagnification in mesopelagic biota

This study investigated the distribution and biomagnification pathways of total mercury (THg) concentrations in three mesopelagic species in the Tasman Sea: two fish (Diaphus hudsoni, Metelectrona ventralis) and one predatory squid (Lycoteuthis lorigera). THg concentrations in muscle tissue ranged from 0.06 to 0.26 μg g⁻¹ dry weight (DW) and were consistently higher in liver tissue (0.15–0.96 μg g⁻¹ DW) across all species. Species-specific variations in THg bioaccumulation and the liver-to-muscle THg ratio suggested differences in mercury metabolism. Stable isotope ratios of carbon (δ¹³C) varied across tissues and species, reflecting a diverse range of bathypelagic habitat utilisation (−19.46 to −16.72 ‰). Bulk and amino acid nitrogen (δ¹⁵N) isotopes provided robust estimates of mean trophic positions of fish (ranging 3.1 to 3.5) and squid (4.3). THg concentrations correlated with isotopic trophic indicators only in L. lorigera, and increased with body size in M. ventralis and L. lorigera, but not in D. hudsoni. THg biomagnification factors between fish and squid ranged between 0.54 and 1.45, indicating limited biomagnification, as confirmed with trophic magnification factors in the liver (1.89) and muscle (0.78). Spatial differences in THg concentrations were evident with higher levels in the eastern region for all species, likely driven by local ecological and environmental conditions. These results highlight the complexity of THg dynamics in mesopelagic food webs and provide important baseline data for future bioaccumulation studies in this and other oceanic regions.