Understanding associations between nitrogen and carbon isotopes and mercury in three Ammodramus sparrows

Mercury concentrations in Seaside Sparrows and Marsh Rice Rats differ across the Mississippi River Estuary

Mercury (Hg) concentrations and their associated toxicological effects in terrestrial ecosystems of the Gulf of Mexico are largely unknown. Compounding this uncertainty, a large input of organic matter from the 2010 Deepwater Horizon oil spill may have altered Hg cycling and bioaccumulation dynamics. To test this idea, we quantified blood concentrations of total mercury (THg) in Seaside Sparrows (Ammospiza maritima) and Marsh Rice Rats (Oryzomys palustris) in marshes west and east of the Mississippi River in 2015 and 2016. We also tested for a difference in THg concentrations between oiled and non-oiled sites. To address the potential confounding effect of diet variation on Hg transfer, we used stable nitrogen (δ15N) and carbon (δ13C) isotope values as proxies of trophic position and the source of primary production, respectively. Our results revealed that five to six years after the spill, THg concentrations were not higher in sites oiled by the spill compared to non-oiled sites. In both species, THg was higher at sites east of the Mississippi River compared to control and oiled sites, located west. In Seaside Sparrows but not in Marsh Rice Rats, THg increased with δ15N values, suggesting Hg trophic biomagnification. Overall, even in sites with the most elevated THg, concentrations were generally low. In Seaside Sparrows, THg concentrations were also lower than previously reported in this and other closely related passerines, with only 7% of tested birds exceeding the lowest observed effect concentration associated with toxic effects across bird species (0.2 µg/g ww). The factors associated with geographic heterogeneity in Hg exposure remain uncertain. Clarification could inform risk assessment and future restoration and management actions in a region facing vast anthropogenic changes.

Levels of Mercury in Feathers of Clapper Rails (Rallus crepitans) over 45 Years in Coastal Salt Marshes of New Hanover County, North Carolina

We sampled clapper rail (Rallus crepitans) feathers from museum specimens collected between 1965 and 2010 to investigate changes in mercury (Hg) availability in coastal marshes of New Hanover County, North Carolina. Stable isotope analysis (δ(13)C and δ(15)N) was conducted to control for dietary shifts that may have influenced Hg exposure. Hg concentrations ranged from 0.96 to 9.22 μg/g (ppm), but showed no significant trend over time; diet (δ(15)N) or foraging habitat (δ(13)C) also provided little to no explanatory power to the variation in Hg concentrations among clapper rails. Our findings suggest the bioavailability of Hg to clapper rails in coastal North Carolina salt marshes has remained consistent over time, despite the global trend of increasing mercury in many other bird species, providing an excellent baseline for any future assessment of Hg availability to salt marsh birds in coastal North Carolina.