Seasonal Characterization of Mercury and Organic Matter From the Contaminated Penobscot River to the Gulf of Maine

Journal article, 2026

Mercury (Hg), in the form of methylmercury (MeHg), is a neurotoxin that bioaccumulates in marine organisms and biomagnifies up the food chain. Understanding its behavior in coastal ecosystems is important, especially in fishing grounds such as the Gulf of Maine (GoM). Elevated inputs of terrestrial organic matter (OM) have been observed in the GoM, driven by increased watershed runoff from climate and land use changes. OM is an important vector for Hg transport in rivers, and OM character and concentration can influence the methylation and bioavailability of MeHg. To evaluate how increased watershed inputs would impact the GoM, the relationship between MeHg, Hg, OM quality and quantity, and other water parameters was assessed between April 2023–2024 during four cruises in the GoM and a major tributary, the historically Hg-contaminated Penobscot River, as well as during two trips further upstream. OM quality was evaluated via fluorescence spectrophotometry and a six-component parallel factor analysis (PARAFAC) model identified two marine and four terrestrial OM components. Terrestrial OM proxies were strong predictors of, and positively correlated with, MeHg in the Penobscot River/Estuary, suggesting that co-transport of MeHg and OM outweighs terrestrial OM's attenuation of in situ methylation in the water column. We concluded however that the Penobscot River was not an important source of Hg to the GoM. Rather, a seasonally consistent hotspot of MeHg at depth in Jordan Basin was identified, correlating with elevated apparent oxygen utilization, nitrate, and other proxies for OM degradation suggesting its formation in situ in the deeper waters.