Terrestrial organic matter input causes dual effects on methylmercury accumulation in coastal planktonic food webs

Methylmercury (MeHg) accumulation in aquatic food webs is a key pathway for human exposure. We examined how terrestrially derived organic matter (tOM) influences MeHg bioaccumulation in coastal planktonic food webs by measuring MeHg bioaccumulation factors (BAFs) for a MeHg isotope tracer added to mesocosms receiving four tOM levels. Combined with two previous studies, the results showed that tOM alters MeHg bioaccumulation through opposing processes. All treatments produced highly heterotrophic food webs, with bacterial production contributing 68–87% of basal production and supporting high MeHg accumulation (log BAFs up to 6) due to complex trophic structure. However, average BAFs declined with increasing tOM among individual treatments, attributed to higher concentrations of thiol compounds associated with the dissolved organic matter (DOM-RSH) that suppress MeHg bioavailability. Together, these findings demonstrate that tOM inputs exert dual effects on MeHg bioaccumulation: enhancing it by promoting heterotrophy while simultaneously reducing it via increased DOM-RSH. Understanding the net outcome of these processes is essential for predicting MeHg dynamics under changing tOM inputs.