A new study from Erik Björn’s research group demonstrates how the input of terrestrial organic matter influences methylmercury accumulation in coastal planktonic food webs through two opposing processes: (1) increased concentration of thiol functional groups associated with the terrestrial organic matter suppresses methylmercury incorporation at the base of the food web, through the formation of thermodynamically stable methylmercury-thiol complexes with low bioavailability; (2) brownification of the water phase enhances methylmercury biomagnification, through enhanced heterotrophic bacteria production causing a longer food web. Understanding the net outcome of these two processes is essential for predicting methylmercury concentrations in coastal food webs under changing inputs of terrestrial organic matter.