Processes affecting the transformation of mercury in the coastal zone in the vicinity of two river mouths in the southern Baltic Sea

Abstract Rivers play an important role in the coastal biogeochemical cycling of mercury (Hg), however, the fate of Hg in these environments remains poorly understood. In this study, we determine how processes such as mixing, flocculation , and settling of particles in the river mouths of the Vistula and the Reda rivers in the southern Baltic Sea, affect the transformation and migration of mercury. The most important process was the mixing of river water with seawater . This process led to a decrease in the concentration of total mercury and particulate mercury with increasing distance from the river mouth. The second most important process affecting the distribution of Hg was flocculation and particle settling. This process most effectively occurred in the middle part of the mouth, where the highest salinity gradient was observed. It was responsible for approximately 32% loss of Hg from the water column with a simultaneous increase in Hg concentration in sediments. In consequence, it significantly limited the transport of Hg to the open Baltic Sea. The assimilation of Hg by marine plankton was also observed. As riverine organic matter stimulated the growth of marine plankton, they actively accumulated Hg and then transferred this metal to the higher trophic level . In both of the river mouths the processes affected the fate of Hg in a similar way. We therefore concluded that the size of the river did not affect the Hg transformation in the river mouth. However, the area of the river catchment , depth of the mouth and hydrological conditions had a significant impact on Hg cycling in the coastal zone in the vicinity of the river mouth.