| Summary: | Methylmercury (MeHg) is of concern because it has the capacity to readily bioaccumulate and biomagnify along trophic levels until humans, exhibiting toxic effects such as neurotoxicity. Increased permafrost melt (that stores large amounts of carbon and mercury (Hg)) and river inputs, are expected to increase the exposure to Hg through uptake and transfer of contaminants through the food web. The main aim of this study was to determine the impacts of seasonal river inputs on the Hg accumulation in Arctic coastal Paticulate Organic Matter (POM) and zooplankton. The study area was Adventfjord located at 78° North. Analysis were carried out for water and zooplankton samples monthly collected in Adventfjord and its main rivers Longyearelva, Adventelva and tributaries from April to August 2018. Physicochemical parameters and Hg were paired with the analysis of zooplankton diet and trophic interactions, based on stable isotope and fatty acid analysis in order to describe Hg trophodynamics. Seasonal river discharge mainly occured in June and July and rivers contained 2 fold-higher SPM, 10 fold-higher Hg, and had a 4 fold-higher C :N ratio than Adventjord waters. However, strong tidal currents and a lack of sill in Adventfjord allowed for a rapid mixing of river inputs throughout the fjord. Although phytoplankton was the most important food source for zooplankton for nearly all sites and study dates, there was some evidence of dietary reliance on allochtonous energy sources during the main river discharge period. Hg- and MeHg-concentrations in zooplankton increased over summer and could be influenced by river inputs, although other processes could also be involved. This study highlights that Adventfjord is a very dynamic system with complex water chemistry and trophic interactions affecting Hg trophodynamics.
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