Alina (ESR 5) and Sonja (ESR 7) conducted a field campaign in the Baltic Sea in June of this year. With the support of Charlotte (ESR 8) and two post-docs from Stockholm University they carried out three day-cruises onboard the R/V Electra af Askö. Along a transect from the Trosaån estuary (HF) to Landsort Deep (BY31), they sampled six stations around Stockholm Archipelago. The transect covers waters with distinct biogeochemical characteristics, from anthropogenically influenced water near a wastewater treatment plant outlet to the deepest point of the Baltic Sea (Landsort Deep). The semi-enclosed Baltic Sea is subject to a strong vertical salinity gradient. This gradient leads to a stratification of the water column resulting in anoxic conditions below the halocline, which favor microbial mercury (Hg) methylation. The Baltic Sea is affected by multiple stressors, such as large nutrient inputs, chemical pollution, warming, O2 depletion, acidification, and invasive species. Besides the ecological relevance, these bear economic and health hazards. Related to these pressures, an expansion of the oxygen depleted zones has been predicted, which could potentially result in elevated MeHg concentrations in seafood, the main threat of Hg poisoning to the general public. Additionally, climate and land use change can increase the amount of Hg entering the marine environment via rivers and surface runoff..

The overall goal of the study is to test the hypothesis that Hg entering the sea from land is mainly unavailable for methylation and biological uptake. In total, they took almost 900L of brackish water from 15 CTD casts, various sediment cores and phytoplankton samples to better understand the biotic and abiotic conditions along the transect. Water samples were not only taken for Hg speciation analysis, but also to analyze a large variety of parameters, such as FDOM, DOC, TOC, nitrate, phosphate, sulfate and thiols. Another important question is what role organic matter and its origin plays for Hg transformations in the water column. Therefore, Sonja extracted DOM from over 400 L of seawater. In addition, Alina carried out incubation experiments to constrain mercury (de-)methylation rates using isotopically enriched tracers in well-oxygenated surface waters and the anoxic bottom water of Landsort Deep.

Looking back at the experience, Alina sums up: “I enjoyed the field campaign with the great infrastructure provided, good company and perfect weather conditions”. Sonja has a similarly positive impression of the collaboration and says: “It was a great experience to plan and conduct our own field campaign under these conditions. Now I am super excited for the results!”.

While Alina and Sonja are now diligently working on the measurements and experiments in the laboratories of the universities of Stockholm and Pau, Charlotte is on her way to another research adventure in the north of Sweden.