Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955–2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat.

No abstract

In the Baltic Sea, cyanobacterial community is mainly composed of filamentous nitrogen-fixing forms, including the toxic Nodularia spumigena, and single-celled picocyanobacteria (Pcy), represented by Synechococcus spp. The main aim of the work was to test the hypothesis that the picocyanobacteria dependend on the presence of the nitrogen-fixing cyanobacteria. In addition, the contamination of blue mussels and fish with nodularin (NOD), the N. spumigena toxin, was examined. In years 2008-2011, the samples for the study were collected in the Southern Baltic Proper using FerryBox system and, occasionally, during research cruises. The analyses showed no correlation between the growth of the nitrogen-fixing cyanobacteria and Synechococcus. Compared with the previously published data, a shift in the composition of Pcy phenotypes was observed. This shift might be an indication of the proceeding changes induced by the reduced nutrient loading and/or climate change. Analyses of NOD revealed differences in the cyanotoxin concentrations between mussels of different shell size. The highest concentration of NOD was detected in the liver of round goby. However, temporarily, also the fish muscles were significantly contaminated with the toxin.

The eutrophication status of the entire Baltic Sea is classified using a multi-metric indicatorbased assessment tool. A total of 189 areas are assessed using indicators where information on reference conditions (RefCon), and acceptable deviation (AcDev) from reference condition could be combined with national monitoring data from the

This study follows up on a previous assessment of eutrophication status in the Baltic Sea, which covered the period 2 2001-2006. The updated assessment is based on new eutrophication targets, an improved eutrophication assessment tool 3 (HEAT 3.0) as well as monitoring data for the period 2007-2011. Based on classifications of eutrophication status in all 4 Baltic Sea sub-basins, we reveal that during the assessment period 2007-2011, the entire open Baltic Sea was affected 5 by eutrophication. This is a different conclusion compared to earlier assessments and studies. Whilst the confidence of 6 the assessment was high or moderate in most basins, there were indications of declining confidence in some assessment 7 units and improved confidence in others. The problems in confidence were mainly related to scarcity of in situ 8 monitoring data on chlorophyll-a and Secchi depth. The potential implications of our results, e.g. the expansion of the 9 eutrophic zone and declining confidence in the classifications of eutrophication status, are discussed in relation to the 10 existing Baltic Sea-wide nutrient management strategy as well as future assessment activities.