Since then, nitrogen inputs have decreased but phosphorus has continued to increase (HELCOM 2013). One of the most conspicuous and environmentally significant effects of environmental deterioration is the establishment of hypoxia and anoxia in near-bottom waters in deep areas (Diaz & Rosenberg 2008). Furthermore, recent findings indicate that hypoxic conditions significantly affect coastal zones as well (Conley et al. 2011), CSF-1R inhibitor mostly because of the combination of increased inputs of nutrients from the land and higher respiration rates caused by elevated

water temperatures (Carstensen et al. 2014). As discussed by e.g. Zillén et al. (2008), anoxic and hypoxic conditions alter nutrient biogeochemical cycles, leading to increased phosphorus release from the sediments and reduced nitrogen losses through bacteria-mediated denitrification (Conley et selleck chemicals al. 2011, Meier et al. 2012, Hietanen et al. 2012, Jäntti & Hietanen 2012). Enhanced phosphorus availability fuels primary production, in particular by diazotrophic cyanobacteria, subsequently increasing the oxygen demand for the decomposition of organic matter to an extent where oxygen depletion restricts nitrification and thus limits denitrification, as a result blocking the natural cycle of nitrogen removal via dinitrogen gas (Hietanen et al. 2012, Jäntti & Hietanen 2012). These distortions and internal

feedbacks in nutrient biogeochemical cycling have been suggested as maintaining eutrophication (Conley et al. 2011) and should also be relevant to the Gulf of Riga, where denitrification is the major pathway of nitrogen removal and sediment-water fluxes

represent the largest phosphorus supply to the water column (Savchuk 2002, Müller-Karulis & Aigars 2011). Given the importance of oxygen as a driver of biogeochemical reactions, a number of studies worldwide and in the Baltic Sea have been conducted to investigate process alterations caused by the transition from oxic to anoxic conditions. However, systems like the Gulf of Riga, where bottom waters exhibit Farnesyltransferase various degrees of hypoxia (1–6 mg l−1) during the summer thermal stratification but never reach anoxic conditions, have been less well studied. Owing to global climate change and the subsequent strengthening of thermal stratification (Graham et al. 2008), there is a growing possibility of more frequent and prolonged periods of hypoxia in the near-bottom waters of the Gulf of Riga and similar shallow ecosystems of the Baltic Sea. Although various models for the Baltic Sea ecosystem have been developed in recent years (e.g. Eilola et al. 2009, Savchuk & Wulff 2009, Müller-Karulis & Aigars 2011), which successfully hindcast changes in nutrient and oxygen concentrations as well as primary production, few direct observations on major nutrient fluxes are available to validate individual model processes.