Model study on present and future eutrophication and nitrogen fixation in the Gulf of Finland, Baltic Sea Gennadi Lessin a,b, ⁎, Urmas Raudsepp a , Ilja Maljutenko a , Jaan Laanemets a , Jelena Passenko a , Andres Jaanus c a Marine Systems Institute, Tallinn University of Technology, Akadeemia tee 15A, 12618 Tallinn, Estonia b European Commission, Joint Research Centre, Institute for Environment and Sustainability, Via E. Fermi, 2749 (TP272), I-21027 Ispra, VA, Italy c Estonian Marine Institute, University of Tartu, Mäealuse 14, 12618 Tallinn, Estonia abstract article info Article history: Received 11 January 2012 Received in revised form 9 August 2013 Accepted 16 August 2013 Available online 13 September 2013 Keywords: Baltic Sea Gulf of Finland Climatic changes Eutrophication Nitrogen fixation Modelling The response of nutrient and chlorophyll fields to climate change by the end of the twenty-first century was evaluated in the Gulf of Finland (Baltic Sea) using comparison of a hindcast simulation for 1997–2006 and future climate forcing, assuming an A1B greenhouse gas emission scenario and business-as-usual riverine nutrient load for 2090–2099. The comparison of simulated oxygen, phosphate and nitrate levels from the hindcast model with the measurements indicated a good performance of the 3D ecosystem model, except for overestimated near- bottom layer nitrates. The mean chlorophyll level was slightly overestimated by the model, whereas the variability in the surface layer chlorophyll level was well reproduced. Future projection simulations indicate no considerable changes in the upper layer oxygen concentrations compared with the hindcast simulation and observations, but deeper near-bottom layers were projected to become anoxic, causing an increase in phosphate and a decrease in nitrate concentrations in these layers. The increase in surface layer phosphate and the decrease in nitrate concentrations lead to an increase in summer cyanobacteria blooms and an increase in nitrogen fixation, which therefore led to an increase in the annual mean chlorophyll content in the upper layer. © 2013 Elsevier B.V. All rights reserved. 1. Introduction According to the HELCOM eutrophication assessment, both the open and coastal waters of the Gulf of Finland are classified as being “affected by eutrophication” (HELCOM, 2009). Eutrophication signals include the winter nitrate and phosphate concentrations in the surface layer, chlorophyll a concentrations, depth distribution of submerged aquatic vegetation and status of benthic invertebrate communities, the latter being connected to near-bottom oxygen levels (HELCOM, 2009). The Baltic Sea Action Plan (BSAP) adopted in 2007 foresees the achievement of a good ecological status of the Baltic Sea by 2021 (HELCOM, 2007). The BSAP foresees the reduction of nitrogen input to the Baltic Sea of ap- proximately 18% and reductions of phosphorus input of approximately 42%, which can be considered as an optimistic scenario. In addition to the BSAP scenario, Gustafsson et al. (2011) defined the following nutrient input scenarios: reference (REF), current legislation (CLEG) and business as usual (BAU). REF assumes the nutrient load remain con- stant at the level observed in recent years (average for 1995–2002). CLEG implies nutrient input reduction according to the EU directives on sewage treatment and the Nitrogen Emission Ceiling (NEC) directive on air emissions. The BAU scenario assumes rapid agriculture growth around the Baltic Sea, especially in transitional countries. Swinnen et al. (2009) have shown that gross agricultural output has increased in Central and Eastern European countries and the Former Soviet Union since the mid-1990s and is reaching the pre-reform output level. However, in the long term, changes in ecosystem status do not de- pend only on the levels of nutrient input, and climate effects must also be taken into account (Meier et al., 2011a; Savchuk and Wulff, 2009). The projections of climate change predict alterations in the physical conditions of the Baltic Sea in future decades (BACC Author Team, 2008). The physical status of the Baltic Sea depends on external forcing, comprised of direct interaction with the atmosphere through air-sea interface, freshwater runoff from the land and interaction with the ocean at the open boundary (Stigebrandt and Gustafsson, 2003). These changing physical conditions will inevitably impact the biogeochemistry of the sea. When the temperature remains above the temperature of maximum water density, deep convection weakens, and the amount of nutrients entering the euphotic zone decreases. In addition, an increase in precipitation increases the nutrient input from land and favours eutrophication in coastal areas (BACC Author Team, 2008). It has been suggested that an increase in precipitation and a decrease in the frequency of major Baltic inflows from the North Sea will result in a shift from marine to more brackish and freshwater species in the Baltic (Philippart et al., 2011). Climate change in the Baltic Sea Basin has been studied using regional climate models (RCM). One of the first modelling studies investigating the impact of climate change on the future physical conditions of the Baltic Sea (2070–2100) using RCM was that of Meier (2006). More recent model simulations include the analysis of the impact of climate Journal of Marine Systems 129 (2014) 76–85 ⁎ Corresponding author. Tel.: +372 6204313. E-mail address: gennadi.lessin@msi.ttu.ee (G. Lessin). 0924-7963/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmarsys.2013.08.006 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys