Description: This paper is devoted to a study on the effects of different parameterisations of vertical turbulence – with a 3D hydrodynamic model COHERENS – on the accuracy of calculated temperature and salinity fields in a hydrodynamically complex test area – the Baltic Sea, Gulf of Finland. Two algebraic parameterisations and k–ε and k-models were used. For k-model four different sets of stability functions were used. Calculated vertical profiles of temperature and salinity were compared against CTD-profiles collected during a measurement campaign in the Gulf of Finland in summer 1996. The dataset has an outstanding spatial and temporal coverage including over 300 measured CTD profiles. The thermocline depth was underestimated throughout summer by all the vertical turbulence schemes. The selection of stability functions had significant effect on the accuracy of the k-model. Generally k-model performed better when the limiting conditions for mixing length were not applied. The k-model with stability functions based on the Munk–Anderson relation without limiting condition for mixing length showed best accuracy in the calculated profiles of temperature and in the thermocline depth. The improvement of the meteorological forcing had an impact on the exactness of the calculated thermocline depth. However, sensitivity tests showed that this impact was relatively small. Generally, calculated salinity was overestimated in the surface layer and underestimated in the bottom layers. Algebraic parameterisations had highest accuracy in the vertical salinity profiles. In the eastern Gulf of Finland the calculated values of salinity were overestimated. The accuracy of initial conditions, river runoff and bathymetry had significant effect on the accuracy of calculated salinity fields. Highlights: We studied the performance of the vertical turbulence parameterisations in the modeling of hydrodynamics in the Baltic Sea. ► We compared the calculated temperature and salinity fields against dataset having outstanding temporal and spatial coverage. ► The vertical turbulence parameterisations showed considerable differences in the accuracy of the calculated temperature. ► The mixed layer depth was underestimated by all vertical turbulence schemes used in this study. ► Selection of stability functions had significant effect on the accuracy of the turbulence closure schemes.

Description: The Boknis Eck (BE) time series station, initiated in 1957, is one of the longest-operated time series stations worldwide. We present the first statistical evaluation of a data set of nine physical, chemical and biological parameters in the period of 1957–2013. In the past three to five decades, all of the measured parameters underwent significant long-term changes. Most striking is an ongoing decline in bottom water oxygen concentration, despite a significant decrease of nutrient and chlorophyll a concentrations. Temperature-enhanced oxygen consumption in the bottom water and a prolongation of the stratification period are discussed as possible reasons for the ongoing oxygen decline despite declining eutrophication. Observations at the BE station were compared with model output of the Kiel Baltic Sea Ice Ocean Model (BSIOM). Reproduced trends were in good agreement with observed trends for temperature and oxygen, but generally the oxygen concentration at the bottom has been overestimated.

Description: We review progress in Baltic Sea physical oceanography (including sea ice and atmosphere–land interactions) and Baltic Sea modelling, focusing on research related to BALTEX Phase II and other relevant work during the 2003–2014 period. The major advances achieved in this period are: • Meteorological databases are now available to the research community, partly as station data, with a growing number of freely available gridded datasets on decadal and centennial time scales. The free availability of meteorological datasets supports the development of more accurate forcing functions for Baltic Sea models. • In the last decade, oceanographic data have become much more accessible and new important measurement platforms, such as FerryBoxes and satellites, have provided better temporally and spatially resolved observations. • Our understanding of how large-scale atmospheric circulation affects the Baltic Sea climate, particularly in winter, has improved. Internal variability is strong illustrating the dominant stochastic behaviour of the atmosphere. • The heat and water cycles of the Baltic Sea are better understood. • The importance of surface waves in air–sea interaction is better understood, and Stokes drift and Langmuir circulation have been identified as likely playing an important role in surface water mixing in sea water. • We better understand sea ice dynamics and thermodynamics in the coastal zone where sea ice interaction between land and sea is crucial. • The Baltic Sea’s various straits and sills are of increasing interest in seeking to understand water exchange and mixing. • There has been increased research into the Baltic Sea coastal zone, particularly into upwelling, in the past decade. • Modelling of the Baltic Sea–North Sea system, including the development of coupled land–sea–atmosphere models, has improved. Despite marked progress in Baltic Sea research over the last decade, several gaps remain in our knowledge and understanding. The current understanding of salinity changes is limited, and future projections of salinity evolution are uncertain. In addition, modelling of the hydrological cycle in atmospheric climate models is severely biased. More detailed investigations of regional precipitation and evaporation patterns (including runoff), atmospheric variability, highly saline water inflows, exchange between sub-basins, circulation, and especially turbulent mixing are still needed. Furthermore, more highly resolved oceanographic models are necessary. In addition, models that incorporate more advanced carbon cycle and ecosystem descriptions and improved description of water–sediment interactions are needed. There is also a need for new climate projections and simulations with improved atmospheric and oceanographic coupled model systems. These and other research challenges are addressed by the recently formed Baltic Earth research programme, the successor of the BALTEX programme, which ended in 2013. Baltic Earth will treat anthropogenic changes and impacts together with their natural drivers. Baltic Earth will serve as a network for earth system sciences in the region, following in the BALTEX tradition but in a wider context.

Description: Throughout the last decades there has been a world-wide, general warming trend. In this study, we use the example of the Baltic Sea to resolve the overall estimated temperature trend into smaller, meso-scale spatial units. Afterwards, we investigate the spatially resolved potential impact of the temperature trend on larval survival for two important fish species, cod and sprat. We used two different sets of hydrographic data: (i) long-term temporally and depth-resolved data measured in situ originating from one geographic position and (ii) long-term horizontally resolved data, originating from a circulation model. In contrast to basin-wide integrated results, our modelling approach revealed different results related to smaller spatial scales. In shallow and coastal areas non-significant long-term temperature trends were observed. In some cases even decreasing temperature trends were found. Average distribution maps (1973–2010) of cod and sprat eggs and larvae confirmed the higher importance of central, deep basins as nursery grounds. Applying the temperature trends when calculating cod larval window of opportunity values, resulted in decreased durations of 1–3 days (~ 3–13%) in most areas. Sprat larval window of opportunity values mainly increased up to 4 days (~ 45%), indicating a potential reproduction advantage of sprat over cod under anticipated future temperature increase. Highlights ► We resolve the overall positive temperature trend in the Baltic into meso-scale spatial units and investigate the impact on larval survival for two important fish species, cod and sprat. ► In shallow and coastal areas non-significant or even negative temperature trends occurred. ► Cod larval window of opportunity values decreased by 1–3 days (~ 3–13%). ► Sprat larval window of opportunity values increased up to 4 days (~ 45%). ► Sprat will have a reproduction advantage over cod under anticipated future temperature change.

Description: Highlights: • Modeling of oxygen conditions of the Baltic Sea for the period 1970–2010 by a coupled hydrodynamic-oxygen consumption model • Determination of the evolution of hypoxic and anoxic areas for the period 1970–2010 for the total Baltic Sea • Determination of the frequencies of the occurrences of areas of oxygen deficiencies for the period 1970–2010 • Calculation of the Baltic Cod reproduction volumes in the Bornholm Basin for the period 1970–2010 Abstract: The Baltic Sea deep waters suffer from extended areas of hypoxia and anoxia. Their intra- and inter-annual variability is mainly determined by saline inflows which transport oxygenated water to deeper layers. During the last decades, oxygen conditions in the Baltic Sea have generally worsened and thus, the extent of hypoxic as well as anoxic bottom water has increased considerably. Climate change may further increase hypoxia due to changes in the atmospheric forcing conditions resulting in less deep water renewal Baltic inflows, decreased oxygen solubility and increased respiration rates. Feedback from climate change can amplify effects from eutrophication. A decline in oxygen conditions has generally a negative impact on marine life in the Baltic Sea. Thus, a detailed description of the evolution of oxygenated, hypoxic and anoxic areas is particularly required when studying oxygen-related processes such as habitat utilization of spawning fish, survival rates of their eggs as well as settlement probability of juveniles. One of today's major challenges is still the modeling of deep water dissolved oxygen, especially for the Baltic Sea with its seasonal and quasi-permanent extended areas of oxygen deficiency. The detailed spatial and temporal evolution of the oxygen concentrations in the entire Baltic Sea have been simulated for the period 1970–2010 by utilizing a hydrodynamic Baltic Sea model coupled to a simple pelagic and benthic oxygen consumption model. Model results are in very good agreement with CTD/O2-profiles taken in different areas of the Baltic Sea. The model proved to be a useful tool to describe the detailed evolution of oxygenated, hypoxic and anoxic areas in the entire Baltic Sea. Model results are further applied to determine frequencies of the occurrence of areas of oxygen deficiency and cod reproduction volumes.

Description: In order to detect shifts in community structure and function associated with global change, the natural background fluctuation in these traits must be known. In a 6 yr study we characterized the composition of young benthic communities at 7 sites along the 300 km coast of the Kiel and Lübeck bights in the German Baltic Sea and we quantified their interannual variability of taxonomic and functional composition. Along the salinity gradient from NW to SE, the relative abundance of primary producers decreased while that of heterotrophs increased. Along the same gradient, annual productivity tended to increase. Taxonomic and functional richness were higher in Kiel Bight as compared to Lübeck Bight. With increasing species richness functional group richness showed saturation indicating an increasing functional redundancy in species rich communities. While taxonomic fluctuations between years were substantial, functionality of the communities seem preserved in most cases. Environmental conditions potentially driving these fluctuations are winter temperatures and current regimes. We tentatively define a confidence range of natural variability in taxonomic and functional composition a departure from which might help identifying an ongoing regime shift driven by global change. In addition, we propose to use RELATE, a statistical procedure in the PRIMER (Plymouth Routines in Multivariate Ecological Research) package to distinguish directional shifts in time ("signal") from natural temporal fluctuations ("noise")