Entire Baltic Sea Research Articles

Changes in numbers and distribution of wintering Long-tailed Ducks Clangula hyemalis in Swedish waters during the last fifty years

The Long-tailed Duck Clangula hyemalis is the most common wintering waterfowl in Swedish marine waters but it is not well covered in the International Midwinter Counts due to its offshore distribution making special surveys necessary for the species. Surveys covering the entire Baltic Sea were organized for the first time in 1992/1993 with follow-up surveys in 2007–2011 and 2016. In Sweden, aerial line transect surveys covered all areas of importance for the Long-tailed Duck during the last two all-Baltic surveys. In the 1970s extensive regional surveys were done from coastguard boats and airplane. The total wintering population of Long-tailed Duck was estimated to be around 370,000 individuals in 2016 compared to 436,000 in 2009, and in the order of 1,400,000 in 1992/1993, indicating an ongoing decrease. Comparing the more recent surveys with the surveys from the 1970s, the most marked changes in wintering numbers were found east of Gotland, where only a small proportion of the numbers estimated in the 1970s remained. Decreases were also found in the waters around Öland, whereas no marked changes were noted for the smaller wintering populations of the east coast archipelagos.

Importance of coastal primary production in the northern Baltic Sea.

In this study, we measured depth-dependent benthic microalgal primary production in a Bothnian Bay estuary to estimate the benthic contribution to total primary production. In addition, we compiled data on benthic microalgal primary production in the entire Baltic Sea. In the estuary, the benthic habitat contributed 17 % to the total annual primary production, and when upscaling our data to the entire Bothnian Bay, the corresponding value was 31 %. This estimated benthic share (31 %) is three times higher compared to past estimates of 10 %. The main reason for this discrepancy is the lack of data regarding benthic primary production in the northern Baltic Sea, but also that past studies overestimated the importance of pelagic primary production by not correcting for system-specific bathymetric variation. Our study thus highlights the importance of benthic communities for the northern Baltic Sea ecosystem in general and for future management strategies and ecosystem studies in particular.Electronic supplementary materialThe online version of this article (doi:10.1007/s13280-016-0778-5) contains supplementary material, which is available to authorized users.

Sources of sedimentary biomarkers and proxies with potential paleoenvironmental significance for the Baltic Sea

The Baltic Sea is a shallow, semi-enclosed and intra-continental shelf sea characterized by anoxic bottom waters in the deepest basins, allowing for the preservation of sedimentary organic matter. In the present study, the most abundant, naturally-occurring lipids in surface sediments from the entire Baltic Sea and the Skagerrak area were identified and their potential sources were assigned. Together with long-chain n-alkanes derived from land plant leaf waxes, diploptene and branched glycerol dialkyl glycerol tetraethers (GDGTs) are of allochthonous origin, while isoprenoid GDGTs, hydroxylated isoprenoid GDGTs (OH-GDGTs), n-C25:1, n-C27:1 and n-C29:1 alkenes are autochthonous lipids. The isoprenoid and OH-GDGTs are probably derived from Thaumarchaeota and the long-chain n-alkenes from phototrophic organisms. Significant correlations were found between indexes based on isoprenoid and OH-GDGTs and Baltic Sea surface and bottom temperatures. The calibrations obtained for surface temperature have statistically similar slopes, but different intercepts than calibrations established for the Nordic Seas. The branched and isoprenoid tetraether index can be used to estimate the percentage of soil (terrestrial) organic matter in the sediments of the Baltic Sea. High values of the Paq’ ratio (defined here as the ratio of odd numbered n-C23 and n-C25 over n-C23 to n-C29 alkanes) in the northern Baltic Sea originate from the presence of both Sphagnum mosses in the drainage basin and submerged macrophytes, such as Potamogeton sp. and Myriophyllum sp., in the freshwater to brackish water of the coastal areas. The Paq’ ratio may thus reflect fluctuations in the regional expansion of freshwater to brackish coastal environments in the Baltic Sea.

Using satellite data to estimate partial pressure of CO2 in the Baltic Sea

AbstractIn this study we focused on estimating the pressure partial of CO2 (pCO2) in the entire Baltic Sea which can be considered a coastal area in its entirety. We used the self‐organizing multiple linear output (SOMLO) method to estimate the ocean surface pCO2 in the Baltic Sea from the remotely sensed sea surface temperature, chlorophyll a, colored dissolved organic matter, net primary production, and mixed‐layer depth. Uncertainties in the estimates include sparsity of in situ data used to train the algorithms, in particular, for some sectors and seasons. For this application we divided the Baltic Sea in four basins. When comparing the results obtained with this division to those obtained in previous studies, we notice a decrease in the root‐mean‐square error (<40 μatm) between the reconstruction of the pCO2 and their corresponding in situ measurements, as well as an increase of the correlation coefficient (> 0.96) between them. The outputs of this research have a horizontal resolution of 4 km and cover the 1998–2011 period. For the first time, a climatological mean distribution of surface water pCO2 over the Baltic Sea was calculated based on the SOMLO method with a mean pCO2 of 368.3 ± 100 μatm, and a range of 234–514 μatm. The seasonal variability is similar throughout the Baltic Sea, being high in winter and low in summer.

The Baltic Sea scale inventory of benthic faunal communities

AbstractThis study provides an inventory of the recent benthic macrofaunal communities in the entire Baltic Sea. The analyses of soft-bottom benthic invertebrate community data based on over 7000 locations in the Baltic Sea suggested the existence of 10 major communities based on species abundances and 17 communities based on species biomasses, respectively. The low-saline northern Baltic, characterized by silty sediments, is dominated by Monoporeia affinis, Marenzelleria spp., and Macoma balthica. Hydrobiidae, Pygospio elegans, and Cerastoderma glaucum dominate the community in sandy habitats off the Estonian west coast and in the southeastern and southern Baltic Sea. Deep parts of the Gulf of Finland and central Baltic Sea often experience hypoxia, and when oxygen levels in these regions recover, Bylgides sarsi was the first species to colonize. The southwestern Baltic Sea, with high salinity, has higher macrofaunal diversity compared with the northern parts. To spatially interpolate the distribution of the major communities, we used the Random Forest method. Substrate data, bathymetric maps, and modelled hydrographical fields were used as predictors. Model predictions were in good agreement with observations, quantified by Cohen's κ of 0.90 for the abundance and 0.89 in the wet weight-based model. Misclassifications were mainly associated with uncommon classes in regions with high spatial variability. Our analysis provides a detailed baseline map of the distribution of benthic communities in the Baltic Sea to be used both in science and management.

Rating species sensitivity throughout gradient systems – a consistent approach for the Baltic Sea

Evaluating the state of benthic communities has played an important role in water quality assessments. Indices incorporating species sensitivities, richness and densities are commonly applied. In Europe, the importance of benthic indices has increased in the last years with the implementation of the European Marine Strategy Framework Directive (MSFD) which at the same time demands the applicability of an index across regional scales. To date, environmental variability is rarely considered in benthic indices and most sensitivity rankings have the disadvantages of static values (i.e. the same value in all areas), expert judgement and a limited geographical range.This study presents species sensitivity values calculated along environmental gradients for the Baltic Sea. Sensitivities were calculated according to the procedure of the Benthic Quality Index (BQI). We created a matrix of subregions, classes of salinity, depth and gear to identify comparable subsets for data analysis. Altogether, 19 subsets were defined within the Baltic Sea basins. Sensitivity values were calculated for 329 species out of a total of 678 species that were recorded in this study. Sensitivity values of taxa vary between subsets as it was expected for different environmental conditions. Most sensitivity values can be assigned to species occurring in euhaline and polyhaline waters. Distribution of species with high and low sensitivity values differed along the salinity gradient. In euhaline waters more species with high sensitivity values occurred than species with low sensitivity values, while in mesohaline waters the ratio of high and low sensitivity values among species was almost equal. In oligohaline waters more species with lower sensitivity values were present.For the first time, sensitivity values were calculated for a large number of species using the same method for the entire Baltic Sea. This results in a Baltic-wide comprehensive set of sensitivity values based on a dataset across subregional borders, and divided along environmental gradients and gear type. The same principles can be applied to transient waters from rivers to coastal lagoons as well as to other environments with gradients of, e.g. hydrodynamic characteristics. Publicly available sensitivity values will increase transparency and support the improvement of state assessments under the MSFD.

Influence of sea level rise on the dynamics of salt inflows in the Baltic Sea

AbstractThe Baltic Sea is a marginal sea, located in a highly industrialized region in Central Northern Europe. Saltwater inflows from the North Sea and associated ventilation of the deep exert crucial control on the entire Baltic Sea ecosystem. This study explores the impact of anticipated sea level changes on the dynamics of those inflows. We use a numerical oceanic general circulation model covering both the Baltic and the North Sea. The model successfully retraces the essential ventilation dynamics throughout the period 1961–2007. A suite of idealized experiments suggests that rising sea level is associated with intensified ventilation as saltwater inflows become stronger, longer, and more frequent. Expressed quantitatively as a salinity increase in the deep central Baltic Sea, we find that a sea level rise of 1 m triggers a saltening of more than 1 PSU. This substantial increase in ventilation is the consequence of the increasing cross section in the Danish Straits amplified by a reduction of vertical mixing.

Modelling the contributions to marine acidification from deposited SOx, NOx, and NHx in the Baltic Sea: Past and present situations

We have examined the effects of historical atmospheric depositions of sulphate, nitrate, and ammonium from land and shipping on the acid–base balance in the Baltic Sea. The modelling considers the 1750–2014 period, when land and ship emissions changed greatly, with increasing carbon dioxide concentrations, SOx, NOx, and NHx emissions, and nutrient loads.The present results indicate that Baltic Sea acidification due to the atmospheric deposition of acids peaked around 1980, with a pH cumulative decrease of approximately 10−2 in surface waters. This is one order of magnitude less than the cumulative acidification due to increased atmospheric CO2. The acidification contribution of shipping is one order of magnitude less than that of land emissions. However, the pH trend due to atmospheric acids has started to reverse due to reduced land emissions, though the effect of shipping is ongoing.The effect of strong atmospheric acids on Baltic Sea water depends on the region and period studied. The largest total alkalinity sink per surface area is in the south-western Baltic Sea where shipping is intense. Considering the entire Baltic Sea over the 2001–2010 period, the pH changes are approximately −3×10−3 to −11×10−3 and −4×10−4 to −16×10−4 pH units attributable to all emissions and ship emissions only, respectively. The corresponding changes in total alkalinity are approximately −10 to −30µmolkg−1 and −1 to −4µmolkg−1 attributable to all emissions and ship emissions only, respectively.

Early 2015 Performance In Baltic Sea Ports: Forecasts Of Estonian Performance For Entire Year

Abstract Many changes have appeared in year 2015. In Europe economic malaise has continued since debt crisis started in year 2010, and although its effects on Northern Europe have started to diminish, new economic dark clouds have appeared through sanctions set by both European Union and Russia to each other during year 2014. Together with these, shipping sector has been under pressure due to strict sulphur regulation implemented from early 2015 onwards in the entire Baltic Sea Region. Due to these factors, sea ports at Baltic Sea have been under pressure during the first months of 2015, this particularly concerning container handling. Based on our regression model forecast, Estonia and Port of Tallinn shall have clearly declining container handling year ahead. However, overall handling at sea port is not so easy to forecast.

Spatial and temporal variability of sea surface temperature in the Baltic Sea based on 32-years (1982–2013) of satellite data

Summary Satellite measurements provide synoptic view of sea surface temperature (SST) and can be used to trace global and regional climate trends. In this study we have examined the multiyear trends and variability of the Baltic Sea SST using 32-years (1982–2013) of satellite data. Our results indicate that there is a statistically significant trend of increasing SST in the entire Baltic Sea, with values ranging from 0.03 to 0.06°C year−1, depending on the location. SSTs averaged over the entire Baltic Sea increase at the rate of 0.05°C year−1. Higher values of SST trend are generally present in the summer months, while trend is not statistically significant in the winter months. The seasonal cycle of SST in the Baltic Sea is characterized by well-defined winter and summer seasons. The average amplitude (16–18°C) of this cycle is significantly larger than in the North Sea waters located at the same latitudes as the Baltic Sea. The analyzed data set also highlights considerable interannual SST variability, which is coherent in different regions of the Baltic Sea and significantly correlated with interannual variability of the air temperature. SST variability in the Baltic Sea in winter can be linked to the North Atlantic Oscillation index.

Large-scale species invasion into a low-diversity system: spatial and temporal distribution of the invasive polychaetes Marenzelleria spp. in the Baltic Sea

Invasive species are considered to be a major threat to both terrestrial and aquatic ecosystems worldwide due to their negative effects on biodiversity, ecosystem functioning and species interactions. This study investigates with the help of large-scale monitoring datasets the spatial and temporal distribution of a successful invader, the polychaete species complex Marenzelleria spp. Mesnil 1896 into a species-poor, brackish water ecosystem in northern Europe, the Baltic Sea. The species complex has spread throughout the entire Baltic Sea and has become a dominant member of the communities especially in deeper, hypoxia-affected areas. The dynamics of the invasion follow a boom-and-bust pattern, with abundances peaking almost simultaneously throughout the Baltic Sea in 2005–2006. Possible mechanisms behind the successful establishment are changes in hydrography and species composition. Hypothesized and observed ecological impacts are reviewed. Evidence suggests a multitude of potential, sometimes contrasting, impacts. Due to the widespread distribution and high abundances, the biology and ecology of Marenzelleria spp. in the Baltic Sea need to be investigated further in order to assess its impact on ecosystem structure and functioning.

Seasonal and Regional Patterns in Performance for a Baltic Sea Drainage Basin Hydrologic Model

AbstractThis study evaluates the ability of the Catchment SIMulation (CSIM) hydrologic model to describe seasonal and regional variations in river discharge over the entire Baltic Sea drainage basin (BSDB) based on 31 years of monthly simulation from 1970 through 2000. To date, the model has been successfully applied to simulate annual fluxes of water from the catchments draining into the Baltic Sea. Here, we consider spatiotemporal bias in the distribution of monthly modeling errors across the BSDB since it could potentially reduce the fidelity of predictions and negatively affect the design and implementation of land‐management strategies. Within the period considered, the CSIM model accurately reproduced the annual flows across the BSDB; however, it tended to underpredict the proportion of discharge during high‐flow periods (i.e., spring months) and overpredict during the summer low flow periods. While the general overpredictions during summer periods are spread across all the subbasins of the BSDB, the underprediction during spring periods is seen largely in the northern regions. By implementing a genetic algorithm calibration procedure and/or seasonal parameterization of subsurface water flows for a subset of the catchments modeled, we demonstrate that it is possible to improve the model performance albeit at the cost of increased parameterization and potential loss of parsimony.

Impact of saltwater inflows on phosphorus cycling and eutrophication in the Baltic Sea: a 3D model study

The impact of dense saltwater inflows on the phosphorus dynamics in the Baltic Sea is studied from tracer experiments with a three-dimensional physical model. Model simulations showed that the coasts of the North West Gotland Basin and the Gulf of Finland, the Estonian coast in the East Gotland Basin are regions where tracers from below the halocline are primarily lifted up above the halocline. After 1 yr tracers are accumulated at the surface along the Swedish east coast and at the western and southern sides of Gotland. Elevated concentrations are also found east and southeast of Gotland, in the northern Bornholm Basin and in the central parts of the East Gotland Basin. The annual supplies of phosphorus from the deeper waters to the productive surface layers are estimated to be of the same order of magnitude as the waterborne inputs of phosphorus to the entire Baltic Sea. The model results suggest that regionally the impact of these nutrients may be quite large, and the largest regional increases in surface concentrations are found after large inflows. However, the overall direct impact of major Baltic inflows on the annual uplift of nutrients from below the halocline to the surface waters is small because vertical transports are comparably large also during periods without major inflows. Our model results suggest that phosphorus released from the sediments between 60 and 100 m depth in the East Gotland Basin contributes to the eutrophication, especially in the coastal regions of the eastern Baltic Proper.

A new approach to model oxygen dependent benthic phosphate fluxes in the Baltic Sea

The new approach to model the oxygen dependent phosphate release by implementing formulations of the oxygen penetration depths (OPD) and mineral bound inorganic phosphorus pools to the Swedish Coastal and Ocean Biogeochemical model (SCOBI) is described. The phosphorus dynamics and the oxygen concentrations in the Baltic proper sediment are studied during the period 1980–2008 using SCOBI coupled to the 3D-Rossby Centre Ocean model. Model data are compared to observations from monitoring stations and experiments.The impact from oxygen consumption on the determination of the OPD is found to be largest in the coastal zones where also the largest OPD are found. In the deep water the low oxygen concentrations mainly determine the OPD. Highest modelled release rate of phosphate from the sediment is about 59×103t P year−1 and is found on anoxic sediment at depths between 60–150m, corresponding to 17% of the Baltic proper total area. The deposition of organic and inorganic phosphorus on sediments with oxic bottom water is larger than the release of phosphorus, about 43×103t P year−1. For anoxic bottoms the release of total phosphorus during the investigated period is larger than the deposition, about 19×103t P year−1. In total the net Baltic proper sediment sink is about 23.7×103t P year−1. The estimated phosphorus sink efficiency of the entire Baltic Sea is on average about 83% during the period.

Nitrate source identification in the Baltic Sea using its isotopic ratios in combination with a Bayesian isotope mixing model

Abstract. Nitrate (NO3−) is the major nutrient responsible for coastal eutrophication worldwide and its production is related to intensive food production and fossil-fuel combustion. In the Baltic Sea NO3− inputs have increased 4-fold over recent decades and now remain constantly high. NO3− source identification is therefore an important consideration in environmental management strategies. In this study focusing on the Baltic Sea, we used a method to estimate the proportional contributions of NO3− from atmospheric deposition, N2 fixation, and runoff from pristine soils as well as from agricultural land. Our approach combines data on the dual isotopes of NO3− (δ15N-NO3− and δ18O-NO3−) in winter surface waters with a Bayesian isotope mixing model (Stable Isotope Analysis in R, SIAR). Based on data gathered from 47 sampling locations over the entire Baltic Sea, the majority of the NO3− in the southern Baltic was shown to derive from runoff from agricultural land (33–100%), whereas in the northern Baltic, i.e. the Gulf of Bothnia, NO3− originates from nitrification in pristine soils (34–100%). Atmospheric deposition accounts for only a small percentage of NO3− levels in the Baltic Sea, except for contributions from northern rivers, where the levels of atmospheric NO3− are higher. An additional important source in the central Baltic Sea is N2 fixation by diazotrophs, which contributes 49–65% of the overall NO3− pool at this site. The results obtained with this method are in good agreement with source estimates based upon δ15N values in sediments and a three-dimensional ecosystem model, ERGOM. We suggest that this approach can be easily modified to determine NO3− sources in other marginal seas or larger near-coastal areas where NO3− is abundant in winter surface waters when fractionation processes are minor.

Policy Goals for Improved Water Quality in the Baltic Sea: When do the Benefits Outweigh the Costs?

This paper develops and applies a spatially explicit bioeconomic model to study transboundary nutrient pollution of the Baltic Sea. We combine catchment, marine and economic models covering the entire Baltic Sea region to weigh the costs of nutrient abatement and the benefits of improved water quality and solve for the optimal level of water protection. The overall benefits of the Baltic Sea Action Plan, the present convention on nutrient abatement, clearly outweigh the costs. Nevertheless, the total cost could be almost halved if the mix of measures and the regional targets were planned in a spatially cost–effective manner and if the consequent reductions of nitrogen and phosphorus, the two nutrients causing eutrophication, were better balanced. Policy optimizations, however, suggest that the optimal level of nutrient abatement is somewhat lower than the more ambitious level envisaged by the convention. The welfare gains from cost sharing that makes the optimal level of nutrient abatement worthwhile for all littoral countries would be 170 million euros annually.

Distribution and estimated release of sediment phosphorus in the northern Baltic Sea archipelagos

Phosphorus contents in the sediments were determined in archipelago areas of the northern Baltic Sea (Svealand in Sweden and Åland, SW Finland and W Uusimaa in Finland) during 2008–2012. Spatial and vertical distribution of phosphorus was studied by analysing sediment samples from 345 stations of different seabed substrate types. A sequential extraction method was applied to evaluate the pool of the potentially mobile phosphorus, i.e., the amount of phosphorus that can be expected to be released from sediments to water with time, and possibly support primary production. In addition, vertical distribution of immobile phosphorus forms in the sediments was used as a tool to assess phosphorus burial. The uppermost 2 cm of sediments were calculated to contain 126,000 tonnes of phosphorus in the study area covering 19,200 km2 of the seafloor. Subtracting the assumed average background content (i.e. that assumed to be buried) of this total phosphorus content gave an estimation of 31,000−37,000 tonnes of potentially mobile phosphorus at the sediment surface. Redox sensitive iron-bound phosphorus accounts for two thirds of this pool. Compared with the total phosphorus input from the catchment of the entire Baltic Sea – 29,000 tonnes in 2009 – it can be concluded that the store of phosphorus that can be released with time from the sediments is large, and that internal phosphorus recycling processes thus may play a key role in phosphorus fluxes in the coastal zone. Spreading of hypoxia in the future, as recent modelling and sediment proxy results suggest, is likely to severely deteriorate the water quality, particularly in the archipelago areas where the water exchange is slow.

Quantifying the heterogeneity of hypoxic and anoxic areas in the Baltic Sea by a simplified coupled hydrodynamic-oxygen consumption model approach

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.

Could submarine groundwater discharge be a significant carbon source to the southern Baltic Sea?

Submarine Groundwater Discharge (SGD) is an important yet poorly recognised pathway of material transport to the marine environment. This work reports on the results of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentrations and loads in the groundwater seeping into the southern Baltic Sea. Most of the research was carried out in the Bay of Puck (2009–2010), while in 2013 the study was extended to include several other groundwater seepage impacted areas situated along the Polish coastline. The annual average concentrations of DIC and DOC in the groundwater were equal to 64.5±10.0mg C L−1 and 5.8±0.9mg C L−1 respectively. The carbon specific flux into the Bay of Puck was estimated at 850mgm−2yr−1. The loads of carbon via SGD were scaled up for the Baltic Sea sub-basins and the entire Baltic Sea. The DIC and DOC fluxes via SGD to the Baltic Sea were estimated at 283.6±66.7 kt yr−1 and 25.5±4.2 kt yr−1. The SGD derived carbon load to the Baltic Sea is an important component of the carbon budget, which gives the sea a firmly heterotrophic status.

Tidal oscillations in the Baltic Sea

Long-term hourly data from 35 tide gauge stations, including 15 stations in the Gulf of Finland, were used to examine tidal sea level oscillations of the Baltic Sea. High-resolution spectral analysis revealed the well-defined fine structure of tidal peaks with diurnal peaks at most stations being higher than semidiurnal. At some stations (e.g., Narva, Daugava, and Wladyslawowo), high frequency radiational tidal peaks with periods multiple of the solar day (3, 4, 5, 6, and 8 cpd) were detected; the respective oscillations are supposed to be caused by seabreeze winds. Harmonic analysis of tides for individual yearly sea level series followed by vector averaging over the entire observational period was used to estimate the amplitudes and phases of 16 tidal constituents. The maximum tidal oscillations of 17–19 cm were found to be observed in the Gulf of Finland and, first of all, in Neva Bay (in the head of the gulf). Diurnal or mixed diurnal tides are predominant in almost the entire Baltic Sea. The comparison of the observed tides with those theoretically computed showed that the existing numerical models of the main tidal harmonics generally quite accurately reproduce the structure of the tides in the Baltic Sea except for some regions of the Gulf of Bothnia.