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The Gulf of Riga is a relatively shallow bay connected to the deeper central Baltic Sea (Baltic Proper) via straits with sills. The decrease in the near-bottom oxygen levels from spring to autumn is a common feature in the gulf, but in 2018, extensive hypoxia was observed. We analyzed temperature, salinity, oxygen, and nutrient data collected in 2018, along with historical data available from environmental databases. Meteorological and hydrological data from the study year were compared with their long-term means and variability. We suggest that pronounced oxygen depletion occurred in 2018 due to a distinct development of vertical stratification. Seasonal stratification developed early and was stronger in spring–summer 2018 than on average due to high heat flux and weak winds. Dominating northeasterly winds in early spring and summer supported the inflow of saltier waters from the Baltic Proper that created an additional deep pycnocline restricting vertical transport between the near-bottom layer (NBL) and the water column above. The estimated oxygen consumption rate in the NBL in spring–summer 2018 was about 1.7 mmolO2m-2h-1, which exceeded the oxygen input to the NBL due to advection and vertical mixing. Such a consumption rate leads to near-bottom hypoxia in all years when vertical mixing in autumn reaches the seabed later than on average according to the long-term (1979–2018) meteorological conditions. The observed increase in phosphate concentrations in the NBL in summer 2018 suggests a significant sediment phosphorus release in hypoxic conditions counteracting the mitigation measures to combat eutrophication. Since climate change projections predict that meteorological conditions comparable to those in 2018 will occur more frequently, extensive hypoxia would be more common in the Gulf of Riga and other coastal basins with similar morphology and human-induced elevated input of nutrients.

Circulation plays an essential role in the creation of physical and biogeochemical fluxes in the Baltic Sea. The main aim of the work was to study the quasi-steady circulation patterns under prevailing forcing conditions. A total of 6 months of continuous vertical profiling and fixed-point measurements of currents, two month-long underwater glider surveys, and numerical modeling were applied in the central Baltic Sea. The vertical structure of currents was strongly linked to the location of the two pycnoclines: the seasonal thermocline and the halocline. The vertical movements of pycnoclines and velocity shear maxima were synchronous. The quasi-steady circulation patterns were in geostrophic balance and highly persistent. The persistent patterns included circulation features such as upwelling, downwelling, and boundary currents, as well as a sub-halocline gravity current. The patterns had a prevailing zonal scale of 5–60 km as well as considerably higher magnitude and different direction than the long-term mean circulation pattern. A northward (southward) geostrophic boundary current in the upper layer was observed along the eastern coast of the central Baltic in the case of southwesterly (northerly) wind. The geostrophic current at the boundary was often a consequence of wind-driven, across-shore advection. The sub-halocline quasi-permanent gravity current with a width of 10–30 km from the Gotland Deep to the north over the narrow sill separating the Fårö Deep and Nothern Deep was detected in the simulation, and it was confirmed by an Argo float trajectory. According to the simulation, a strong flow, mostly to the north, with a zonal scale of 5 km occurred at the sill. This current is an important deeper limb of the overturning circulation of the Baltic Sea. The current was stronger with northerly winds and restricted by the southwesterly winds. The circulation regime had an annual cycle due to seasonality in the forcing. The boundary current was stronger and more frequent northward during the winter period. The sub-halocline current towards the north was strongest in March–May and weakest in November–December.

In this study, we conduct numerical simulations to investigate the transport and accumulation of microplastics (MPs) in the Gulf of Finland (eastern Baltic Sea). The transport of MPs is simulated using a Lagrangian particle tracking model, developed by the authors, which accounts for the physical transport and transformational processes specific to MPs, such as diffusion, beaching, resuspension, and biofouling.Positively and negatively buoyant particles were included to represent different types of MPs. High-resolution (250 m) 3D hydrodynamic and biogeochemical model data are used as input to the MPs model. Major rivers and wastewater treatment plants around the Gulf of Finland are the sources of MPs in the model.The main objectives of the study are 1) to perform a sensitivity analysis of four processes (beaching, mixing, resuspension, and biofouling) to investigate the importance of each process on the distribution of MPs in the Gulf of Finland; and 2) to execute a multi-year simulation using realistic MPs input to locate the main accumulation areas in sediments and beaches around the Gulf of Finland.Results of the initial sensitivity analysis indicate that the changes in the parametrization of beaching have a stronger effect on the horizontal distribution of MPs in off-shore sediments than the changes in the parametrization of resuspension. The maximum MPs concentration is observed near the sea surface; however, MPs have also spread into the water column.

Stratification plays an essential role in the marine ecosystem, with a shallow mixed layer being one of the preconditions for enhanced primary production in the ocean. In the Baltic Sea, the general understanding is that the upper mixed layer (UML) is well below the euphotic zone in winter. In this study, we demonstrate that wintertime shallow stratification is common in the Gulf of Finland and it forms at a depth comparable to the euphotic zone in January–February. The onset of restratification is likely associated with the annual cycle of westerly winds, which ease off in late January–early February. Stratification is first invoked along the northern coast by the westward advection of riverine water forced by easterly winds and is expanded to the south when the prevailing wind direction changes from an easterly to a westerly direction. Haline stratification emerges approximately 1 month later in the southern part of the gulf. Winter restratification can occur in the entire gulf and also in the absence of ice; thus, it is a regular seasonal feature in the area. Interannual variations in the wintertime UML correspond with variations in the North Atlantic Oscillation. Chlorophyll a concentrations in winter can be comparable to mid-summer; the limiting factor for phytoplankton bloom in winter is likely insufficient solar radiation.

The development of submesoscale phenomena in seas is recognized with the refinement of observational and model grids. While in situ measurements are discrete and limited in time and/or space, numerical models could fill the entire sea with simulation data. Three high-resolution glider missions were conducted in the Gulf of Finland, Baltic Sea, in 2018–2019 with an aim to detect submesoscale processes. We captured frontal submesoscale features affecting water column structure in spring 2018. In this study, a three-dimensional hydrodynamic model GETM (General Estuarine Transport Model) with eddy-resolving grid spacing of 0.5 nautical miles and submesoscale permitting grid spacing of 0.125 nautical miles is used to simulate the dynamics in the study area. We compare the model results with the measurements, present differences, and propose probable explanations. The submesoscale processes are related to the energetic mesoscale flow field and they contribute to the energy transfer at smaller scales. We investigate the statistical features to characterize the submesoscale structures in the simulated area during the glider missions. The regions of order one Rossby and Richardson numbers characterize the spots with active submesoscale dynamics. Further, horizontal buoyancy gradients are one of the primary sources of submesoscale processes. We propose that the smaller-scale tracer patterns at a lateral scale up to a km demonstrated an ageostrophic secondary circulation. The phenomenon illustrates the relation between horizontal and vertical structures. The high-resolution simulation allows us to have a three-dimensional view of the submesoscale features and describe the events that permitted the smaller-scale structures to arise along with some estimate for the re-occurrence of the event.

Modern research methods enable unfolding the structure of the water column with higher resolution than ever, revealing the importance of submesoscale. Submesoscale processes have intermediate space and time scales of

Highlights • Hazardous substances (HS) assessed in the bottom sediments of the Gulf of Finland. • Organotins (mainly TBT) are the most distributed HS along the shipping routes. • Contamination by PAHs increased near the ports indicating fuel incineration. • High ecological risk attributed to the heavy metals in the deep offshore areas. • Simulated HS accumulation pattern in qualitative agreement with field observations. Contamination by hazardous substances is one of the main environmental problems in the eastern Gulf of Finland, Baltic Sea. A trilateral effort to sample and analyse heavy metals (HMs), polycyclic aromatic hydrocarbons (PAHs), and organotins from bottom sediments in 2019–2020 were conducted along with harvesting historical data in Russian, Estonian and Finnish waters. We suggest that the input of organotins still occurs along the ship traffic routes. The tributyltin content exceeded the established quality criteria up to more than 300 times. High contamination by PAHs found near the ports, most likely originate from incomplete fuel incineration processes. The Neva River Estuary and Luga Bay might potentially suffer from severe cadmium contamination. The high ecological risk attributed to the HMs was detected at deep offshore areas. The simulated accumulation pattern qualitatively agrees with field observations of HMs in sediments, demonstrating the potential of numerical tools to tackle the hazardous substances problems.

Stratification plays an essential role in the marine system. The shallow mixed layer is one of the preconditions for the enhanced primary production in the ocean. The general understanding is that the mixed layer is well deeper than the euphotic zone in the Baltic Sea during winter. In this work, we demonstrate the wintertime stratification is a common phenomenon in the Gulf of Finland. Shallow haline stratification at the depth comparable to the euphotic zone depth forms in late January–early February. Stratification is evoked by the positive buoyancy flux created by the westward advection of riverine water along the northern coast of the gulf. Fresher water and haline stratification appeared approximately one month later in the southern part of the gulf. The phenomena can occur in the whole gulf and also without ice. Chl a concentration and phytoplankton biomass in winter can be comparable to mid-summer. The limiting factor for phytoplankton bloom in winter is likely insufficient solar radiation.

In this study, we present dissolved oxygen (DO) concentration data collected during an observational program conducted in the central Baltic Sea, six months after a Major Baltic Inflow (MBI) event from December 2014 has ended a long period of stagnation. The main working area was the Eastern Gotland Basin (EGB). The spatial distribution of DO was obtained by two ship based CTD-systems (classical CTD probe, towed undulating CTD probe) and by using a glider (Slocum), while the temporal variability of DO concentrations was revealed by a profiling mooring (GODESS - Gotland Deep Environmental Sampling Station). In this paper, we compare the performance of all platforms used and show that integrating modern oceanographic tools into an existing observation network can help to enhance the capability of the network to capture the spatiotemporal variability of DO in the Baltic Sea on both a large (seasonal; 10–500 km) and a small (diurnal; 1–10 km) scale. It was found that in comparison to a classical standard CTD system, a towed undulating CTD system (ScanFish) is better suited to resolve the meso- and submeso-scale distribution patterns of DO concentrations. In particular, in the sub-halocline layers where the variability of DO concentrations was consistently high (1–70 µM). The highly dynamic post-inflow situation was also observed by the glider and the profiling mooring. For comparison purposes four different sub-halocline layers in a depth range from 80 m to 180 m depth were defined and the mean oxygen content of these layers was calculated for both platforms. It turned out, that the mean DO concentrations for the different layers, which were determined from glider and mooring measurements, differ only slightly from each other. Differences were greatest (up to 7 µM) below the halocline (80–120m) and could be explained by the spatial and temporal variability of DO concentrations in this area and by the large response time of the gliders oxygen optode. However, the temporal development of inflow events could be determined and investigated in more detail by using autonomous platforms. The frequency and size of smaller inflows, for instance, that move along the halocline or in deeper layers thus could be evaluated.

Establishment of distinct circulation patterns in the Gulf of Finland was observed by a targeted measurement campaign in winter 2013–2014. Strong and long enough up-estuary wind events caused a collapse of vertical stratification and development of a barotropic flow system consisting of an outflow in the open part and inflow along the coasts. In the periods without such unidirectional wind forcing, but when the water column remained weakly stratified, the residual barotropic inflow in the open gulf and outflow along the coasts was observed. In the case of moderate wind forcing, the three-layer vertical stratification and flow structure developed in the gulf. It is shown that the along-gulf expansion of the fresher water tongue in the surface layer as well as the up-estuary penetration of the saltwater wedge in the near-bottom layer followed well the long-term (monthly) changes in the cumulative along-gulf wind stress. The dynamics of the near-bottom saltwater wedge determined the extent of hypoxic bottoms and, as suggested by the firm correlation between the near-bottom phosphate concentration and salinity, the nutrient conditions in the near-bottom layer. The lateral transport of phosphorus, strengthened vertical stratification in the Northern Baltic Proper and suggested increase in the frequency of stratification collapses in the Gulf of Finland in winter predict that the eutrophication effects would not diminish significantly in this estuary in the nearest future.

Bottom-mounted ADCP measurements from 10 installations, collected between 2009 and 2014 and each lasting several months, are analysed in order to distinguish between different flow regimes, and to detect variability (a) along the thalweg of the elongated basin, with different regimes in summer and in winter, and (b) on the coastal slopes. In the deep thalweg area the mean flow speed amounts to 6–13 cm s−1, whereas the maximum speeds appear in winter near the bottom of the basin, and in summer within the halocline (around 70 m depth). The mean zonal flow component reveals a nearly depth uniform inflow during winter, and a layered inflow-outflow during summer. In years where up-estuary (W to SW) winds are stronger during the summer, inflow dominates in upper layers, and anti-estuarine outflow dominates in deeper layers. This causes the export of a salt wedge, and the weakening of haline stratification. Infra-low frequency zonal currents (i.e. excluding topographic waves etc. with periods of less than 10 days) have a structure which is uniform with depth for 53% of the time in winter; in summer, a layered structure is present 65% of the time. However, during both periods the reversed estuarine flow (inflow in upper layers and outflow in the bottom layer) appears, on average, for 30% of the time. The deep flow zonal component is well correlated with westward winds during summer (r = 0.84), and south-westward winds during winter (r = 0.77). On the coastal slopes, the speed of the currents are lower than in the thalweg region, and they decay with depth. In the vertical the flow exhibits a layered structure in both the winter and summer seasons.

Eutrophication and consequent increase in biomass production and sedimentation of organic material cause oxygen depletion of the deep layers and an increase in hypoxic bottom areas in the Baltic Sea.The Baltic Sea – a semi-enclosed brackish sea – has restricted water exchange with the North Sea. High fresh water runoff and sporadic inflows of saline water through the Danish Straits maintain stratification. Seasonal thermocline and quasi-permanent halocline, their vertical location, shape and strength are sensitive to atmospheric forcing and influence the oxygen depletion in the near-bottom layer. Physical processes altering deoxygenation in the three sub-basins of the Baltic Sea (Baltic Proper, Gulf of Finland and Gulf of Riga) are under scope of the present overview. Permanent halocline is present in the deep Baltic Proper, while in the Gulf of Finland, it occasionally vanishes during winter. Complete mixing occurs in each winter in the shallow Gulf of Riga separated from the Baltic Proper by the sill. We show that the bathymetry, combined with physical drivers, causes distinct spatial and temporal patterns of oxygen depletion in the basins. The results presented here are a summary of in-situ measurement campaigns conducted by the research vessel, underwater glider, autonomous vertical profiler and bottom moorings in 2011–2020.Large barotropic inflows from the North Sea temporarily ventilate the deep layer of the Central Baltic Proper, but rather intensify hypoxia in the Northern Baltic Proper and the Gulf of Finland. Wind-driven estuarine circulation alterations shape the hypoxic area and volume in the Gulf of Finland considerably. Seaward winds support estuarine circulation and the advection of hypoxic saltier water of the Northern Baltic Proper into the gulf deep layer. The landward wind can reverse estuarine circulation, the collapse of stratification and mixing of the whole water column in winter (when the seasonal thermocline is absent), thus, temporarily improving oxygen conditions in the deep layer of the gulf. Intrusion of cold saltier water of the Baltic Proper over the sill into the Gulf of Riga deep layer strengthens water column stratification and supports hypoxia formation in summer. Such a water exchange regime is related to the northerly wind forced upwelling along the eastern coast of the Baltic Proper. The role of submesoscale processes on vertical mixing and deep layer ventilation is still unclear, and the data of high-resolution in situ measurements in the Baltic Sea is limited yet. Preliminary results from the dedicated underwater glider surveys conducted at the coastal slope of Eastern Baltic Proper in 2019-2020 will be presented.

Water samples from the different basins of the Baltic Sea and North Sea were collected during four cruises taken place in 2016–2017. Altogether 60 water samples for 3H activity concentration and 120 water samples for stable isotopes ratio (δ18O and δ2H) were analysed. Exceptionally strong inflow activity from North Sea to the Baltic Sea occurred prior our surveys. The data obtained enabled comprehensive evaluation of tritium and stable isotopes distribution in waters of Baltic Sea and contributing inputs, including riverine and atmospheric freshwater flux and saline water inflow through the Danish Straits. By comparing with the previous studies, we noticed a tendency toward an increase in the δ18O and δ2H values of zero-salinity component over time, which may be due to climate change. In the case of tritium, the influence of artificial sources has also been noticed.

The area off the Changjiang Estuary is under strong impact of fresh water and anthropogenic nutrient load from the Changjiang River. The seasonal hypoxia in the area has variable location and range, but the decadal trend reveals expansion and intensification of the dissolved oxygen (DO) depletion. Two oceanographic cruises, conducted in summer 2015 and 2017, revealed very different stratification and DO conditions in the area. Strongly inclined oxycline well correlated with the thermocline in both years. Southerly wind caused reversal of the Chinese Coastal Current and as a result, spreading of the CDW (Changjiang Diluted Water) caused pronounced hypoxic zone in the area east and northeast of the river mouth in 2017. Hypoxic layer started right below the CDW layer at 5–8 m depth and extended down to bottom. Strong DO depletion was also observed in the shallow coastal slope in the southern part of the study area. High DO utilization there closely coincided with the interaction zone of the upwelling and fresher surface water. The stratification and hypoxia pattern observed in the area in 2017 is prevailing phenomena during summers if considering the long-term wind statistics. Northeasterly winds supported southward transport of the CDW before the survey in 2015. Consequently, low DO was found in the southern part of the study area while subsurface layer in the northern part was ventilated. Weaker than long-term average summer monsoon is required for the existence of such pattern. Importance of the wind forcing was confirmed by remotely sensed sea surface salinity fields and by circulation simulation. We suggest wind forcing, together with river run-off are likely main contributors of determining the synoptic, seasonal and inter-annual time scale variations of the extent and location of low DO areas off the Changjiang Estuary.

High-resolution numerical modeling, remote sensing, and in situ data have revealed significant role of sub-mesoscale features in shaping the distribution pattern of tracers in the ocean's upper layer. However, in situ measurements are difficult to conduct with the required resolution and coverage in time and space to resolve the sub-mesoscale, especially in such relatively shallow basins as the Gulf of Finland, where the typical baroclinic Rossby radius is 2–5 km. To map the multi-scale spatiotemporal variability in the gulf, we initiated continuous measurements with autonomous devices, including a moored profiler and Ferrybox system, which were complemented by dedicated research-vessel-based surveys. The analysis of collected high-resolution data in the summers of 2009–2012 revealed pronounced variability at the sub-mesoscale in the presence of mesoscale upwelling/downwelling, fronts, and eddies. The horizontal wavenumber spectra of temperature variance in the surface layer had slopes close to −2 between the lateral scales from 10 to 0.5 km. Similar tendency towards the −2 slopes of horizontal wavenumber spectra of temperature variance was found in the seasonal thermocline between the lateral scales from 10 to 1 km. It suggests that the ageostrophic sub-mesoscale processes could contribute considerably to the energy cascade in such a stratified sea basin. We showed that the intrusions of water with different salinity, which indicate the occurrence of a layered flow structure, could appear in the process of upwelling/downwelling development and relaxation in response to variable wind forcing. We suggest that the sub-mesoscale processes play a major role in feeding surface blooms in the conditions of coupled coastal upwelling and downwelling events in the Gulf of Finland.

Major Baltic Inflows (MBI) have a significant impact on physics, biogeochemistry and marine life in the Baltic Sea. Spreading of the North Sea water from the Danish Straits to the Eastern Gotland Basin has been rigorously studied in recent decades. Investigations of lateral signal propagation using in-situ measurements, which cover the area from the Eastern Gotland Basin to the Gulf of Finland, are missing. Estonian-Swedish-German-Finnish oceanographic data from January 2014 to March 2017 were merged and analyzed to fill the gap. Recent MBIs caused considerable changes in water column properties, and salinity reached the highest values of the last 40–60 years. The arrivals of MBI waters were detected as peaks in the salinity and temperature time-series in the near-bottom layer of the Gotland Deep 4–5 months after the MBI events. Similar peaks were also identified in the Faro Deep, Northern Deep and Kopu West (Northern Baltic Proper) with a further delay of 2–3 months, 3–5 months and 4–6 months, respectively. The first impact of the 2014 December MBI occurred in the Gulf of Finland in nine months as the arrival of the former Northern Baltic Proper deep layer water. Water renewal in the Faro Deep occurred as a gravity current over the sill between Faro and Gotland Deep. Deep layer water in the Northern Baltic Proper and the Gulf of Finland originated from the sub-halocline layer (110–120 m) of the Eastern Gotland Basin. The pre-condition for such mid-layer advection was a denser deep layer in the Gotland and Faro Deep. Fresh oxygen, which arrived in the Gotland Deep in April 2015 and February 2016, was consumed in the near-bottom layer within 3–6 months. Since summer 2016, oxygenated waters occurred in the Gotland Deep in the layer from the halocline to 160 m depth. This oxygen did not reach the area further in the north, except a slight sign of ventilation of the Faro Deep in February 2017. Thus, MBIs did not improve the oxygen conditions in the area north of the Gotland Deep and oxygen conditions rather worsened in the Northern Baltic Proper and the Gulf of Finland.

Two consecutive summer upwelling events, each lasting for less than 24 h, where surveyed in high temporal and vertical resolution at the Boknis Eck time series station (BE) in the western Belt Sea (Baltic Sea) in summer 2010 with an autonomous glider. Driven only by moderate offshore winds, both events resulted in strong cooling of surface waters (up to 5 K). Only for the second event, significant irreversible changes in the vertical stratification were observed and the appearance of low oxygen waters at the bottom indicated that the upwelling had an impact on the water column as a whole. A combination of wind and seasurface temperature data revealed that summer (June to September) upwelling at BE occurs for wind directions between 190 ° to 260 ° and with hourly averaged wind speed exceeding 4 m s−1. For the period 1982 to 2012 BE experience about 18 days of upwelling favourable wind conditions on average. Large interannual variability exist ranging from only 7.7 days in 2006 to more than 28 days in 1985. Surface (1 m depth) and deep water (below 25 m depth) anomalies of salinity and oxygen at the BE follow extended periods of strong upwelling favourable winds. Although seasurface temperature is good indicator for the existence of summer upwelling, the upwelling intensity does not correlate with the temperature anomaly.

The reversal of estuarine circulation caused by southwesterly wind forcing may lead to vanishing of stratification and subsequently to oxygenation of deep layers during the winter in the Gulf of Finland. Six conductivity, temperature, depth (CTD)+oxygen transects (130 km long, 10 stations) were conducted along the thalweg from the western boundary to the central gulf (21 December 2011–8 May 2012). Two bottom-mounted ADCP were installed, one near the western border and the second in the central gulf. A CTD with a dissolved oxygen sensor was deployed close to the western ADCP. Periods of typical estuarine circulation were characterized by strong stratification, high salinity, hypoxic conditions and inflow to the gulf in the near-bottom layer. Two circulation reversals were observed: one in December–January and one in February. The first reversal event was well developed; it caused the disappearance of the stratification and an increase in the oxygen concentration from hypoxic values to 270 μmol L−1 (to 6 mL L−1) throughout the water column along the thalweg and lasted approximately 1.5 months. Shifts from estuarine circulation to reversed circulation and vice versa were both associated with strong longitudinal (east–west) gulf currents (up to 40 cm s−1) in the deep layer. The change from oxygenated to hypoxic conditions in the western near-entrance area of the gulf occurred very rapidly, within less than a day, due to the intrusion of the hypoxic salt wedge from the NE Baltic Proper. In the eastern part of the gulf, good oxygen conditions caused by reversals remained for a few months.

The contributions of autonomous underwater gliders as an observing platform in the in-situ global ocean observing system (GOOS) are investigated. The assessment is done in two ways: First, the existing in-situ observing platforms contributing to GOOS (floats, surface drifters, moorings, research/commercial ships) are characterized in terms of their current capabilities in sampling key physical and bio-geochemical oceanic processes. Next the gliders’ capabilities are evaluated in the context of key applications. This includes an evaluation of 140 references presented in the peer-reviewed literature. It is found that GOOS has adequate coverage of sampling in the open ocean for several physical processes. There is a lack of data in the present GOOS in the transition regions between the open ocean and shelf seas. However, most of the documented scientific glider applications operate in this region, suggesting that a sustained glider component in the GOOS could fill that gap. Glider data are included for routine product generation (e.g. alerts, maps). Other noteworthy process-oriented applications where gliders are important survey tools include local sampling of the (sub)mesoscale, sampling in shallow coastal areas, measurements in hazardous environments, and operational monitoring. In most cases, the glider studies address investigations and monitoring of processes across multiple disciplines, making use of the ease to implement a wide range of sensors to gliders. The maturity of glider operations, the wide range of applications that map onto growing GOOS regional needs, and the maturity of glider data flow all justify the formal implementation of gliders into the GOOS. Remaining challenges include the execution of coordinated multinational missions in a sustained mode as well as considering capacity-building aspects in glider operations as well as glider data use., Comments from two anonymous reviewers and the Associate Editor G. Griffiths are appreciated. This work was supported by the European Union 7th Framework Programme (FP7 2007–2013), under grant agreement number 284321 GROOM (www.groom-fp7.eu); and the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement number 633211 AtlantOS.

We present and analyze high-resolution observational data of thermohaline structure and currents acquired in the Gulf of Finland (Baltic Sea), using an autonomous buoy profiler and bottom-mounted acoustic Doppler current profiler during July–August 2009. Vertical profiles of temperature and salinity were measured in the upper 50-m layer with a 3 h time resolution, and vertical profiles of current velocity and direction were recorded with a 10 min time resolution. Although large temporal variations of vertical temperature and salinity distributions were revealed, it was possible to define several periods with quasi-stationary vertical thermohaline structure. These quasi-stationary stratification patterns persisted for 4–15 days and were dominated by certain physical processes: upwelling, relaxation of upwelling, estuarine circulation and its wind-induced reversal, and downwelling. Vertical profiles of current velocities supported the concept of synoptic-scale, quasi-stationary periods of hydrophysical fields, characterized by distinct layered flow structures and current oscillations. To estimate the contribution of different processes to the changes in stratification, a simple conceptual model was developed. The model accounts for heat flux through the sea surface, wind mixing, wind-induced transport (parallel to the horizontal salinity gradient) in the upper layer, and estuarine circulation. It reproduced observed changes in vertical stratification reasonably well. The largest discrepancies between observations and model results were found when water motions across the Gulf and associated vertical displacements of isopycnals (upwelling or downwelling) were dominant processes.

We present the results of multiparametric observations designed to follow the phytoplankton dynamics and interrelated physical, chemical and biological processes in the Gulf of Finland (Baltic Sea). Data were acquired by an autonomous moored water column profiler, an acoustic Doppler current profiler, a flow-through system installed aboard a ferry and by profiling and discrete water sampling aboard research vessels in July and August 2009. The main aim of the study was to investigate the processes responsible for the formation and maintenance of sub-surface maxima of phytoplankton biomass. We suggest that the environmental conditions caused by the prevailing atmospheric and oceanographic forcing (wind; vertical stratification; basin-wide, mesoscale and sub-mesoscale processes) are preferred by certain species/taxonomic groups and explain the migration patterns of phytoplankton. Nocturnal downward migration of phytoplankton with a swimming speed up to 1.6 m h−1 occurred when the community was dominated by the dinoflagellate Heterocapsa triquetra. The observed splitting of the population into two vertically separated biomass maxima suggests that the H. triquetra cells, which reached the sub-surface layers with high nutrient concentrations, experienced bi-diurnal or asynchronous (when swimming upwards) vertical migration. The most intense sub-surface biomass maxima, on some occasions with the biomass much higher than that in the surface layer, were detected in connection to the sub-mesoscale intrusions below the depth of the strongest vertical density gradient.

Temporal variation and distribution of chlorophyll a and nutrients concentration was evaluated on the basis of field observations in August 2006 in the Gulf of Finland. Strong easterly winds in August 2006 generated an upwelling event along the Estonian coast of the Gulf of Finland. It caused a drop of the water-surface temperature and nutrient enrichment of the upper layer. At first, the chlorophyll a declined in the area affected by the upwelled water due to the strong advective transport of the chlorophyll a rich waters towards the northern coast and due to the intensive water mixing and low seed population in the upwelling waters. After stabilization of the upwelling, nutrients from the upper mixed layer were consumed fast: there were no nitrites + nitrates left one week later, and phosphate concentration was under the detection limit 2 weeks later. The smaller phytoplankton size fraction showed faster response to the upwelled nutrients compared with the bigger size fraction, showing the increase in chlorophyll a content already during the stabilization of the upwelling. The increase in chlorophyll a concentration in >20-μm size fraction at stations influenced by upwelling was observed only after the relaxation of the upwelling and formation of stratification.

Weekly mapping of vertical temperature and salinity fields was carried out across the Gulf of Finland in summer 2006 and spring 2007. Using successive cross-gulf vertical sections of salinity and wind data from the region the variations of estuarine and transverse circulation are described. Changes of deep layer phosphate-phosphorus concentrations are found to be related to the described variations in circulation patterns in a season with strong vertical stratification of the water column. Cumulative volume transport estimates were obtained using the results of a 3D baroclinic circulation model (HIROMB). We suggest that the north-easterly winds, which intensify the estuarine circulation and lead to the upwelling events along the southern coast of the Gulf of Finland, could have a major impact to the Gulfpsilas ecosystem by importing more saline and phosphorus rich waters. These events with many-fold more intense upward movement and mixing of deep waters (upward diapycnal transport) could contribute significantly to the ventilation of deep layers of the northern Baltic proper.