The data set analyzed in the present study was gathered using an observational network applied by the Marine Systems Institute at Tallinn University of Technology in the Gulf of Finland, Baltic Sea. It includes autonomous measurements and sampling on board a ferry traveling between Tallinn and Helsinki and autonomous measurements at a profiling buoy station close to the ferry route. Additionally, research-vessel-based measurements and sampling, as well as surveys using a towed undulating vehicle (ScanFish), are employed (Fig. 1).

The Ferrybox system records temperature (T), salinity (S), and chlorophyll a (Chl a) fluorescence in the surface layer (water intake is approximately at 4 m depth) twice a day along the ferry route Tallinn–Helsinki (the system is described in detail by Kikas and Lips, 2015). The time resolution of measurements of 20 s corresponds to an average spatial resolution of 160 m. For temperature measurements, a PT100 temperature sensor with a measuring range from -2 to +40 ∘C with an accuracy of ±0.1 % is used. The sensor is installed close to the water intake to diminish the effect of warming of water while flowing through the tubes onboard. For salinity measurements a FSI Excell thermosalinograph (temperature and conductivity meter) is used, and the data quality is checked by water sampling and analysis of samples by a high-precision salinometer Portasal 8410A (Guildline Instruments) 2–4 times a year. For Chl a fluorescence and turbidity (turbidity data not presented here) measurements, a SCUFA (Self-Contained Underwater Fluorescence Apparatus) submersible fluorimeter (Turner Designs) with a flow-through cap is used. An acid-washing cleaning system is applied to prevent biofouling and 11–17 water samples along the ferry route are collected once a week for laboratory analysis of Chl a content to calibrate the fluorimeter data.

The autonomous profiler deployed in the summers of 2009–2012 at station AP5 (Fig. 1) recorded vertical profiles of temperature, salinity, and Chl a fluorescence in the water layer from 2 to 50 m with a time resolution of 3 h and a vertical resolution of 10 cm. The sensor set at the buoy profiler consisted of an OS316plus CTD probe (Idronaut S.r.l.) equipped with a Seapoint Chl a fluorimeter. To avoid biofouling of sensors, the parking depth well below the euphotic layer depth and electrochemical antifouling system were applied. Ship-borne measurements and sampling close to the buoy profiler were arranged bi-weekly to check the quality of data (compare the vertical profiles from the buoy with those from the research vessel) and to calibrate the Chl a fluorimeter by laboratory analyses of Chl a content from the water samples.

The data set used also includes ScanFish surveys of temperature, salinity, and Chl a fluorescence conducted to map the horizontal distribution of T, S, and Chl a in the water column from 2 to 45 m (see location of sections in Fig. 1). The average distance between the consecutive ScanFish cycles, including down- and upcast while the vessel was moving with a speed of 7 knots, was 600 m. Data were recorded continuously (both down- and upcast are used) and the processed data were stored with a vertical resolution of 0.5 m. The ScanFish sensor set consisted of a Neil Brown Mark III CTD probe and TriOS microFlu-chl-A fluorimeter. Ship-borne CTD measurements and water sampling was conducted before and after the ScanFish surveys to control the quality of ScanFish data and calibrate the fluorimeter.

To calibrate the used (different) Chl a sensors, the Chl a concentration in the water samples was determined in the laboratory. Whatman GF/F glass fiber filters and extraction at room temperature in the dark with 96 % ethanol for 24 h were used. The Chl a content from the extract was measured spectrophotometrically (HELCOM, 1988) by Thermo Helios γ.

The data set from July-August 2009-2012 analyzed in the present study consists of 461 ferry crossings (Tallinn-Helsinki), 968 CTD and Chl a profiles collected at station AP5, and 6 ScanFish surveys.

The results in the following sections are presented as graphs of pre-processed observational data and horizontal wavenumber spectra of temperature variance calculated from the Ferrybox and ScanFish measurements as well as the estimated characteristics of vertical stratification at station AP5. The use of spatial spectra of temperature (instead of density) was based on the assumptions that in summer in the surface and thermocline layer of the Gulf of Finland (GoF), the water density is mainly controlled by temperature and it is measured by one sensor while density has to be estimated from the readings of two separate sensors. The following approaches are used in the calculations.

Horizontal wavenumber spectra of temperature variance were calculated for each ferry crossing between Tallinn and Helsinki assuming that the distance between the data points along the ferry route was constantly 160 m. The areas close to the harbors, where the ferry speed varied, were excluded, and only the data along the ferry route between the latitudes 59.48 and 60.12∘ N were used. The mean spectra for a certain period with quasi-stationary variability were obtained by averaging of single spectra over this period. The spectral slopes between the spatial scales of 10 and 0.5 km were estimated. The overall variability was characterized by daily standard deviations of temperature along the ferry route.

Horizontal wavenumber spectra of temperature variance in the sub-surface layer were calculated using the data of ScanFish surveys. Since the distance between the consecutive profiles varies depending on the depth, the ScanFish data were first interpolated to the grid with a constant horizontal step of 300 m, which corresponds to the average distance between the up- and downward casts. Then the individual spectra for every depth (with 0.5 m step) were calculated, and the mean spectra in 10 m thick water layers were obtained by averaging all spectra in those layers containing 21 individual spectra. The spectral slopes between the spatial scales of 10 and 1 km were estimated.

Vertical stratification was described by estimating the potential energy anomaly P (Simpson and Bowers, 1981; Simpson et al., 1990) as P=1h∫-h0(ρA-ρ)gzdz,ρA=1h∫-h0ρdz;, where ρ(z) is the density profile over the water column of depth h. The stratification parameter P(J m-3) is the work required to bring about the complete mixing of the water column under consideration. Similarly to Liblik and Lips (2012), the integration was conducted from the sea surface until 40 m depth. If the surface data were missing (upper 2 m where the buoy profiler did not measure), the uppermost available density value was extrapolated to the surface.

The intrusion index was calculated as a sum of negative salinity gradients (g kg-1 m-1) in the water layer from the sea surface to 40 m depth. Before calculations, the salinity profiles were smoothed by 2.5 m window. The idea behind the method comes from the fact that on the background of vertical salinity gradient with a fresher surface layer and more saline deep layer, lateral salinity gradients exist in the study area. In general, fresher water originates from the east (the Neva River and other larger rivers in the GoF) while more saline water originates from the Baltic proper. This general lateral salinity gradient could be enhanced locally as a result of meso- and sub-mesoscale dynamics. If the water layers with a thickness of a few to 10 m move in different directions, vertical salinity inversions could be generated in the water column where the vertical density gradient is mostly maintained by the temperature distribution. Thus, high values of intrusion index indicate the occurrence of layered flow structures.

The Chl a fluorescence data acquired with different sensors attached to the Ferrybox system, buoy profiler, and ScanFish were converted into Chl a content values using equations of linear regression between the fluorescence readings and results of laboratory analyses of water samples. The conversation equation of Chl a= 2.47 × F (r2= 0.41, p< 0.05) was used for the buoy profiler fluorescence data analyzed in this paper to convert fluorescence (F; in arbitrary units) into Chl a content in mg m-3. Interpretation of Ferrybox fluorescence data was sometimes difficult due to some problems with biofouling. In the present study, we used only data from July 2010 when the found regression line had the following parameters: Chl a= 2.34 × F - 2.41 (r2= 0.77, p< 0.05) and from summer 2012 by applying the following regression line equation: Chl a= 1.06 × F - 4.11 (r2= 0.80, p< 0.05). Data only from evening crossings were used to diminish the fluorescence quenching effect.

The study period in July–August of 2009–2012 was characterized by distinct inter-annual differences in wind conditions and distribution patterns of temperature and salinity in the central part of the Gulf of Finland. Based on HIRLAM (High-Resolution Limited Area Model) wind data, the average wind speed in July–August 2009–2012 in the GoF area was 6.0 m s-1, and the prevailing wind direction was from the south-southeast with an average velocity of the airflow of 1.4 m s-1. While the winds from the southwest prevailed in July–August 2009 and 2012 (average direction from 217 and 214∘ , respectively), the dominating wind direction was from the southeast in 2010 and 2011 (average direction from 160 and 122∘, respectively). Both in 2010 and in 2011 the monthly average wind direction differed between the 2 analyzed months being from 192∘ in July to 115∘ in August 2010 and from 73∘ in July to 177∘ in August 2011.

On the synoptic scale (several days – a couple of weeks), mostly westerly wind pulses occurred in 2009 except for a period in the first half of July with a relatively strong wind pulse from the south-southeast (see time series of wind stress vectors in Fig. 2). In 2010, moderate winds from the southwest were prevailing in the first half of July while several wind pulses from the east, northeast and south occurred during the rest of the study period. Typical for 2011 was a consecutive appearance of relatively strong wind pulses from southwest and east-northeast. In 2012, westerly winds clearly prevailed with only two short periods when the wind pulses from the east (early July) and northeast (second half of August) occurred.

In general, as described by Kikas and Lips (2015) based on Ferrybox data, the surface layer temperature was clearly higher in 2010 and 2011 than in 2009 and 2012. The surface layer salinity was the highest at the beginning of the study period in 2011; the less saline water occupied the central gulf in 2009 and the second half of summer in 2010 and 2011 while the surface layer salinity stayed relatively high in July–August 2012.

Based on the combined figures of horizontal and vertical distributions of temperature and salinity in July–August 2009 (Fig. 3a and b), the following characteristic features could be identified. In the first half of July, an upwelling event developed near the southern coast resulting in large variations of temperature (7.8–17.8 ∘C) and salinity (4.2–6.1 g kg-1) across the gulf. Deepening of the thermocline occurred after the upwelling relaxation in the southern part of the gulf. A shallow and warm upper layer (temperature between 17.3 and 20.4 ∘C) with very low variations of salinity across the gulf (between 4.6 and 5.0 g kg-1) appeared in the study area due to a period of weak winds in the first half of August. Upwelling near the southern coast occurred in the second half of August with increased across-gulf variability of temperature (from 12.9 to 17.7 ∘C) and salinity (4.7–5.6 g kg-1).

At the beginning of the study period in 2010, when mainly weak or variable moderate winds prevailed, the variations of temperature (mostly being between 20 and 22 ∘C) and salinity in the surface layer were very low across the gulf (Fig. 3c and d). This calm period was followed by a relatively weak upwelling event off the northern coast and deepening of the thermocline from 10 m to 15 m in the southern part. A strong upwelling event near the southern coast with the high spatial variability of temperature (varying between 11.1 and 21.6 ∘C) and salinity (varying between 4.0 and 6.3 g kg-1) across the gulf occurred in late July. The seasonal thermocline had a much shallower position in 2010 (for the period with available data until early August) than in 2009.

Study period in July–August 2011 started with an upwelling event near the southern coast with the surface layer temperature varying across the gulf from 12.8 to 20.6 ∘C and salinity varying from 5.1 to 6.5 g kg-1 (Fig. 3e and f). During the next extensive upwelling event in late July–early August off the southern coast, high variability of the surface layer temperature (varying between 13.5 and 21.4 ∘C), and salinity (varying between 4.4 and 6.4 g kg-1) across the gulf was observed. Relaxation of the latter event was accompanied by a moderate deepening of the thermocline in the southern part of the gulf. Strong winds from variable directions in the second half of August caused strong vertical mixing and downwelling with a drastic deepening of the seasonal thermocline to 45 m depth in the southern part of the gulf.

The first half of July 2012 was characterized by relatively low spatial variability of temperature and salinity in the surface layer of the study area (Fig. 3g and h). An upwelling event occurred near the northern coast at the end of July, creating a temperature difference across the gulf from 10.3 to 17.2 ∘C, and accompanied by the deepening of the seasonal thermocline in the southern part (to 45 m). After a short period with low variability, the second upwelling event appeared near the northern coast while the surface layer temperature stayed quite high in the rest of the study transect (up to 20 ∘C). In the period between the two upwelling events, strong intrusions of more saline water were observed in the subsurface layer at the buoy station. The position of the seasonal thermocline in the southern part of the gulf was the deepest in 2012 among the analyzed years.

Overall horizontal variability of temperature characterized as the standard deviation of temperature along the ferry route varied in quite large ranges in time, from 0.2 to 3.7 ∘C (Fig. 4). High values of standard deviation of temperature in the surface layer were related to the observed coastal upwelling events and, as a rule, the upwelling events near the southern coast resulted in larger spatial variations of temperature than those near the northern coast. During the upwelling event in August 2010, the standard deviation of temperature was as high as 3.7 ∘C while during the other upwelling events within the study period in July–August 2009–2012, the values of standard deviation of temperature did not exceed 2.5 ∘C. Despite the high temporal variability of standard deviations of temperature calculated based on data from single crossings, the average values of standard deviations for the studied 4 years did not differ much; a minimum of 0.71 ∘C was found in 2009 and a maximum of 0.83 ∘C in 2010 (Table 1).

The calculated horizontal wavenumber spectra of temperature variance had also relatively large variability if to compare the spectra estimated based on data from single crossings. The spectral slope between the lateral scales of 10 and 0.5 km varied between -1.8 and -3.7 (in logarithmic scales). Note that the spectral curves were approximately linear (Fig. 5) between the scales of 15–20 km (the latter corresponds to a horizontal wavenumber of 0.05 km-1 or in a logarithmic scale -1.3 in Fig. 5) and 0.5 km (corresponds to wavenumber of 2 km-1 or in logarithmic scale 0.3 in Fig. 5); thus, linear approximation of their slopes is feasible. Within the periods of the high spatial variability of temperature, mostly related to upwelling events affecting the distribution of temperature in the surface layer of the Gulf of Finland, the estimated slopes were between -1.8 and -2. When the spatial variability of temperature was low in the surface layer, the slopes varied mostly between -2 and -3 (Fig. 4). At the same time, the average spectra for the entire period under consideration in the studied years were quite close to each other (Fig. 5, bold lines) and the spectral slopes on average were close to -2 (from -2.1 to -2.2; see also Table 1).

Based on the presented lateral variability of temperature, some distinct periods when the standard deviation of temperature was high and spectral slope was close to -2 can be distinguished. We selected 3–4 periods with the almost quasi-stationary character of variability in each year to quantitatively describe the character of variability within these periods; the periods are marked in the Fig. 4 by dashed lines.

In 2009, two periods of high spatial variability caused by coastal upwelling events existed (periods 1 and 3 marked in Fig. 4). During both periods, the spectral lines had a higher position, and their slopes were shallower than the average for the entire study period in 2009 (see Fig. 5 and Table 1). In 2010, two periods, which were also associated with the upwelling events (periods 2 and 4; see Fig. 4), had much higher spatial variability, and the spectral slopes were shallower than the average in 2010. All time intervals comprising upwelling events in 2011 (periods 1 and 3; see Fig. 4) and 2012 (period 2; see Fig. 4) were also characterized by a higher position and shallower slope of spectral lines than the lines representing the average for July–August 2011 and 2012. The noticed divergence of spectral slopes from the high-variability and low-variability periods resulted in a clearly larger separation between the spectral curves at the sub-mesoscale than at the mesoscale. While the spectral density of spatial variations of temperature at the spatial scale of 1 km varied more than 1.5 in magnitude, it varied in ranges of 1 magnitude at the spatial scale of 10 km (Fig. 5).

The vertical distributions of temperature and salinity at the buoy station varied considerably in time similarly to the horizontal distributions of temperature and salinity along the ferry route. The variations were revealed as changes in the magnitude of vertical gradients, depth of the upper mixed layer and seasonal thermocline, fast deepening or surfacing of the thermocline, and occurrence of intrusions leading in certain cases to local inversions in vertical salinity distribution (Fig. 3).

Temporal changes in vertical stratification in the Gulf of Finland could be related to the differences in the heat flux through the sea surface and to the prevailing wind forcing that influences both the estuarine circulation alterations and the intensity of vertical mixing (see e.g., Liblik and Lips, 2012). Note that the autonomous buoy station in the present study was located in the southern part of the open Gulf of Finland. Thus, in addition to the seasonal course of stratification and its dependence on the estuarine circulation, the vertical stratification at this location could be significantly influenced both by the upwelling and by the downwelling along the southern coast.

Stratification parameter (P) calculated for the water layer between the sea surface and 40 m depth increased in July 2010 and 2011 in accordance with the strengthening of the seasonal thermocline (Fig. 6). In July–August of these years, the winds from the southeast prevailed supporting the estuarine circulation and, in turn, keeping up the strong vertical stratification; the maximum of P= 370 J m-3 was observed at the beginning of August 2010. This continuous increase of P in both years was disrupted only due to the coastal upwelling events (in 2010 also due to a weak downwelling event) leading to rapid changes in the stratification parameter mostly because of vertical movements of the thermocline. In contrast to 2010 and 2011, the stratification parameter did not increase much during the study window in 2009 and 2012 in accordance with the prevailing southwesterly winds. In 2009, the stratification parameter being relatively high due to the vertical salinity stratification had the maximum in the first half of August (P= 300 J m-3) and decreased rapidly afterwards when the downwelling influence reached the buoy station. In 2012, the vertical stratification in the water layer from the surface to 40 m depth almost vanished at the measurement site AP5 by 20 July due to a very strong downwelling event, which appeared along the southern coast of the Gulf of Finland. Later on, the stratification at the buoy station strengthened, but the stratification parameter was clearly the lowest in 2012 if compared to the other years due to the deepest position of the seasonal thermocline.

Vertical profiles of temperature and salinity collected at the buoy station often exposed variability with vertical scales of a few to 10 m, which could be interpreted as intrusions related to the sub-mesoscale dynamics. Since the temperature was the main contributor to the vertical density distribution in the seasonal thermocline, such intrusions could create local inversions in the vertical distribution of salinity as mentioned above and seen in Fig. 3. The calculated intrusion index showing how much the vertical stratification is weakened due to local salinity inversions varied mostly between 0 and 0.05. However, every year one or a few periods were detected when the index exceeded 0.05, whereas the maximum index value obtained on 1–2 August 2012 reached 0.36.

In 2009, the only period with relatively high intrusion index values was detected during and just after the period of estuarine circulation reversal (Liblik and Lips, 2012). The maximum of the intrusion index coincided with the last day of the upwelling event near the northern coast that was followed by the event near the southern coast and rapid decrease of the intrusion index. In 2011, the index values > 0.05 were detected a few times in July, whereas the highest values on 12–14 August (exceeding 0.24 on 14 August) were related to the relaxation of an intense upwelling event near the southern coast and short-term deepening of the thermocline at the buoy station during a weak upwelling event near the northern coast. The mentioned highest intrusion index value on 1–2 August 2012 was detected within the period when two consecutive major upwelling events occurred near the northern coast, whereas this maximum emerged between the upwelling events just before the second one.

Thus, the intrusions were most intense (in the sense of salinity inversions) at the buoy station AP5 in connection with the relaxation of upwelling events near the southern coast, development of upwelling events near the northern coast and estuarine circulation reversals. All these situations correspond to the periods when the thermocline was deepening or was already at a deep position at the buoy station in the southern part of the open Gulf of Finland. In addition, the stratification parameter values were low or decreasing when the temporal maximum of intrusion index was detected. When relating intrusion index values with the lateral variability in the surface layer one could conclude that the found temporal maxima of intrusion index corresponded to the periods of moderate lateral variability in the surface layer. Nevertheless, during such periods, the slopes of horizontal wavenumber spectra of temperature variance were close to -2 as during the periods of high lateral variability, and approximately a week before the highest intrusion index values, the lateral variability in the surface layer was also high (Figs. 4 and 6).

We analyzed the data of ScanFish surveys in the Gulf of Finland conducted across the gulf in the open, deeper part and along the southern coast in the summers of 2010 and 2012. The hydrographic background of surveys in 2010 is characterized by the development of a weak upwelling along the northern coast of the gulf on 22 July 2010, a strong upwelling event along the southern coast on 27 July 2010 and relaxation of it by 2 August 2010 when the last survey was conducted (see Fig. 3c, d). In summer 2012, when the upwelling events along the northern coast dominated, the survey on 4 July 2012 characterizes the situation before those upwelling events, on 20 July 2012 the development of upwelling and on 31 July 2012, which was conducted along the gulf, the situation related to a temporal relaxation of upwelling (see Fig. 3g, h).

A clear cross-gulf inclination of the thermocline was revealed on 22 July 2010 (Fig. 7a), although the ScanFish section did not reach the upwelling area near the northern coast. At the same time, the isopycnals had opposite inclination below the 20 m depth resulting in a weakening of the vertical stratification from south to north (Fig. 8). A well-pronounced, less saline water zone with a width of less than 5 km was observed in the surface layer. The extension of an associated intrusion of lower salinity in the thermocline was wider in the horizontal dimension and its thickness, decreasing from north to south in accordance with the strength of the vertical stratification, was less than 5 m.

The along-gulf ScanFish section on 27 July 2010 from the buoy station AP5 to the southwest crossed the meandering upwelling front (Fig. 7b). The observed variability is characterized by clear mesoscale meanders of the front with spatial scales of 10–15 km, strong stratification at the warm, less saline side of the front and much weaker stratification at the cold, more saline side of it, and very low intrusion index almost along the entire section (Fig. 8b). A remarkable variability at the sub-mesoscale, also resulting in intrusions of water with different salinity seen in Fig. 7c and expressed in high values of intrusion index (Fig. 8c), was observed on 2 August 2010. A less saline water zone was well visible almost at the same location as on 22 July 2010, but the surface layer salinity in it was much lower, less than 4.5 g kg-1 (Fig. 7c).

The average vertical gradients of all parameters – temperature, salinity, and density – in the upper 40 m water layer were clearly lower in summer 2012 (Fig. 7d–f) than in summer 2010 (Fig. 7a–c). A less saline water zone in the central part and slightly stronger vertical stratification in the southern part of the section were observed on 4 July 2012 (Figs. 7d and 8d). Development of the upwelling along the northern coast and downwelling in the southern part caused strong inclination of the thermocline across the gulf and a clear strengthening of vertical stratification from south to north on 20 July 2012 (Figs. 7e and 8e). In the area of strong inclination of the thermocline, relatively large horizontal gradients of salinity and intense sub-mesoscale variability, also seen as intrusions of water with different salinity (Fig. 7e), were observed. On 31 July 2012, the ScanFish survey revealed a mesoscale eddy-like feature (Fig. 7f), which could be formed in the process of downwelling relaxation as also observed earlier along the southern coast of the gulf (in its mouth area; see Lips et al., 2005). This mesoscale feature was characterized by relatively weak vertical stratification in its central part and high intrusion index values, especially at its periphery (Fig. 8f). Note that the pronounced intrusion of more saline water detected at the western end of the section was also registered at the buoy station during several days (Fig. 3h).

The horizontal wavenumber spectra of temperature variance shown in Fig. 9 vary considerably between the analyzed six surveys, both in the spectral level and the shape of the curves. The highest variability at the meso- and sub-mesoscale was related to the surveys, which were conducted when pronounced upwelling events dominated in the study area, on 27 July 2010 and 20 July 2012. The former survey crossed the meandering upwelling front, and the peak of the spectral density was found at the lateral scale of about 15 km. The latter survey was conducted when an upwelling event along the northern coast was developing, and the ScanFish section crossed the inclined thermocline. A similar situation with a weaker upwelling development and slightly lower spectral densities was mapped on 22 July 2010.

The spectral slopes between the horizontal scales of 10 and 1 km for spectra averaged over depth intervals with the thickness of 10 m were mostly shallower than -2; the average values of spectral slopes for each survey (shown in Fig. 9) varied between -1.7 and -2.0. The slopes close to -2 were obtained for the surveys with the most pronounced mesoscale features. Local vanishing of the spectral slope could be detected between the horizontal scales from 3 to 1 km on 2 August 2010 and at scales from 3 to 2 km on 31 July 2012. High intrusion index values were characteristic for the both mentioned surveys, especially for the survey on 31 July 2012 (Fig. 8c, f), which is in accordance with the index estimates based on the buoy profiler data (Fig. 6d). High intrusion index values were also found for the survey on 20 July 2012 when a patch of more saline water appeared in the sub-surface layer at the warm side of the upwelling front (Figs. 7d and 8d).

Temporal variability of chlorophyll a at the scales of days is usually much higher than that of temperature and salinity since in addition to the advection and mixing, the phytoplankton growth (and decay) could increase (decrease) the biomass and consequently chlorophyll a content rapidly. Despite such high variability and other factors that could influence the comparability of acquired chlorophyll a fluorescence data, e.g., fluorescence quenching, the presented combined plots of changes in horizontal and vertical distributions agree reasonably well (Fig. 10).

In the first half of July 2010, a sub-surface bloom developed in the southern part of the Gulf of Finland, which occasionally was also seen in the surface layer with higher chlorophyll a values off the southern coast (Fig. 10a). When an upwelling event along the southern coast started to dominate (see Fig. 3c), the chlorophyll a content decreased in the southern part and increased in the northern part of the study area. Before the bloom near the northern coast, relatively deep chlorophyll a maxima were detected at the buoy station at depths below 20 m (since the maxima layers were thin they are not well seen in Fig. 10) and at the ScanFish section on 22 July 2010, especially at its northern part (Fig. 11). When the upwelling developed along the southern coast of the gulf, the sub-surface chlorophyll a maxima were situated at a shallower position in the warmer side of the front, but they almost disappeared by 2 August 2010 after relaxation of the upwelling event. In between the two surveys, the bloom developed near the northern coast in the convergence/downwelling area. It was suggested that this bloom could be related to the observed sub-surface maxima of chlorophyll a, which contained the dinoflagellate Heterocapsa triquetra in very high numbers (Lips and Lips, 2014).

In 2012, the chlorophyll a content was higher in the northern half of the study area than in its southern part in the first half of July. Later, when the two consecutive upwelling events appeared along the northern coast, the highest chlorophyll a values were observed close to the southern coast. At the buoy station, where the downwelling influence was visible (see Fig. 3g), the chlorophyll a content also increased in the sub-surface layer. Occasionally, high chlorophyll a values were detected close to the upwelling front in the northern part of the study area. While the sub-surface maxima of chlorophyll a were observed at the buoy station and the ScanFish transect at the beginning of July 2012 (Fig. 11), they disappeared when the upwelling events occurred near the northern coast. During the upwelling development, both the chlorophyll a content in the surface layer and the thickness of the surface layer with elevated chlorophyll a increased from north to south. However, those tendencies had pronounced intermittency at lateral scales of 1 to a few kilometers. A similar distribution pattern was registered in the phase of the upwelling/downwelling relaxation on 31 July 2012.