The main characteristic of the Galician coastline, located in the northwest of the Iberian Peninsula, is the Rías Baixas, four long coastal estuaries or rias (> 2.5 km3) between 42 and 43∘ N (Fig. 1). The water exchange between the Rías Baixas and open waters is drastically affected by the coastal wind pattern as part of the Canary Current upwelling system (Wooster et al., 1976; Fraga 1981; Arístegui et al., 2004). Under the predominance of northeasterly winds (Blanton et al., 1984) during spring–summer, the surface offshore transport of surface waters leads to a rising cold, nutrient-rich, deep water mass called the Eastern North Atlantic Central Water (ENACW) (Ríos et al., 1992). Under these conditions, the Rías Baixas act as an extension of the continental shelf (Rosón et al., 1995; Souto et al., 2003; Gilcoto et al., 2017), where upwelling filaments extending westward export primary production from the coast into the ocean (Álvarez-Salgado et al., 2001). In the opposite direction, the prevalence of northward winds (Blanton et al., 1984) moves the surface waters towards the coast, where they accumulate, sink and thus isolate the coast. This process, known as downwelling, is typical during the autumn–winter along with other characteristics such as the warm, salty waters from the Iberian Poleward Current (IPC) of subtropical origin (Fraga et al., 1982; Alvarez-Salgado et al., 2006) that flows constrained to the Iberian shelf break (Frouin et al., 1990). The runoff from local rivers also contributes to the presence of river plumes over the shelf (Otero et al., 2008). These hydrodynamic conditions, the meteorological forcings and the alternation of periods of upwelling and downwelling (Álvarez, 1999; Gago et al., 2003c; Cobo-Viveros et al., 2013) stimulate the development of intense primary production and high rates of recycling and downward carbon export (Alonso-Pérez and Castro, 2014). The result of this biogeochemical variability in terms of air–sea CO2 exchange is that the surface waters act as a net CO2 sink that is especially intense and variable over the shelf compared to offshore or in the inner Rías Baixas (Padin et al., 2010).

Besides the short-term and seasonal variability, significant changes in the long-term scale have been reported in this region. In addition to changes such as the weakening and shortening of the upwelling events (Lemos and Sansó, 2006; Pérez et al., 2010; Alvarez-Salgado et al., 2008), the warming (González-Pola et al., 2005; Pérez et al., 2010), and changes in the composition of phytoplankton (Bode et al., 2009; Pérez et al., 2010), the acidification in the first 700 m for the geographical area from the Iberian Peninsula to the 20∘ W meridian and from 36 to 43∘ N has also been observed at a rate of -0.0164 pH units per decade (Ríos et al., 2001; Castro et al., 2009).

According to the type of region under study, different areas were identified in order to classify the measurements gathered in the oceanographic cruises (Fig. 1). The latitude of 43∘ N where Cape Finisterre is located was used as the dividing line between northern and southern waters. Subsequently, a criterion of depth also split the waters to the north of 43∘ N into north oceanic (below 250 m), north shelf (between 205 and 75 m) and north coast (75 m to the surface). The southern shelf waters were divided by latitude 42∘ N into Portuguese and the Rías Baixas (RB) shelves, whereas the shallower waters were identified by the main rias, where three different zones were defined using longitude boundaries (outer, middle and inner) according to Gago et al. (2003c) in the Ría de Vigo and just two zones in the other rias (Ría de Pontevedra, Ría de Arousa, Ría de Muros). Southern waters between the isobath at 75 m and the mouth of the estuaries were identified as the Portuguese and RB coast.

The discrete measurements gathered in the ARIOS dataset were mainly found in different regions' waters around 42∘ N latitude (Figs. 1 and 2a), especially in the outer and middle areas of the Ría de Vigo, which accounted for 15 % and 21 % of the total measurements respectively due to the proximity to the Instituto de Investigaciones Marinas (IIM-CSIC). Most of the measurements (85 %) carried out by many of these cruises to study the coastal ecosystems concentrated on shallow waters between the seawater surface and 75 m in depth (Fig. 2b). Although waters below 4900 m depth were also sampled, observations below 900 m only account for 1 % of the ARIOS database.

The observations made over more than 40 years in every region of the ARIOS database were irregular on both an interannual and seasonal scale (Fig. 2a). The period of most sampling activity was the 1980s and 1990s, whereas samples were especially scarce in the early 2010s. On a seasonal scale, summer and autumn were the preferred seasons to address the different research purposes, with 37 % and 36 % of the total samples respectively. The observations taken during less favourable winter conditions, especially aboard the coastal vessels usually available, only accounted for the 10 % of the ARIOS database.

The ARIOS database is a compilation of biogeochemical properties with discrete measurements of temperature, salinity, oxygen, nutrients, alkalinity, pH and chlorophyll that were sampled in waters off the northwest of the Iberian Peninsula from 1976 to 2018 and measured by IIM-CSIC (Table 1). This data collection is part of the research by 24 projects and oceanographic cruises conducted in response to different aims. The different sampling strategies built up an irregular biogeochemical database whose particular frequency and spatial coverage is shown in Fig. 2.

The contribution to the ARIOS database from the oceanographic cruises and projects over the different decades is described below.

To assess of the level of acidification in the ocean adjacent to the Galician coast, variables of the carbon system (pH and alkalinity), nutrient concentration, dissolved oxygen, chlorophyll a, salinity and temperature were measured in each cruise. The variables measured in each oceanographic cruise gathered in the ARIOS dataset are shown in Table 1. The main changes in the materials and methods throughout these years are detailed below.

The vertical profile of the temperature, salinity, pH on total scale at in situ temperature (pHT), NO3- and oxygen concentration in the ocean region between 41 and 43∘ N was estimated for each oceanographic station as the mean value of the depth ranges described in Fig. 2b. These measurements were gathered during the collection periods (December–February, March–May, June–August and September–November) and averaged to describe winter, spring, summer and autumn respectively (Fig. 3, Table A1 in the Appendix).

The vertical distribution of the temperature (Fig. 3a) showed the presence of warmer saline waters throughout the water column in winter with the exception of the surface waters during summer, which showed intense heating due to the radiant solar energy. Below the maximum temperature observed during the summer, cold central waters of subpolar origin occupied the water columns with lower salinity (Fig. 3b). The vertical variation of temperature is typical for a temperate region with relatively homogenous deep water below the seasonal thermocline, reaching maximum sea surface temperature (SST) values in summer and autumn and minimums in spring and winter. The winter temperature profile is relatively warmer than in spring because of the presence of the IPC (Alvarez-Salgado et al., 2006), which reaches a depth of 300 m. The maximum salinity is also found in winter due to the presence of the IPC, whereas the minimum values are found in autumn (Fig. 3b). Below 500 m in depth, the increase in salinity points to the presence of Mediterranean water. These differences reach a minimum at 500 m depth, where the salinity values coincided. From this depth down to 1100 m, the differences in temperature and salinity throughout the four seasons were minimal, with the mean values converging to 11.03±0.07 ∘C and 36.117±0.009 psu, respectively (Fig. 3a, b).

The vertical profiles of pHT, NO3- and oxygen concentration (Fig. 3c–e) also showed a variation lower than 1 % within this depth range, with annual means of 15.2±0.1 µmol kg-1, 8.025±0.005 and 188±1 µmol kg-1 respectively. The pH values from a maximum subsurface located at around 40 m depth showed a clear inverse correlation with the depth down to a depth of 500 m throughout the seasonal cycle, where the annual minimum value of 8.018±0.005 was reached. The higher pH values could be attributed to the biological reduction of CO2 by phytoplankton activity, which brought the pH to a maximum value of 8.13 to 40 m during the spring bloom. After the intense photosynthetic activity observed in surface waters during spring and summer, pH values reached minimum values in the first 200 m of depth during autumn due to respiration of organic matter. However, it was at a depth of 500 m that the minimum pH values were measured in all seasons where the subpolar Eastern North Atlantic Central Water proceeding from the northeastern cyclonic gyre is found (Harvey, 1982; Ríos et al., 1992). The influence of phytoplankton growth on biogeochemistry during spring can be also evidenced by the oxygen concentration pattern (Fig. 3e). In the upper layer above 250 m depth, spring oxygen levels exceeded those in winter, whereas a decrease in oxygen concentration was found from this depth down to 1000 m, possibly due to enhanced respiration from cascading organic matter. The impact on the growth of the phytoplankton community during the spring was also evident, judging by the oxygen concentration. So, in the upper waters the spring oxygen concentration values exceeded those of the winter values, while oxygen consumption was found from a depth of 300 to 1000 m due to respiration from organic matter arriving from above. The minimum values for oxygen concentration throughout the water column were found during summer and autumn. The nitrate concentration displayed a particularly vertical distribution, growing with depth from minimum values in the upper layer of the ocean region, which was practically zero during the first 50 m. Below 100 m, the nitrate concentration showed the maximum values in the vertical distribution during summer and autumn, coinciding with the presence of waters of subpolar and subtropical origin respectively, whereas the minimum values appeared in winter. Towards the bottom, the seasonal values of NO3- concentration were almost coincident at a mean value of 15.2±0.1 µmol kg-1.

The seasonal cycle of the biogeochemical properties (temperature, salinity, pHT, oxygen concentration, nitrate concentration and chlorophyll) in the surface waters (0 to 5 m) of five geographical boxes was estimated as a monthly average previously filtering values outside of 2 standard deviations of the mean (Table A2 in the Appendix). Five regions that were located as a longitudinal transect between the inner Ría de Vigo and the ocean zone are shown in Fig. 4.

In general terms, the seasonal variability of the temperature was very similar in every area, ranging between 12 and 19 ∘C (Fig. 4a). Only particular features observed on a short-term scale as in the examples below differ between each region. The warmer waters were usually found in the oceanic zone, reaching a maximum monthly averaged temperature of 18.6 ∘C in September, while the coldest surface waters of 12.6 ∘C were located in the inner stations closer to the mouth of the Ría de Vigo in January. Another secondary minimum averaged temperature was also found in the shelf and the outer area of the Ría de Vigo, which was remarkably low in August due to the entry of cold upwelled waters in the surface layer (Alvarez-Salgado, 1993).

The monthly salinity averages (Fig. 4b) clearly showed significant differences between the offshore and coastal waters. Sharp salinity changes were seen in the estuary during winter, especially in the inner area where values lower than 28 psu were reached with the arrival of continental inputs in December. The weak seasonal cycle of salinity in the shelf and ocean waters showed high values in December due to the influence of warm saline water from the IPC, usually located on the shelf slope even though it may even enter the rias depending on the relative intensity of shelf winds and the intensity of the continental runoff (Alvarez-Salgado et al., 2003). In this sense, the slight salinity minimum observed in the shelf waters in March could be consequence of the offshore spreading of the maximum discharges from the River Miño and Douro (Otero et al., 2008) at the end of downwelling season. After this, the shelf and ocean waters showed minimum values in summer due to the arrival of cooler and fresher subpolar waters (Rios et al., 1992; Alvarez-Salgado et al., 2003, 2006). In August, coinciding with the maximum salinity of the surface waters in the interior of the Ría de Vigo due to the minimum river runoff, the surface waters between the inner Ría de Vigo and the ocean region were almost homogeneous, with minimum differences in salinity of 0.2 psu.

Like salinity, there was little seasonal variability in pH in the offshore waters but large seasonal variability in coastal waters, with maximum and minimum pH values in spring and autumn, respectively, and in all regions (Fig. 4c). The net balance between production and respiration of organic matter and the estuarine circulation caused a maximum pH of 8.19 in the outer region of the Ría de Vigo in May and a minimum of 7.96 in the inner waters in November.

The oxygen concentration (Fig. 4d) in the coastal ecosystems is also controlled by the remineralization of the organic matter and photosynthetic activity of the phytoplankton community, with the effect of salinity and temperature on the oxygen saturation level. The variability in the oxygen concentration, like the pH distribution, showed a growing seasonal amplitude towards the coastline, with maximum values in the outer and middle Ría de Vigo and lower values in the inner waters, especially during the second half of the seasonal cycle. Hence, the dissolved oxygen concentration mirrored the seasonal cycle of pH, showing growing seasonal amplitude towards the coastline with a range between 284 µmol kg-1 found in the outer region of the Ría de Vigo in May and 205 µmol kg-1 in the inner waters in November. These results seem to reinforce the importance of the oxygen consumption in this shallow area, where the water column is less than 10 m deep, and therefore it would also be influenced by benthic respiration (Alonso-Pérez and Castro, 2014).

The monthly means of nitrate concentration (Fig. 4e) could be summarized as high values during autumn and winter due to the nutrients delivered from the continent and the vertical mixing, and as minimum nitrate values from March to September because of phytoplankton consumption. The nitrate concentration was markedly higher in the inner Ría de Vigo, where it exceeded 9 µmol kg-1 in February and decreased towards the open ocean, where the highest monthly value was seen to be 2.5 µmol kg-1. Some notable aspects can be seen in Fig. 5d, such as water poor in nitrate in the ocean region between the two peaks of 3.5 µmol kg-1 in March and 1.3 µmol kg-1 in October. This shows the presence of the IPC waters, which are warmer and saltier than the shelf waters. Also noteworthy was the particular fact that while the nitrate concentration in other areas was practically zero in summer, the nitrate amount in the surface waters within the Ría de Vigo, and especially in the inner Ría de Vigo, was not completely consumed. This indicates a constant supply throughout the year, either through upwelling events or the continental inputs. This in turn means that while the chlorophyll values were at a minimum in the offshore waters in summer, the phytoplankton community in the estuary grew in summer during the upwelling relaxation periods (Pérez et al., 2000). The nutrient concentration during spring and summer was only detectable in the newly upwelled waters that can show values up to 6 µmol L-1 (Fraga, 1981; Castro et al., 1994). During the cessation of the upwelling season in September and October, the chlorophyll concentration (Fig. 5f) increased again, sustained by nutrients that entered from deeper waters through vertical mixing. It should be noted that there was a coincidence of high chlorophyll in the water column and low oxygen concentration in the inner Ría de Vigo from May to November, indicating the potential importance of benthic fluxes and vertical fluxes (reference).

The long-term trends of the biogeochemical properties in the surface waters along the described longitudinal transect between the inner Ría de Vigo and the ocean zone were estimated to be the interannual linear rate of the deseasonalized time series, previously removing the monthly means in these regions and assuming a null spatial variability. The significant trends in the ARIOS database, meaning long-term variability, should be interpreted as a combination of the natural variability on a decadal scale (Pérez et al., 2010; Padin et al., 2010) and anthropogenic forcings (Wolf-Gladrow et al., 1999; Anderson and Mackenzie 2012; Bakun et al., 2010).

No long-term temperature variability was found in the surface waters of any region despite the known warming previously reported on the northern Iberian coast (Pérez et al., 2010; Gómez-Gesteira et al., 2011; González-Pola et al., 2005). Unlike the temperature, the other expected consequence of climate change in marine ecosystems, namely ocean acidification (Caldeira and Wickett, 2003), was observed along the longitudinal transect, with a greater decrease in the long-term trend of pH towards the coast (Table 2). The long-term pH variation of -0.0039±0.0005 yr-1 in the inner waters was about 3-fold higher than the change observed in the ocean zone, equivalent to -0.0012±0.0002 yr-1 in the ocean zone, explaining the 34 % and 22 % variation in pH in situ, respectively, and representing 1 %–3 % of the seasonal pH variation in all zones. These pH decrease rates found in both coastal and open-ocean regions of the Iberian upwelling system lie within the range of other acidification rates estimated in different sites of the North Atlantic Ocean (Lauvset and Gruber, 2014; Bates et al., 2014), being also coherent with the mean rates calculated for the global ocean and for the eastern North Atlantic and equal to -0.018 and -0.0164 decade-1, respectively (Lauvset et al., 2015; Ríos et al., 2001).

The long-term trend in salinity was also seen to be evidently dependent on the distance to the mouth of the Ría de Vigo. The interannual rate of sea surface salinity in the outer and inner ria previously reported by Rosón et al. (2009) was 0.0426±0.016 and 0.0193±0.0056 psu yr-1 respectively. These changes were observed in parallel to an interannual alkalinity increase that is cancelled out in the normalized alkalinity, estimated as the difference between the alkalinity measured and the alkalinity calculated using the linear regression with salinity in each region. Therefore, the interannual salinity increase was the forcing that explains the increase in the buffer capacity of the surface waters (Sarmiento and Gruber, 2006).

Other significant long-term variations were found in other biogeochemical parameters in the ARIOS database. The long-term trend of the concentrations of nutrients in the inner Ría de Vigo that had been previously reported for the period 2001–2011 by Doval et al. (2016) showed a significant increase in nitrate, phosphate and ammonium concentrations of 0.0559±0.0158, 0.0076±0.0016 and 0.0560±0.0011 µmol kg-1 yr-1 respectively. This fertilization on a long-term scale in the surface waters of the inner ria estimated from the ARIOS database was observed in parallel to the deoxygenation of -0.7±0.2 µmol kg-1 yr-1. The apparent oxygen utilization (AOU), calculated using the concentration of O2 at saturation calculated according to Benson and Krause (1984), underwent an equivalent significant long-term change of 0.7±0.2 µmol kg-1 yr-1, indicating that either the biological consumption rates, a change in the amount of time that the waters are ventilated, or even its interaction or exchange with the sediment cause the long-term reduction of oxygen.

This fertilization on a long-term scale estimated from the ARIOS database in the surface waters of the inner ria was observed in parallel to the deoxygenation of -0.7±0.2 µmol kg-1 yr-1. The apparent oxygen utilization (AOU), calculated using the concentration of oxygen at saturation calculated according to Benson and Krause (1984), underwent a long-term change of 0.7±0.2 µmol kg-1 yr-1 equal to that observed in the measurements of oxygen concentration. This coincidence may indicate that the long-term reduction of oxygen is due to the changes in the biological consumption rates, in the rates of the waters ventilation or even in sediment–water interactions rather than due to the effect of temperature and salinity on oxygen saturation.

These findings found in the shallower waters of the Ría de Vigo allow us to hypothesize that the long-term increase in salinity would produce an increasingly weak vertical salinity gradient in the water column that would favour the vertical fluxes between the bottom and surface waters. Therefore the observed changes of oxygen and remineralized nutrient inputs in the surface waters could be due to an increasing footprint of benthic respiration, which has a major importance in the net ecosystem metabolism of this coastal region (Alonso-Pérez et al., 2015). This hypothesis would also explain the intense acidification in the inner waters in spite of growing alkalinity buffering.

The mean values at each station of the ARIOS database estimated for each depth range described in Fig. 2, resulting in 8384 values, were used to estimate a general value of the long-term trend in pH. The historical pH values in situ from the ARIOS database showed a general decrease in seawater pH in the Iberian upwelling between 1976 and 2018, with an acidification rate of -0.012±0.002 yr-1 that significantly explains 2 % of the total pH variation (Fig. 5a). The apparent oxygen utilization was also shown as function of pH over time, revealing the association of higher AOU values with lower pH. The relationship between pH and AOU (Fig. 5b) showed an inverse linear correlation of -399±5 µmol kg-1 and a coefficient of determination (r2) of 0.52. The strong biological activity of the upwelling systems is the main driver of pH changes, explaining 52 % of the observed variation in the discrete measurements. The distribution of nitrate seen in relation to the distribution of pH and AOU (Fig. 5b) showed the association of higher pH values with negative AOU values and a nitrate decrease, reinforcing the importance of biological processes in these marine carbonate system. Although the different processes controlling the AOU values were not separated in this analysis, the oxygen concentration in addition to the remineralization of the organic matter and the photosynthesis is conditioned by changes in temperature and salinity, ventilation events, water masses mixing and other processes (Sarmiento and Gruber, 2006). Therefore, the long-term drop in seawater pH measurements estimated from the ARIOS database presented here confirms that the future evolution of ocean acidification in this productive region is likely to depend on both the CO2 increase in the atmosphere and other long-term changes (of natural and/or anthropogenic origin) affecting the seawater's carbonate system.