Drivers of the Carbonate System Seasonal Variations in a Mediterranean Gulf

Estuarine, Coastal and Shelf Science 168 (2016) 58e70

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Estuarine, Coastal and Shelf Science

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Drivers of the carbonate system seasonal variations in a Mediterranean gulf

* Gianmarco Ingrosso a, b, , Michele Giani a, Cinzia Comici a, Martina Kralj a, Salvatore Piacentino c, Cinzia De Vittor a, Paola Del Negro a a Istituto Nazionale di Oceanograﬁa e di Geoﬁsica Sperimentale (OGS), Oceanography Section, Via A. Piccard 54, 34151 S. Croce (TS), Italy b University of Trieste, Department of Life Sciences, Via Giorgieri 10, 34127 Trieste, Italy c ENEA-CLIMOSS, Via Catania 2, 90141 Palermo, Italy article info abstract

Article history: The effects of different physical and biogeochemical drivers on the carbonate system were investigated in Received 20 November 2014 a semi-enclosed coastal area characterized by high alkalinity riverine discharge (Gulf of Trieste, Northern Received in revised form Adriatic Sea, Mediterranean Sea). Our 2-year time-series showed that a large part of the seasonal car- 6 August 2015 bonate chemistry variation was controlled by the large seasonal change of seawater temperature, though Accepted 2 November 2015 air-sea CO exchange, biological activity (primary production-respiration), and riverine inputs also Available online 6 November 2015 2 exerted a significant influence. With the exception of summer, the Gulf of Trieste was a sink of atmospheric carbon dioxide, showing a Keywords: fl 2 1 pH very strong CO2 uxes from atmosphere into the sea ( 16.10 mmol m day ) during high wind speed fl Carbonate system event of north easterly Bora wind. The CO2 in ux was particularly evident in winter, when the biological Buffer capacity activity was at minimum and the low seawater temperature enhanced CO2 solubility. During spring, the River drawdown of CO2 by primary production overwhelmed the CO2 physical pump, driving a significant fi Ocean acidi cation decrease of dissolved inorganic carbon (DIC), [CO2], and increase of pHT25 C. In summer the primary Gulf of Trieste production in surface waters occurred with the same intensity as respiration in the bottom layer, so the Adriatic Sea net biological effect on the carbonate system was very low and the further reduction of seawater CO2 Mediterranean Sea concentration observed was mainly due to carbon dioxide degassing induced by high seawater temperature. Finally, during autumn the respiration was the predominant process, which determined an overall increase of DIC, [CO2], and decrease of pHT25 C. This was particularly evident when the breakdown of summer stratification occurred and a large amount of CO2, generated by respiration and segregated below the pycnocline, was released back to the whole water column. Local rivers also significantly affected the carbonate system by direct input of total alkalinity (AT) coming from the chemical weathering of carbonate rocks, which dominate the river watershed. Our finding clearly 1 demonstrates a high AT concentration in low salinity surface waters (AT max ¼ 2742.8 mmol kg ) and a negative AT-salinity correlation. As a result the Gulf of Trieste revealed a low Revelle factor (10.1) and one 1 of the highest buffer capacities of the Mediterranean Sea (ßDIC ¼ 0.31 mmol kg ), which allows the system to store a significant amount of atmospheric CO2 with a small decrease of seawater pH. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction 2004; Le Quer e et al., 2009, 2010; 2015; McKinley et al., 2011). However, the oceanic uptake of anthropogenic CO2 also leads to a The ocean currently absorbs approximately a quarter of the CO2 gradual acidification of the world's ocean, which is now a well added to the atmosphere from human activities each year, greatly known phenomena commonly referred to as “ocean acidification” reducing the impact of this greenhouse gas on climate (Sabine et al., (Caldeira and Wickett, 2003; Doney et al., 2009). Since the begin- ning of the industrial era, the ocean surface pH has decreased by 0.1 pH units and the global pH reduction actually ranges 1 * Corresponding author. Present address: Department of Biological and Envi- between 0.0014 and 0.0024 yr (IPCC, 2013). In the NW ronmental Science and Technology, University of Salento, 73100 Lecce, Italy. Mediterranean Sea the ocean acidification process is also occurring E-mail address: [email protected] (G. Ingrosso). http://dx.doi.org/10.1016/j.ecss.2015.11.001 0272-7714/© 2015 Elsevier Ltd. All rights reserved. G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70 59

(De Carlo et al., 2013) and the acidification rate has been estimated gulf (Querin et al., 2007). to be 0.003 pH units yr 1 (Geri et al., 2014). The water column is subject to a strong variation in tempera- The projections of ocean acidification in the coastal area, where ture, salinity, and vertical stratification during the year (Malacic and paradoxically some of the most vulnerable taxa live, are still lacking Petelin, 2001). Seawater temperature shows a seasonal cycle from and they are remarkably difficult to predict due to the great winter minima of 6 C to summer maxima of >29 C(Cardin and complexity of these ecosystems. Here, the CO2 levels are regulated Celio, 1997; Celio et al., 2006), whereas the salinity ranges from by different physical and biological drivers such as air-sea CO2 ex- 33 to 38 (Malacic et al., 2006). During spring the freshwater input changes, ecosystem metabolism, net community calcification rate, and surface heating cause the thermohaline stratification, which riverine inputs of carbonate weathering products, as well as hy- increases in strength during summer. In autumn and winter, drological features that determine the mixing between open ocean convective and mechanical mixing, induced by water cooling and and coastal waters (Duarte et al., 2013). These processes can cause wind, disrupts the vertical stratification leading to a mostly ho- changes in pH from seasonal to decadal and longer timescales, mogeneous water column. Also in this period, strong Bora wind enhancing or reducing the ocean acidification. For example, in events, a regional ENE cold wind, cause the formation of dense temperate eutrophic waters with high degree of stratification and waters, which are a fundamental component for the generation of reduced ventilation in summer/autumn months (Melzner et al., North Adriatic Dense Water (Artegiani et al., 1997; Malacic and 2012), the oxidation of organic matter in subsurface waters Petelin, 2001). lowers dissolved oxygen concentrations, adds CO2 to solution, re- River watersheds along the Gulf of Trieste are composed mainly duces pH, and, over longer time scales, can exacerbate the ocean of carbonate rocks (e.g., limestone and dolomite) (Plenicar et al., acidification process (Byrne et al., 2010; Cai et al., 2011). Conversely, 2009) and they have some of the highest carbonate-weathering an increase of total alkalinity export from river to the sea could in the world (Szramek et al., 2007, 2011). The riverine inputs then increase the seawater buffer capacity and mitigate regionally the represent a very important source of inorganic carbon, which can effect of ocean acidification (Raymond and Cole, 2003; Raymond significantly affect the marine carbonate system and the whole C et al., 2008; Watanabe et al., 2009). cycle. The Northern Adriatic Sea (NAd), situated in the northernmost region of the Mediterranean Sea, receives freshwaters from many 2.2. Sampling and analyses rivers and is subject to wide variations of temperature and salinity. Though oceanographic studies carried out in this area date back to The study was performed at the C1 station in the northeastern the 18th century, long time series of chemical oceanography pa- part of the Gulf of Trieste (4542.050Ne1342.600E), a coastal time rameters began only in the 1970s and the carbonate chemistry series station (bottom depth of 17.5 m) where a long term ecolog- observations were set up only recently (Turk et al., 2010). ical monitoring has been in place since 1970 (http://nettuno.ogs. Climatic and anthropogenic driven changes have occurred in the trieste.it/ilter/BIO/). The data presented here refer to the period of last decades in the NAd. An oligotrophication trend was observed March 2011eFebruary 2013. Discrete water samples were collected by different authors (Solidoro et al., 2009; Mozetic et al., 2010; monthly with 5-L Niskin bottles at four depths (0.5, 5, 10, 15 m) in Giani et al., 2012), who ascribed this process to a reduction of order to measure total alkalinity (AT), pH, dissolved oxygen, and annual river discharges and river-borne nutrient inputs. Moreover, nutrients. Luchetta et al. (2010) have recently detected a pH decrease in the Depth profiles of salinity (Practical Salinity Scale of 1978) and NAd, underlining the importance to follow also in this area the temperature (C) were determined using a Conductivity- ocean acidification process. Temperature-Depth/Pressure (CTD) recorders (Idronaut 316 and The objective of the present work is to understand how physical SBE 19 Plus Seacat probe). and biogeochemical drivers could influence the seasonal cycles of For the total alkalinity, samples were pre-filtered on glass-fibre marine carbonate system in a shallow Mediterranean gulf. Specif- filters (Whatman GF/F) into a 500 mL narrow-necked borosilicate ically, both the air-sea CO2 exchanges and thermal/not-thermal glass bottle. Filtration was performed to remove phytoplankton contributions to the seasonality of the CO2 system are quantified. cells and particles of CaCO3, derived from karstic watershed or We also investigate the buffer capacity of the area, which is affected calcifying organisms, which respectively can interact with HCl by the products of carbonate mineral weathering coming from titrant solution and dissolve during measurements due to acid freshwater discharges of the local rivers. additions, producing a non-negligible error in the exact estimation of the AT concentration (Gattuso et al., 2010; Hydes et al., 2010; 2. Materials and methods Bockemon and Dickson, 2014). Each bottle was poisoned with 100 mL of saturated mercuric chloride (HgCl2) to halt biological 2.1. Study area activity, sealed with glass stoppers and stored at 4 C in the dark until analysis. Total alkalinity was determined by potentiometric The Gulf of Trieste (Fig. 1) is a shallow semi-enclosed basin titration in an open cell (SOP 3b, Dickson et al., 2007) using a non- located in the northernmost part of the Adriatic Sea, with a linear least squares approach. The titration procedure was per- maximum depth of 25 m and a surface area of about 600 km2 formed with the titration unit Mettler Toledo G20 interfaced with a (Malej et al., 1995). It is an example of a coastal region of freshwater computer, using the data-acquisition software LabX. After titration influence (ROFI, Simpson, 1997), characterized by a high spatial and the data were processed and AT calculated with a computer pro- temporal variability of its physical, chemical, and biological prop- gram developed at OGS (Istituto Nazionale di Oceanografiaedi erties. Four rivers discharge waters into the gulf: Isonzo, Timavo, Geofisica Sperimentale) similar to that listed in SOP 3 of DOE (1994) Rizana, and Dragonja. The main freshwater input is through the and adapted to work in association with the Mettler Toledo LabX Isonzo river from the north-west Italian coast (Cozzi et al., 2012), software. The HCl titrant solution (0.1 mol kg 1) was prepared in which is characterized by high discharges rate during spring and NaCl background, to approximate the ionic strength of the samples, autumn, while drought periods occur in winter and summer. These and calibrated against certified reference seawater (CRM, Batch discharges, due to the prevailing cyclonic circulation, affect mainly #107, provided by A.G. Dickson, Scripps Institution of Oceanog- the north-western part of the gulf and only during the freshets in raphy, USA). Accuracy and precision of the AT measurements on comitance with southerly winds the plume spread throughout the CRM were determined to be less than ±2.0 mmol kg 1. Additionally, 60 G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70

Fig. 1. Map of the Gulf of Trieste in the North Adriatic Sea, with the C1 sampling site. Other time series stations of the carbonate system are shown in the map: Paloma in the Gulf of Trieste, Point B and DYFAMED in the western Mediterranean Sea. repeated measurements (n 3) on in-house standards of natural Bran þ Luebbe Autoanalyzer 3, according to Grasshoff et al. (1999). seawater were undertaken every day prior to sample analysis in Detection limits for the procedure were 0.003 mM, 0.01 mM, order to monitor the instrument's accuracy and precision 0.04 mM, 0.02 mM, and 0.02 mM for NO2,NO3,NH4,PO4, and H4SiO4, frequently. respectively. The pH was measured using a double-wavelength spectropho- CO2 monitoring in air was run on a weekly basis and with tometer (Cary 100 Scan UVeVisible) and the indicator dye m-cresol continuous measurements at ENEA Station for Climate Observa- purple (Merck-105228) following SOP 6b of Dickson et al. (2007).To tions on the island of Lampedusa (Italy). Duplicate air samples were avoid CO2 gain or loss, unfiltered seawater samples were collected collected and measured using a Siemens Ultramat 5E non- directly into quartz cuvettes with a 10 cm pathlength, leaving no dispersive infrared analyzer (NDIR). Sampling and measurement head space. pH was always measured within a few hours from procedures are described elsewhere (Chamard et al., 2003). sampling and values are reported on total scale (pHT). The analyt- Determination of CO2 mixing ratio on flask samples was obtained ical precision was estimated to be ±0.002 pHT units, determined by with a standard deviation of about 0.04 ppm. To ensure accurate replicates from the same Niskin bottle. The accuracy was assessed measurements, eight CO2 mixtures, certified by NOAA Climate analysing the pHT of three different CRM Batch (#107, #124, #125) Monitoring and Diagnostic Laboratory (CMDL), were used as pri- and comparing it with the pHT reference value calculated from mary standards. certificated AT-DIC and the dissociation constant from Mehrbach et al. (1973) refitted by Dickson and Millero (1987). The pHT measured on these reference materials was lower than the theo- 2.3. Calculation of derived parameters retical value by 0.028 ± 0.006 pH units. This difference probably fi derived from the use of an unpuri ed indicator, which could The other carbonate system parameters, including seawater determine an error as large as 0.02 pH units (Yao et al., 2007; Liu partial pressure of CO2 (pCO2; matm), dissolved inorganic carbon et al., 2011). The pH results at C1 station were therefore cor- 1 T (DIC; mmol kg ), and aragonite saturation state (UAr), were fi rected with the linear t equation between calculated Batch pHT calculated using the CO2SYS program (Lewis and Wallace, 1998) and measured Batch pH (pH ¼ 1.0111pH 0.0589; T T Calc. T Meas. through in situ AT,pHT (25 C), temperature, salinity, phosphate, R2 ¼ 0.998) in order to reduce the offset found between “true” pH T and silicate data. Carbonic acid dissociation constants (i.e., pK1 and and measured pH with unpurified indicator. T pK2)ofMillero (2010) were used for the computation, as well as the For the determination of dissolved oxygen concentration (DO; Dickson constant for the ion HSO (Dickson, 1990) and borate 1 4 mmol kg ) the Winkler method was followed (Grasshoff et al., dissociation constant of Lee et al. (2010). 1999) using an automated titration system (Mettler Toledo G20) The apparent oxygen utilization (AOU; mmol kg 1) was deter- with potentiometric end-point detection (Outdot et al., 1988). The mined as the difference between the saturated oxygen concentra- ± 1 analytical precision and accuracy was 1.5 mmol kg . tion and the observed oxygen concentration, providing an The samples to measure the dissolved inorganic nutrient con- approximation to the balance between biological processes of pri- centration (nitrite, NO2, nitrate, NO3, ammonium, NH4, phosphate mary production and respiration. PO , and silicic acid, H SiO ; mmol kg 1) were prefiltered on 0.7 mm 2 1 4 4 4 The air-sea surface flux of CO2 (FCO2; mmol m day )was pore size glass-fibre filters (Whatman GF/F) immediately after the estimated from the difference between the partial pressure of CO2 sampling and stored at 20 C in polyethylene vials until analysis. at sea surface (pCO2) and the concentration of carbon dioxide in the The samples were defrosted and analysed colorimetrically with a atmosphere (pCO2 atm) according to the equation: G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70 61

present (up to DS z 3.5 in June 2012). ¼ ð Þ FCO2 kK0 pCO2 pCO2 atm Dense water masses with density anomaly greater than 29.0 kg m 3 were observed in winter (Fig. 2a). These waters where k is the gas transfer velocity of CO2 and K0 is the solubility represent the Northern Adriatic Dense Water (NAdDW), which fi coef cient of CO2 in seawater at in situ temperature and salinity spread as a dense current along the western Adriatic shelf, filling up (Weiss, 1974). We use the gas transfer velocity parameterization as the middle and south Adriatic depressions. The highest density function of quadratic wind speed, following the empirical expres- anomaly value (30.46 kg m 3) was detected during February 2012 sion of Wanninkhof (1992) for short-term or steady wind speed. 1 as a consequence of an extreme cooling event due to prolonged The wind speed data (u; m s ) was recorded at 10 m height in the blowing of the cold ENE wind (Bora), which determined on the e nearby weather PALOMA ISMAR CNR station and daily average Adriatic shelf the extraordinary dense water formation event of values were used for the calculation of gas transfer velocity. The winter 2012 (Mihanovic et al., 2013). partial pressure of atmospheric pCO2 was derived from the CO2 Total alkalinity concentration spanned over a wide range. The molar fraction measurements at the remote island of Lampedusa, highest value was detected at the surface in August 2011 using daily barometric pressure, sea surface temperature and (2742.8 mmol kg 1), whereas the minimum was reached at the salinity measured during experimental activity. Negative FCO2 bottom in January 2012 (2657.2 mmol kg 1)(Fig. 2c). These re- value indicates air-to-sea flux, whereas positive value represents a sults reflect the AT seasonal variation, which presented higher net CO2 outgassing from the water body to the atmosphere. and comparable average values in spring and summer To distinguish the effect of seasonal temperature change from (2695.5 ± 16.3 and 2696.2 ± 21.3 mmol kg 1, respectively), and the effect of the other processes, the method of Takahashi et al. lower and comparable concentrations in winter and autumn (2002) was applied on surface and bottom pCO2 data. The effect (2677.7 ± 12.7 mmol kg 1 and 2678.7 ± 11.1 mmol kg 1,respec- of temperature changes on pCO2 (pCO2thermal) was estimated ac- tively) (Fig. 3). cording to the equation In winter the pHT at in situ temperature was elevated and homogeneous along the water column, with an average value of 8.203 pCO ¼ pCO2mean$exp½0:0423ðT TmeanÞ 2thermal obs (SD ¼ 0.03) (Figs. 2d and 3). From spring to summer, the mean where pCO was the biannual mean pCO , and (T T ) seasonal pHT gradually decreased. In these periods, characterized 2mean 2 obs mean fi was the difference between the observed and mean temperature by a strong thermohaline strati cation, the pHT in the bottom waters was generally lower (up to 0.208 pHT units in late summer) over two years. The resulting pCO2thermal indicates the values of than at the surface. In both years, the lowest pHT values were pCO2 that would be expected if a parcel of water, having a mean detected in the water masses under the pycnocline of September pCO2 of the two years, is subjected to seasonal temperature changes under isochemical conditions. (7.906 in 2011; 7.969 in 2012). After this month, with the breakdown of the vertical stratification, the pHT increased and became To remove the thermal effect from the observed pCO2 (pCO2obs), the values were normalised to a constant seawater temperature again uniform in the whole water column, with an average value of ± using the equation 8.129 0.04. The dissolved oxygen concentration (Figs. 2e and 3) showed pCO ¼ pCO $exp½0:0423ðTmean T Þ seasonal variations very similar to the in situ pHT. High DO con- 2not thermal 2obs obs centrations (279.6 mmol kg 1,SD¼ 20.7) characterized the winter The not-thermal pCO2 (pCO2not-thermal) then represents the pCO2 months, whereas during spring and summer the concentrations variation that can be driven by biological processes (primary pro- were substantially lower, reaching the minimum values in fi duction, respiration, and calci cation), advection of water masses, September at the bottom (137.0 mmol kg 1 in 2011; 155.1 mmol kg 1 and air-sea CO2 exchange. in 2012). The seasonal change of dissolved inorganic carbon (Figs. 2f and 2.4. Statistical analysis 3) followed a pattern of lower concentrations during spring and summer periods (2361.5 ± 27.3 mmol kg 1 and The normality of the data was assessed with the ShapiroeWilk 2332.1 ± 42.6 mmol kg 1, respectively) and higher values in winter test (Shapiro and Wilk, 1965). Due to their non-normal distribution, (2379.2 ± 9.6 mmol kg 1). Generally, from May to late-July, a gradual a non parametric approach was adopted. Statistically significant decrease at the surface was followed by an increase in the water differences between seasons for each parameter were tested with masses under the pycnocline, which started in late-spring and the KruskaleWallis rank sum test (Kruskal and Wallis, 1952), reached maximum in the summertime period. whereas the multiple comparison test (Siegel and Castellan, 1988) Ammonia varied from 0.23 mmol kg 1 to 3.61 mmol kg 1 was used to determine which pair of seasons were different. Ana- (Fig. 4a), with low concentration mainly during spring lyses were performed with R program and the package pgirmess. (0.86 mmol kg 1,SD¼ 0.54 mmol kg 1) and high values at the bottom waters in summer (3.61 mmol kg 1 in Aug. 2011; 1 3. Results 3.26 mmol kg in Sept. 2012). PO4 concentrations were low in all seasons, spanning between non-detectable (<0.01 mmol kg 1)to Over the two years of study, the seawater temperature (Fig. 2b) 0.15 mmol kg 1, with higher values above the pycnocline during varied from 4.79 C (February 2012) to 26.16 C (July 2011), springesummer (Fig. 4b). exhibiting a DTof21.4C and of 18.5 C in the 2011 and 2012 respectively. Seawater temperature was significantly different 4. Discussion (p < 0.001) among each season (Table 1), with the only exception of spring and autumn, which instead showed a comparable mean In the river-influenced coastal areas, the seasonal variability of value. the carbonate system is the result of several physical and biological In winter, the salinity reached a mean value of 37.40 (± 0.99) and processes such as seasonal changes in seawater temperature, net it was remarkably higher than the surface waters of spring community production, carbonate mineral dissolution/formation, (36.21 ± 0.94) and summer (36.18 ± 0.98). As a result, from May to air-sea CO2 gas exchange, riverine inputs, and mixing between September of both years a strong vertical halocline gradient was waters with different characteristics. In the following sections we 62 G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70

Fig. 2. Monthly variations at the C1 station of: (a) salinity, density anomaly (st e white isolines), (b) temperature, (c) total alkalinity (AT), (d) in situ pHT, (e) dissolved oxygen (DO), (f) dissolved inorganic carbon (DIC). analize these factors separately in order to understand how they The winter season was characterized by a homogeneous water affect the marine carbonate system in the Gulf of Trieste. column and showed a very low biological activity (AOU ¼ 1 1.6 ± 17.1 mmol kg ; Figs. 3 and 4c) and pHT25 C (7.957 ± 0.015; 4.1. Influence of physical and biological processes on the carbonate Figs. 3 and 4d). From spring to summer, with the formation of the system pycnocline, the water column acted as a two-layer system, with primary production processes prevailing in the upper layer and The concentrations of individual species of the carbonate system respiration of organic matter in the lower layer. As a consequence, change with temperature, salinity, and pressure (because of varia- surface waters presented a higher pHT25 C and were supersatu- tion of the equilibrium constants with T, S, and p) (Zeebe and Wolf- rated in oxygen (AOU < 0). In July 2012, for example, pHT25 C and 1 Gladrow, 2001). For example, colder seawater has higher [CO2] and AOU at 5 m depth were 8.145 and 40.1 mmol kg , respectively, 2 ½HCO3 and lower ½CO3 at a given DIC and AT, and they present whereas the total dissolved inorganic carbon concentration was 1 higher pHT values. very low (2260 mmol kg ) compared to other periods (Fig. 4c and fl To remove the thermal effect on the observed pHT and to eval- d). This strongly negative AOU value re ects a pronounced primary uate how the biological processes of primary production and productivity, which influences the carbonate system reducing pCO2 respiration can influence seasonal variability of pHT, it is possible to and DIC, and increasing the pHT25 C. Otherwise, in spring and consider the seasonal variation of pHT measured at constant tem- summer bottom waters, the pHT25 C was generally lower than at perature of 25.0 C (pHT25 C) with respect to the apparent oxygen the surface, and in particular three periods with a very low pHT25 C utilization (AOU). were observed: September 2011, June 2012, and September 2012 G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70 63

Table 1 Results of KruskaleWallis rank sum test and multiple comparison test among seasons on different physicalechemical parameters. In bold are reported the pair of seasons signiﬁcantly different.

KruskaleWallis rank sum test Temperature Salinity DO AT pHT in situ c2 p-value c2 p-value c2 p-value c2 p-value c2 p-value

77.0414 <0.001 15.1352 0.002 61.6022 <0.001 22.3682 <0.001 60.3131 <0.001

Multiple comparison test (p ¼ 0.05) Obs. dif Critical. dif Obs. dif Critical. dif Obs. dif Critical. dif Obs. dif Critical. dif Obs. dif Critical. dif

Autumnespring 6.90 21.22 14.96 21.22 33.44 21.22 27.90 21.00 25.67 21.00 Autumnesummer 30.65 21.22 10.48 21.22 8.98 21.00 26.00 21.00 14.13 21.00 Autumnewinter 39.58 21.22 13.77 21.22 43.98 21.00 0.47 21.22 42.68 21.22 Springesummer 37.54 21.22 4.48 21.22 42.42 21.22 1.90 21.00 39.79 21.00 Springewinter 32.69 21.22 28.73 21.22 10.54 21.22 27.43 21.22 17.02 21.22 Summerewinter 70.23 21.22 24.25 21.22 52.96 21.00 25.53 21.22 56.81 21.22

KruskaleWallis rank sum test DIC AOU pHT at 25 CCO2 FCO2 c2 p-value c2 p-value c2 p-value c2 p-value c2 p-value

34.6226 <0.001 33.1994 <0.001 41.4368 <0.001 26.18 <0.001 8.19 0.04231

Multiple comparison test (p ¼ 0.05)a Obs. dif Critical. dif Obs. dif Critical. dif Obs. dif Critical. dif Obs. dif Critical. dif Obs. dif Critical. dif

Autumnespring 7.19 21.00 45.48 21.22 10.04 21.00 16.48 21.00 5.00 9.77 Autumnesummer 18.21 21.00 29.88 21.00 23.96 21.00 22.79 21.00 5.33 9.77 Autumnewinter 28.54 21.22 23.77 21.00 26.18 21.22 14.47 21.22 4.33 9.77 Springesummer 25.40 21.00 15.60 21.22 13.92 21.00 6.31 21.00 10.33 9.77 Springewinter 21.35 21.22 21.70 21.22 36.23 21.22 30.95 21.22 0.67 9.77 Summerewinter 46.75 21.22 6.10 21.00 50.14 21.22 37.27 21.22 9.67 9.77

a For FCO2 p ¼ 0.1.

Fig. 3. Box-and-whisker plot presenting the seasonal average of physical and chemical parameters measured in the water column at C1 station. Seasons were chosen as 3 months periods: winter (JanuaryeMarch), spring (AprileJune), summer (JulyeSeptember), and autumn (OctobereDecember).

(Fig. 4d). Concurrently, also a large positive AOU and high con- a substantial amount of CO2, generated by the heterotrophic ac- centration of dissolved inorganic nutrients (e.g., in September 2011, tivity, spread from the bottom to the surface, causing a general 1 1 pHT25 C ¼ 7.805, AOU ¼ 85.2 mmol kg ,NH4 ¼ 3.26 mmol kg , increase of DIC. 1 PO4 ¼ 0.15 mmol kg ) were detected, due to a high organic matter The observed seasonal pattern between pHT25 C and biological degradation which consumes oxygen, remineralizes nutrients, activity, and the decoupling of production and respiration due to produces carbon dioxide, and leads to a higher concentration of the presence of the pycnocline during springesummer seasons, are DIC. very similar to that found by Cantoni et al. (2012) in the same area During October, the decrease of seawater temperature and the (Fig. 1), even if they reported a higher acidiﬁcation of bottom wa- water mixing, triggered by strong winds, caused the breakdown of ters. This difference can be ascribed to the greater depth of their the pycnocline and the homogenization of the water column. Thus, sampling station (25 m) and to the more intense remineralisation 64 G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70

Fig. 4. Monthly variations at the C1 station of: (a) ammonia (NH4), (b) phosphates (PO4), (c) apparent oxygen utilization (AOU), (d) pHT at 25 C, (e) Revelle factor (RF) and (f) aragonite saturation state (UAr). processes registered in the bottom waters during August 2008, Another relevant process, which influences the global cycle of the which led to a hypoxic event, to a rise of pCO2, and to a decrease of carbonate system, is the exchange of carbon dioxide between atmo- pH. sphere and seawater. On annual scale, the coastal waters of the Gulf of 2 1 It has been suggested that hypoxia in coastal habitats will Trieste acted as a CO2 sink at a rate of 2.14 ± 2.55 mmol m day in enhance the future ocean acidification (Feely et al., 2010; Cai et al., 2012 and 3.27 ± 5.80 mmol m 2 day 1 in 2013 (Fig. 5). From January 2011; Melzner et al., 2012). Strong hypoxic conditions have been to March, in both years, our results showed a clear influx of CO2 to- observed in the Northern Adriatic Sea, as consequence of marked ward the sea (2.84 ± 2.24 mmol m 2 day 1). The average magnitude stratification, restricted mixing, prolonged bottom water residence of the flux during winter was 3.48 ± 3.02 mmol m 2 day 1 in 2012 time, and organic matter respiration (Ott, 1992; Giani et al., 1992; and 2.21 ± 1.49 mmol m 2 day 1 in 2013. This condition was driven Degobbis et al., 2000). However, after the year 2000 only few by the low seawater temperature, which led undersaturation of sur- cases of dissolved oxygen saturation lower than 20% have been face pCO2 with respect to the atmosphere. observed (Socal et al., 2008; Solidoro et al., 2009; Giani et al., 2012) During spring, thermally driven pCO2 increase was counter- and considering this trend an exacerbation of ocean acidification balanced by the biologically driven pCO2 decrease. The surface processes due to hypoxic events is unlikely. The biologically waters were still undersaturated with respect to the atmospheric induced acidification of bottom oxygen-depleted water in summer CO2 and air-sea surface fluxes persisted toward the sea. In produces a short-term but very sharp pH drop of approximately 0.3 particular, a very high absorption of atmospheric CO2 occurred in pH units on a seasonal time scale, which lies in the normal range of May 2012, when the combined effects of primary production and seasonal pH variation of coastal habitats (Duarte et al., 2013). strong wind speed resulted in an air-sea CO2 influx G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70 65

are lower than those estimated in the present work at the C1 station and confirm the higher capacity of the Gulf of Trieste to absorb the atmospheric CO2, due to its higher primary productivity and the very low seawater temperature during winter dense water formation episodes. At C1 station, the seasonal air-sea CO2 exchange pattern (Fig. 3) is consistent with the previous works in marginal seas (Chen and Borges, 2009) and also with recent studies in other areas of the Gulf of Trieste (Cantoni et al., 2012; Turk et al., 2013), which indicated a CO2 sink from autumn to spring, and a source in summer. In our study, the seasonal mean CO2 fluxes are comparable among all seasons, except between spring and summer, which instead presented a significantly different mean value (Table 1 and Fig. 3; 4.80 ± 5.22 mmol m 2 day 1 in spring and 0.15 ± 1.08 in summer). The magnitude of the flux during the two Bora wind events (14.22 mmol m 2 day 1 in May 2012 and 16.10 mmol m 2 day 1 Fig. 5. Monthly air-sea CO2 fluxes (FCO2), daily average wind speed at 10 m above sea level (u), and sea surface temperature (Temp.) at C1 station. in November 2012) falls within the range of previous estimates (Cantoni et al., 2012; Turk et al., 2013) and confirms the strong fl in uence of these events on the air-sea CO2 exchange. However, on of 14.22 mmol m 2 day 1. annual time scale, the increase of CO2 flux from the atmosphere to In late summer, the strong seasonal warming drove pCO2 su- the sea generated by Bora episodes was estimated to be around 5% persaturation in surface waters and outgassing of carbon dioxide in (Turk et al., 2013) and it could slightly enhance the ocean acidifi- the atmosphere. These CO2 evasions were particularly evident cation process. 2 1 during September 2012 (1.70 mmol m day ) when the CO2 In order to estimate which is the predominant driver (photo- concentration throughout the water column increased, likely as synthesis/respiration, carbonate dissolution/formation, and CO2 consequence of mixing with deeper layers enriched in CO2 by release/invasion) affecting the marine carbonate system in the respiration. Winds also played an important role in the atmo- different seasons, it is possible to consider a graphical approach spheric carbon dioxide sequestration as showed by the strong Bora based on a propertyeproperty plot of DIC and A (Fig. 6), where the 1 T blowing in November 2012 (up to 12 m s ), when the absolute reaction path can take on variable slopes depending on the ratio of fl 2 1 maximum ux ( 16.10 mmol m day ) was observed. different processes (Deffeyes, 1965; Suzuki and Kawahata, 2003; Our annual average of CO2 flux is significantly lower than the Albright et al., 2013; Andersson and Gledhill, 2013; Nomura et al., magnitude of 6 mmol m 2 day 1 reported by Turk et al. (2010) in 2014). DIC and AT were normalized (nDIC and nAT) to a constant the southern part of the Gulf of Trieste; this dissimilarity could stem salinity, by multiplying DIC and AT data by the ratio of 37.81 (the from differences in the sampling periods and geographical loca- mean value of winter bottom layer which is representative of the tions of the sampling stations, which are subject to different offshore waters) to the measured in situ salinity, in order to remove oceanographic conditions. However, the high-frequency measure- the influence of freshwater input, mixing, and evaporation/pre- ments of sea surface pCO2 reported by Turk et al. (2010) could be cipitation. Moreover, the contribution of calcification and dissolu- more suitable in the coastal zone (Borges and Gypens, 2010) and tion to the carbonate system seasonal variation has been assumed probably could explain the difference in annual air-sea CO2 ex- irrelevant with reasonable approximation. This because at the change between these two neighbouring sites. Flux estimates saturation state level, salinity, and temperature observed in the derived from data collected daily, weekly, or monthly, are less ac- Gulf of Trieste, the natural abiotic precipitation/dissolution of curate than those obtained by (near-)continuous determinations CaCO3 does not occur (Morse and He, 1993; Morse et al., 2007; (De Carlo et al., 2013), because in highly dynamic coastal environ- Marion et al., 2009). The influence of CaCO3 precipitation could ments there are a number of processes that display short-term derive, then, only from calcifying organisms. However, Mozetic variability in the carbonate system which easily can be missed by et al. (1998) reported in the Gulf of Trieste a very low abundance low-frequency sampling. of coccolithophores, the most important calcifying phytoplankton fl It is also interesting to compare the CO2 ux measured in the group in temperate sea, and Cantoni et al. (2012) did not found a Gulf of Trieste and those of other time series in the northwestern clear sing of pelagic calcification rate able to alter the carbonate Mediterranean Sea (Fig. 1): the DYFAMED site (with monthly system. No information is available on the potential contribution of sampling, Begovic and Copin-Montegut, 2002), and Point B (with the benthic calcifying organisms, but this could be also small weekly sampling, De Carlo et al., 2013). Compared to our work, because the study area is characterized mainly by a sandy-muddy these studies used a different wind speed parameterization to seabed with sporadic coralligenous assemblages (Casellato and calculate the air-sea CO2 exchange. Begovic and Copin-Montegut Stefanon, 2008). (2002) used the parameterization of Wanninkhof and McGillis In winter, the slope of the regression line between nDIC and nAT fl (1999), whereas De Carlo et al. (2013) calculated the ux was 0.98 (Fig. 6), so the dissolved inorganic carbon increased following that of Ho et al. (2006). However, the annual average slightly compared to the alkalinity. This relation probably was wind speed in the three cases was relatively modest (5 m s 1 at determined by the atmospheric CO2 uptake, which increased DIC 1 1 DYFAMED, 3.9 m s at Point B, 4.17 m s at C1 station), and when without affecting total alkalinity. As shown in Fig. 3, during winter, e 1 the wind speed is in the range of 2 5ms the choice of param- when the biological activity was minimal (AOU z 0), the very low eterization does not have a large effect on the estimation of the seawater temperature supported a marked invasion of atmospheric fluxes (De Carlo et al., 2013). At the DYFAMED site the authors re- CO2, determining a high concentration of DIC and low pHT 25.0 ported an annual average fluxes of 0.87 mmol m 2 day 1 for 1998 values. and 1.86 mmol m 2 day 1 for 1999, whereas at the Point B they In spring, the slope of the nDIC-nAT regression line was 1.67 2 1 fl found an annual CO2 uptake of 0.53 mmol m day . These uxes (Fig. 6) and the ANCOVA test for the Homogeneity of Slopes- 66 G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70

Fig. 6. Relationships between the salinity-normalized DIC (nDIC) and salinity-normalized AT (nAT) by seasons. The vectors in the inset indicate theoretical modiﬁcation of DIC and AT owing to biogeochemical processes of photosynthesis/respiration, carbonate dissolution/formation, and CO2 release/invasion. Isolines represent the pHT 25.0 calculated at the different AT-DIC with the R package seacarb (Gattuso and Lavigne, 2009).

Intercept found that the winter and spring linear regression were could lead to acidification (Nixon, 2009), adding to the ocean pH significantly different (F ¼ 21.06, p < 0.001). This case is a clear reduction derived by anthropogenic CO2 uptake. indication of the influence of primary production, as evidenced also An oligotrophication trend has been recognised in the Gulf of by a negative AOU (12.8 mmol kg 1, Fig. 3), which caused the Trieste (Solidoro et al., 2009; Mozetic et al., 2010; Giani et al., 2012), decrease of dissolved inorganic carbon concentration and the in- which can be ascribed to a reduction in outflow and nutrients loads crease of total alkalinity and pHT 25.0. from the Isonzo River, due to increased frequency of drought pe- During summer, the primary production persisted in the sur- riods generated by the ongoing climate changes or by a larger hu- face layer, but it was counterbalanced by the strong respiration of man usage of continental waters (Cozzi and Giani, 2011). If this organic matter in the bottom waters. As a result, the net process continues in the future, the primary production could ecosystem production was close to 0 (AOU ¼ 0.7 ± 30.0 mmol kg 1) decrease and, in this way, the ocean acidification process could be and the influence of biological processes on carbonate was very enhanced. Moreover, a decrease in alkalinity discharge would low. Therefore, the nDIC-nAT slope of 1.06 (Fig. 6), significantly reduce the buffer capacity of the system. different respect to the spring (F ¼ 4.92, p ¼ 0.01), was probably due to the CO2 release towards the atmosphere, which determines 4.2. pCO2 seasonal variation a reduction of DIC concentration and a rise of pHT 25.0. Finally in autumn, respiration predominated over photosynthesis Like other carbonate system parameters also the pCO2 seasonal 1 (AOU ¼ 18.3 ± 9.6 mmol kg ). The change of nDIC-nAT regression variation is affected by physical (i.e. temperature, mixing, and air- fl fi (Fig. 6)reected this condition (signi cant difference between sea CO2 exchange) and biological processes (i.e. photosynthesis, summer and autumn Y-intercept: F ¼ 15.36, p < 0.001), as well as respiration, and calcification). To distinguish the effect of seasonal the carbonate system: during the transition from summer to temperature change from the effect of the other processes, the autumn the pHT25 C and total alkalinity decreased, whereas the method of Takahashi et al. (2002) was applied on surface and DIC and dissolved CO2 increases (Fig. 3). bottom pCO2 data. The not-thermal pCO2 (pCO2not-thermal) then fi These results con rm that primary production and respiration represents the pCO2 variation that can be driven by biological processes are relevant drivers of the large annual pH variation in processes (primary production, respiration, calcification), advec- the coastal ecosystem (Duarte et al., 2013). When they are in tion of water masses, and air-sea CO2 exchange. equilibrium, the net CO2 balance is essentially neutral. However, on In Fig. 7, the thermal and not-thermal pCO2 variations at C1 longer timescales, if one of these processes prevails over the other, station are reported for the surface and bottom layers. The results the net community metabolism changes and the pH is affected are shown as differences (dpCO2) with respect to the February 2012, drastically. It has been reported that the eutrophication could chosen as reference because it presented the lowest temperature, counteract or enhance the effects of ocean acidification by the minimum biological activity, and the weakest riverine inputs. increasing pH when primary production CO2 uptake prevails The seasonal thermal effect (dpCO2thermal) was predominant (Borges and Gypens, 2010), or by decreasing pH when respiratory both at surface and in the bottom waters. The highest values were CO2 release prevails (Cai et al., 2011). Oligotrophication, instead, reached at the surface in July 2011 and August 2012 (329 and G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70 67

1 (AT 2680.6 mmol kg ) and strong negative correlation with salinity (y ¼43.63x þ 4258.68 r ¼0.90, p ¼ 0.02) (Fig. 8). Considering all AT data, the relationships between AT and salinity was still evident but with a weaker correlation (Fig. 8, y ¼12.33x þ 3144.13 r ¼0.58, p < 0.001). This probably resulted from the influence of different river end-members (Tamse et al., 2014), as well as the high temporal variability of riverine AT inputs (Cantoni et al., 2012). Moreover, the total alkalinity concentration could be magnified in a highly productive coastal system (Hydes et al., 2010). In particular, the presence of organic carbon, denitrification, biological carbonate mineral precipitation, photosynthesis and respiration, could all cause perturbation of AT concentrations without affecting salinity, and then alter the AT-S relationship. 1 The AT value extrapolated at 0 salinity was 4258.7 mmol kg during the event of high freshwater inputs, whereas considering all data it was 3144.1 mmol kg 1. These two results are consistent with the previous data on the total alkalinity concentration of Isonzo and Timavo watershed (Szramek et al., 2011; Tamse et al., 2014), the two most significant sources of freshwater to the Gulf (Cozzi et al., 2012). In the same area, a recent work using stable isotope signa- Fig. 7. Monthly variations of surface and bottom values of dpCO2, dpCO2thermal, and dpCO2not-thermal at C1 station. ture of dissolved inorganic carbon suggested that freshwater inputs during spring, which is a period of freshets, can represent about 16% of marine DIC (Tamse et al., 2014). 323 matm, respectively). This thermodynamic increase of pCO2 was damped by the not-thermal effect, probably due to the biological 4.4. Buffer capacity of the system drawdown of CO2 (dpCO2not-thermal < 0), which was shown to be elevated above the pycnocline from winter to late summer. In The Revelle factor is a measure of the seawater's capacity to particular, only in March 2012 the not-thermal effect was greater absorb CO2 from the atmosphere and it is defined as the ratio of the than the thermal variation in the whole water column, maybe as a fractional change in the partial pressure of carbon dioxide in the fl consequence of the spring phytoplankton bloom in uence. atmosphere to the fractional increase of the total inorganic carbon > Respiration processes at the bottom (dpCO2not-thermal 0) were in the seawater (Revelle and Suess, 1957). A low Revelle factor also evident. In particular, during September 2011, the dpCO2not- implies that, for a given increase in atmospheric CO2, the concen- therm was up to 144 matm, and the dpCO2 reached the maximum tration of dissolved inorganic carbon will be higher than in waters value of 405 matm. This event indicated the importance of the with a high Revelle factor. remineralisation processes to generate an excess of carbon dioxide At the C1 station the Revelle factor (RF) was highly variable, in the deeper layer, which together with the thermal pCO2 increase ranging from 8.8 to 12.1 (Fig 4e). Due to the strong temperature and determined a high seawater concentration of CO2 during summer. alkalinity dependency, the lowest values (between 8.8 and 9.1) To evaluate the relative importance of temperature with respect were found in spring and summer warm surface waters, when the to mixing, air-sea CO2 exchange and biological processes on the highest temperature and total alkalinity concentration were annual pCO2 cycle, it is useful to consider the T/B ratio, where detected. During autumn and winter, the lowering of seawater T ¼ pCO2thermal max pCO2thermal min and B ¼ pCO2not-thermal max pCO2not-thermal min. In marine ecosystem where the not- thermal effect on the pCO2 dynamics exceeds the thermal effect, the T/B ratio varies from 0 to 1, whereas in areas where the influence of temperature changes on pCO2 cycle is predominant, the T/B ratio is greater than 1 (Takahashi et al., 2002). At C1 station the T/B is 1.01, which indicates a similar importance between thermal and not-thermal effect. These results differ from those reported by Cantoni et al. (2012) who found at PALOMA station, in the center of the Gulf of Trieste, a T/B ratio of 1.35, and therefore a stronger control of temperature on pCO2 cycle. These two different results could be explained by the different locations of the two stations: the PALOMA station is an offshore site, whereas the C1 station is very close to the coast. As shown previously, the air-sea CO2 exchanges between these two sites are very similar, so probably the T/ B ratio at the C1 station could be a result of a more relevant influence of biological processes and freshwater advection on the annual pCO2 variation.

4.3. Effects of river inputs on the carbonate system

In our study the A samples collected in low salinity surface T Fig. 8. A -Salinity and Revelle factor-Salinity plots at C1 station. The lines represent the e e T waters (depth 5 m) of June July August 2011, subsequently to a linear relationships, black dots and dashed lines fitting refer to the data collected after high freshwater input (Fig. 2a and c), showed high concentrations high riverine discharges. 68 G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70 temperature drove an increase of Revelle factor (10.2 ± 0.3 and Table 2 10.7 ± 0.3 respectively), but the highest value was found in the Statistical summary of several buffer factors in the different seasons at C1 station (March 2011eFebruary 2013). Mean, standard deviation (SD), minimum (Min) and bottom water of September 2011 (max ¼ 12.1), due to the high CO 2 maximum (Max) are reported. concentration produced by strong respiration processes. The two-year average RF value that we found (10.1 ± 0.6) is in Buffer factor Seasons Mean SD Min Max agreement with previous studies for the Adriatic Sea (Luchetta RF Winter 10.7 0.3 10.3 11.5 et al., 2010; Alvarez et al., 2014). This confirms that the Gulf of Spring 10.1 0.5 9.1 11.4 Trieste has a relatively high uptake capacity for the anthropogenic Summer 9.6 0.8 8.8 12.1 Autumn 10.2 0.3 9.7 10.7 CO2. However, considering the average RF in the different seasons, All data 10.1 0.6 8.8 12.1 two extreme and opposite situations can be detected: the Revelle gDIC (mmol kg 1) Winter 0.24 0.01 0.23 0.25 factor was 9.6 in summer and 10.7 in winter, which represent Spring 0.25 0.01 0.23 0.28 respectively the lowest and the highest values that we can find in Summer 0.27 0.02 0.22 0.28 Autumn 0.25 0.01 0.24 0.26 the open Mediterranean Sea (Alvarez et al., 2014). Thus, the RF of All data 0.25 0.01 0.22 0.28 the system is more likely to change seasonally than in open waters, ßDIC (mmol kg 1) Winter 0.29 0.01 0.27 0.30 ranging between a very high and very low capacity to take up Spring 0.31 0.02 0.27 0.35 Summer 0.33 0.03 0.25 0.36 carbon for a given atmospheric CO2 increase. The influence of freshwater on the carbonate system of coastal Autumn 0.30 0.01 0.29 0.32 All data 0.31 0.02 0.25 0.36 waters has implications with regard to their buffer capacity. The uDIC (mmol kg 1) Winter 0.31 0.01 0.28 0.33 estuarine buffer capacity is typically lower than the one in the open Spring 0.35 0.03 0.29 0.41 ocean, due to low alkalinity discharge by rivers (Aufdenkampe Summer 0.38 0.04 0.26 0.43 et al., 2011). However, this is not the case of our sampling area. Autumn 0.34 0.02 0.31 0.37 All data 0.34 0.04 0.26 0.43 The riverine input of total alkalinity has a lowering effect on the Revelle factor, i.e. it increases the buffer capacity of the system. As shown in Fig. 8, during a high river discharge event (black dots) the fi system was well buffered (RF z 9) and the Revelle factor was very offsets a portion of regional coastal acidi cation (Raymond and related to the river inputs of alkalinity. Cole, 2003; Raymond et al., 2008). In the Gulf of Trieste, long However, an explicit determination of carbonate system time records of river alkalinity are not present, but recent studies showed a marked decrease of river flows (Cozzi et al., 2012) which response as a result of CO2 uptake from the atmosphere cannot be determined using the Revelle factor. Therefore, we choose the probably could lead to a reduction of river alkalinity export and buffer factors formulated by Egleston et al. (2010). These factors buffer capacity of the system. provide a more direct quantification of seawater's ability to buffer changes in the [CO2](gDIC), pH (ßDIC), and UAr (uDIC) due to the 5. Conclusions accumulation of atmospheric CO2. The buffer factors gDIC,ßDIC, and þ 2 uDIC express the fractional variation of [CO2], [H ], and ½CO3 The goal of this study was to understand how physical and respectively, when DIC changes at constant AT (air-sea CO2 ex- biogeochemical drivers could influence seasonal cycles of marine change). The values of gDIC and ßDIC are positive, while uDIC is carbonate system in a shallow coastal area of the North Adriatic Sea. negative because the addition of CO2 to seawater increases the The experimental results indicate that a large part of seasonal pCO2 concentration of dissolved carbon dioxide and hydrogen ion, but variation was driven by change in seawater temperature, never- decreases the concentration of carbonate. The three buffer factors, theless also the other processes of air-sea CO2 exchange, biological thus defined, have dimensions of mmol kg 1 and the results for C1 activity, and mixing play an important role. station are shown in Table 2. By examining the buffer factors in the The Gulf of Trieste showed a significant CO2 uptake in all seasons different periods, they presented similar seasonal variations. The with the exception of summer, when a weak degassing was highest values were observed in summer surface waters, due to the detected. This demonstrates that the Gulf of Trieste is a sink over riverine inputs of total alkalinity and high seawater temperature the bi-annual observational period, but with marked seasonal that shifts in acid-base dissociation constants. Instead low buffer variations. During winter, when the biological activity was very capacity occurred in cold winter waters and also in the bottom low, the atmospheric CO2 uptake was set mainly by low seawater waters during September as a result of the strong seasonal pH temperature. This thermally driven carbon dioxide sequestration decrease induced by remineralisation of organic matter. continued in the spring season and it was reinforced by the strong Comparing the two-year average values of several buffer factors effect of primary production, which caused a reduction of dissolved with those calculated by Alvarez et al. (2014) in different Mediter- seawater CO2 and a greater flux of atmospheric carbon dioxide ranean sub-basins, the Gulf of Trieste presented the highest values toward the sea. During summer, the primary production in surface of gDIC and ßDIC, whereas uDIC value was very similar to that found waters was counterbalanced by respiration of organic matter in the in the Adriatic Sea. This is a very interesting result, because it shows bottom layer, so the net biological effect on the carbonate system that the waters of the gulf are the most resistant of all Mediterra- was very low and the overall reduction of seawater CO2 concen- nean Sea to changes in [CO2] and pH induced by ocean acidification tration observed was mainly due to CO2 degassing generated from 2 process. They also present a good capacity to buffer the ½CO3 higher seawater temperatures. When the water column turned decrease and the reduction of aragonite saturation state (Fig. 4e). from strongly-stratified in summer to well-mixed in autumn, the The reason for the high buffer capacity of the Gulf of Trieste is high amount of CO2, produced by respiration and that was segre- the high total alkalinity concentration resulting from carbonate gated below the pycnocline, was released and spread throughout weathering discharge delivered by local rivers. In general, changes the column increasing the seawater CO2 concentration. in land use, precipitation, and runoff can alter the alkalinity export Although two years are not enough to detect trends of acidifi- from land to coastal areas and their buffer capacity. For example, cation in this coastal system, this study showed that the very low the Mississippi River in the last 50e100 years exported ~50% more seawater temperature favour the physical pump and absorption of alkalinity in the Gulf of Mexico, due to increasing areas of cropland CO2, especially during dense water formation events, which can and increasing precipitation over the watershed, which currently transport anthropogenic CO2 towards the Southern Adriatic Sea G. Ingrosso et al. / Estuarine, Coastal and Shelf Science 168 (2016) 58e70 69 and the Eastern Mediterranean Deep Waters. By contrast, the Sci. 115, 51e62. fi riverine inputs of total alkalinity contributes to setting a low Revelle Cardin, V., Celio, M., 1997. Cluster analysis as a statistical method for identi cation of the water bodies present in the Gulf of Trieste (Northern Adriatic Sea). Boll. factor and a great capacity to store atmospheric CO2 with a low Geofis. Teor. Appl. 38, 119e135. effect on the pH of the system, which, as noted, is one of the most Casellato, S., Stefanon, A., 2008. Coralligenous habitat in the northern Adriatic Sea: e buffered areas of the whole Mediterranean Sea. an overview. Mar. Ecol. 29, 321 341. http://dx.doi.org/10.1111/j.1439- 0485.2008.00236.x. Our results contribute to a very limited set of observations Celio, M., Malacic, V., Bussani, A., Cermelj, B., Comici, C., Petelin, B., 2006. The coastal regarding the ocean acidification in the coastal Mediterranean scale observing system component of ADRICOSM: Gulf of Trieste network. Acta areas and the role of physical and biogeochemical processes on Adriat. 47 (Suppl. l), 65e79. 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