Carbonate system variability in the Gulf of Trieste (North Adriatic Sea)

Estuarine, Coastal and Shelf Science 115 (2012) 51e62

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Carbonate system variability in the Gulf of Trieste (North Adriatic Sea)q Carolina Cantoni a, *, Anna Luchetta a,1, Massimo Celio b, 2, Stefano Cozzi a,1, Fabio Raicich a,1, Giulio Catalano a,1 a b

CNR e National Research Council of Italy, ISMAR e Marine Sciences Institute in Trieste, Viale Romolo Gessi 2, 34123 Trieste, Italy Regional Environmental Protection Agency (ARPA) of Friuli Venezia Giulia, Via Cairoli, 14, 33057 Palmanova (UD), Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 August 2011 Accepted 11 July 2012 Available online 22 July 2012

The seasonal variability of the carbonate system in the waters of the Gulf of Trieste (GoT) was studied at PALOMA station from 2008 to 2009, in order to highlight the effects of biological processes, meteorological forcings and river loads on the dynamics of pHT, CO2 partial pressure (pCO2), dissolved inorganic carbon (DIC), carbonate ion concentration ðCO¼ 3 Þ, aragonite saturation state (UAr) and total alkalinity (AT). During winter, low seawater temperature (9.0  0.4  C) and a weak biological activity (10.7 < AOU < 15.7 mmol O2 kg1) in a homogeneous water column led to the lowest average values of pCO2 (328  19 matm) and UAr (2.91  0.14). In summer, the water column in the area acted as a two-layer system, with production processes prevailing in the upper layer (average AOU ¼ 29.3 mmol O2 kg1) and respiration processes in the lower layer (average AOU ¼ 26.8 mmol O2 kg1). These conditions caused the decrease of DIC (50 mmol kg1) and the increase of UAr (1.0) values in the upper layer, whereas opposite trends were observed in the bottom waters. In August 2008, during a hypoxic event (dissolved oxygen DO ¼ 86.9 mmol O2 kg1), the intense remineralisation of organic carbon caused the rise of pCO2 (1043 matm) and the decreases of pHT and UAr values down to 7.732 and 1.79 respectively. On an annual basis, surface pCO2 was mainly regulated by the pronounced seasonal cycle of seawater temperature. In winter, surface waters in the GoT were under-saturated with respect to atmospheric CO2, thus acting as a sink of CO2, in particular when strong-wind events enhanced airesea gas exchange (FCO2 up to 11.9 mmol m2 d1). During summer, the temperature-driven increase of pCO2 was dampened by biological CO2 uptake, as consequence a slight over-saturation (pCO2 ¼ 409 matm) turned out. River plumes were generally associated to higher AT and pCO2 values (up to 2859 mmol kg1 and 606 matm respectively), but their effect was highly variable in space and time. During winter, the ambient conditions that favour the formation of dense waters on this continental shelf, also favour a high absorption of CO2 in seawater and its consequent acidification (pHT decrease of 0.006 units during a 7-day Bora wind event). This finding indicates a high vulnerability of North Adriatic Dense Water to atmospheric CO2 increase and ocean acidification process. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: coastal waters carbon dioxide acidification carbonate minerals Italy North Adriatic Gulf of Trieste

1. Introduction During the last two century, about two thirds of the anthropogenic CO2 emissions have remained in the atmosphere, causing the rise of its concentration levels from about 280 to nearly 384 ppm in q The authors state that this paper is original and has not been submitted for publication elsewhere. * Corresponding author. E-mail addresses: [email protected] (C. Cantoni), anna.luchetta@ ts.ismar.cnr.it (A. Luchetta), [email protected] (M. Celio), stefano.cozzi@ ts.ismar.cnr.it (S. Cozzi), [email protected] (F. Raicich), giulio.catalano@ ts.ismar.cnr.it (G. Catalano). 1 Tel.: þ39 040305312; fax: þ39 040308941. 2 Tel.: þ39 0432922668. 0272-7714/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecss.2012.07.006

2007 (Solomon et al., 2007). In this context, the ocean has constituted a net sink as it has taken up more than 30% of the anthropogenic emissions of CO2 (Sabine et al., 2004). However, the uptake of CO2 by the oceans leads to a decrease in pH and carbonate ion ðCO¼ 3 Þ concentration in a process known as “ocean acidification”. Since preindustrial times, the average ocean surface water pH has decreased approximately from 8.21 to 8.10 (Royal Society, 2005), and in the next 100 years it is expected a further decrease by 0.3e0.4 pH units in case of a rise of atmospheric CO2 concentrations up to 800 ppm (Orr et al., 2005). These consequent changes in seawater chemistry are of major concern for their potential effect on marine life at both species and ecosystem levels (Orr et al., 2005; Riebesell et al., 2007; Doney et al., 2009; Gattuso and Hansson, 2011).

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Recent studies report that carbon of anthropogenic origin occurs throughout the water column over the entire Mediterranean Sea (Touratier and Goyet, 2009; Rivaro et al., 2010) and that about 90% has been taken up directly from the atmosphere via gas exchange (Schneider et al., 2010). Touratier and Goyet (2011) claim that Mediterranean waters have already been acidified with a pH decrease of 0.05e0.14 pH units, that is even higher than the average oceanic decrease. This estimate was based on the TrOCA approach that slightly overestimates anthropogenic carbon content, therefore it reasonably represents an upper limit (Krasakopoulou et al., 2011). In this scenario the areas of dense water formation such as the Gulf of Lion, the Aegean Sea and the Adriatic Sea are supposed to play an important role for the transfer of waters enriched in anthropogenic CO2 into the deep thermohaline cells of the Mediterranean (Krasakopoulou et al., 2009, 2011; Schneider et al., 2010). In the shallow northern part of the Adriatic basin, in winter, cold and dry continental air associated to Bora wind events, increase surface heat loss and evaporation leading to the formation of the North Adriatic Dense Water (NAdDW) (density anomaly >29.2 kg m3) (Jeffries and Lee, 2007), which flows southwards and contributes to the formation of Eastern Mediterranean deep waters (Ga ci c et al., 2001). Meteorological and oceanographic conditions favouring heat loss can also favour CO2 dissolution when surface seawater is undersaturated with respect to the overlying atmosphere. The comparison of pH values in the NAdDW between 1983 and 2008, revealed a pH decrease by 0.063 pH units, corresponding to a mean acidification rate of 0.0025 pH units yr1, a value slightly higher than those measured in other oceanic regions (Luchetta et al., 2010). This finding further underpins the sensitivity of the Northern Adriatic Sea to acidification processes. The Gulf of Trieste (GoT) is located in the northernmost part of the Adriatic Sea and presents, on a smaller scale, oceanographic properties that are similar to those of the whole continental shelf of the Northern Adriatic (Mala ci c and Petelin, 2001). Therefore, knowing the main processes controlling air-sea CO2 fluxes in the GoT and the consequent shifts in seawater chemical composition can provide key information that is useful at both local and to some extent Mediterranean scale. To gather more information on the potential vulnerability of coastal areas to ocean acidification is also particularly important, as many potentially sensitive calcifying marine species (including mollusks and crustaceans) are of commercial interest in these regions. However, the complexity of marine biogeochemical processes and their high spatial and temporal variability make it difficult to predict carbonate system shifts that can be expected in coastal zones (Borges and Gypens, 2010). This study focuses on the seasonal variability of the carbonate system in the offshore waters of the GoT, at PALOMA station, from January 2008 to November 2009. The results are discussed with regard to biological processes, river loads and meteorological forcings to evidence (1) how interactions among these factors control the variability of inorganic carbon system parameters (2) the potential vulnerability of the GoT to ocean acidification, with particular reference to winter dense waters.

Fig. 1. Gulf of Trieste and location of PALOMA pylon, Vida buoy and the stations sampled in March and May 2009.

The annual thermal stratification occurs from spring to autumn and is enhanced by the relatively high sea surface temperature and freshwater advection, especially in the shallowest northern area of the gulf that is most affected by the riverine inflows of the Isonzo and Timavo rivers (Cozzi et al., 2012). In winter the water column is mostly homogeneous due to surface cooling and frequent mixing induced by strong Bora wind events (ENE). The overall circulation is mostly cyclonic and strongly depends on water column stratification and wind stress. The main water outflow occurs along the shallow northern coast, after mixing with river waters (Mala ci c and Petelin, 2001). In late autumn and winter, cold air spells cause huge heat losses from the sea surface and the formation of dense waters that afterwards contribute to the larger generation of the North Adriatic Dense Water (NAdDW; Artegiani et al., 1997; Mala ci c and Petelin, 2001). Nutrient advection by the Isonzo River has a significant impact on productivity and plankton community structure, as it triggers intense phytoplankton bloom when the river nutrient supply is combined with favourable ambient conditions (Malej et al., 1995; Cantoni et al., 2003). Other allochthonous sources of land-borne nutrients, such as sewage loads (Cozzi et al., 2008) and atmospheric deposition (Malej et al., 1997), also contribute to the overall nutrient stocks in the coastal zone.

2. Methods

2.2. Field activities and analytical methods

2.1. Study site

This study was carried out in the years 2008e2009 at the site of PALOMA pylon (Piattaforma Avanzata Laboratorio Oceanografico Mare Adriatico, CNR e ISMAR), which is located on a 25 m deep water column in the centre of the Gulf of Trieste (45 370600 N, 13 330 5500 E) and hosts a meteorological station. The study primarily makes use of the monthly hydrological and biochemical monitoring of the water column as well as of the hourly meteorological data measured at 10 m above sea surface [air temperature, wind speed

The Gulf of Trieste is a shallow bay lying in the northernmost part of the Adriatic Sea (<25 m), connected with the Adriatic on the SW side (Fig. 1). Meteorological conditions in the region exhibit a pronounced seasonal cycle, which determines strong variations in seawater temperature, salinity and water column stratification during the year (Mala ci c and Petelin, 2001).

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(u10), and direction]. In March and May 2009, surface seawater samples were collected in 8 additional stations (Fig. 1), in order to better characterise the role of rivers in total alkalinity variability. Field operations were carried out using the vessel M/N EFFEVIGI belonging to the Regional Environmental Protection Agency (ARPA) of Friuli Venezia Giulia. CTD profiles of salinity (S) and temperature (T;  C) were acquired using a multiparametric Idronaut 316 probe. Seawater samples were collected with 5-litre Niskin bottles at the surface, at the bottom and at two intermediate depths, which changed every month according to the vertical gradients of water properties. The samples to determine dissolved oxygen (DO), pH expressed on the ‘total hydrogen ion scale’ (pHT) and total alkalinity (AT) were drawn from the Niskin bottles into 60 ml BOD bottles and into 100 and 300 ml borosilicate glass flasks respectively. DO samples were kept in the dark at seawater temperature until they were analysed on shore. pHT and AT samples were poisoned with mercuric chloride, tightly closed and stored at 4  C. The samples to determine dissolved inorganic nutrients were filtered, frozen at 20  C immediately after collection and stored in the dark. pHT, AT and nutrient samples were analysed on shore within one month after their collection. Dissolved oxygen was determined using an automated potentiometric Winkler titration system. Nitrate plus nitrite (NO3 þ NO2; hereafter cited as NO3), ammonium (NH4), reactive phosphorus (PO4) and reactive silicon (SiO2) were determined using a FlowSolution III autoanalyser (OI-Analytical), following standard colorimetric methods in line with those reported by Grasshoff et al. (1999). AT was determined by potentiometric titration in an open cell with a difference derivative readout (Hernandez-Ayon et al., 1999). The titrating HCl solution was calibrated against certified reference seawater for DIC and AT (batch no. 83, Scripps Institute of Oceanography; USA). Accuracy was checked by the titration of replicates (n ¼ 6) of reference seawater (batch no. 89), obtaining an experimental average AT value of 2213.2  1.7 mmol kg1 (certified value AT ¼ 2214.06  0.24 mmol kg1). Long-term system performance was monitored by a daily analysis of reference seawater samples (n ¼ 56, standard deviation ¼ 3.4 mmol kg1). pHT was determined spectrophotometrically using m-cresol purple as indicator (Clayton and Byrne, 1993; Dickson et al., 2007) and samples were measured in 10 cm cells thermostated at 25  0.05  C (pHT25). The analytical precision was estimated to be 0.002 pHT units, determined by the triplicate analysis of samples. Long-term performance was also monitored by analysing the same batch of reference seawater used to control AT measures. 2.3. Derived parameters and modelling The carbonate system parameters [pHT, partial pressure of carbon dioxide (pCO2), carbonate ion concentration ðCO¼ 3 Þ, dissolved inorganic carbon (DIC), calcite (UCa) and aragonite (UAr) saturation states] were calculated at 16.8  C that corresponds to the mean annual surface temperature and at the in situ temperature based on the experimental data of pHT25, AT, T, S, PO4, SiO2 with CO2SYS program (Lewis and Wallace, 1998). The constants of Mehrbach et al. (1973) as refitted by Dickson and Millero (1987) for the dissociation of carbonic acid and those of Dickson (1990) for KSO4 e equilibrium, were used. The estimated uncertainties for the computed values were: 1.5%CV for pCO2, 0.3%CV for DIC and 1.5%CV for CO¼ 3 concentrations and calcium carbonate saturation states. The thermal and non-thermal effects on surface pCO2 were estimated by the model of Takahashi et al. (2002). The thermal effect on pCO2 (pCO2T) is defined as the change in pCO2 due to temperature changes on a parcel of seawater with constant

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chemical composition equal to the annual sea surface mean composition. It is estimated according to the equation below:

pCO2T ¼ ðpCO2 Þmean $exp½0:0423ðTobs  Tmean Þ

(1)

where T is the sea surface temperature ( C) and the subscripts “mean” and “obs” indicate the annual average and the observed values respectively. The non-thermal effect on pCO2 (i.e. net biology effect) (pCO2NT) is defined as the change in pCO2 due to changes in DIC concentration at a fixed temperature equal to the annual average sea surface temperature (SST), and is computed according to:

pCO2NT ¼ ðpCO2NT Þobs $exp½0:0423ðTmean  Tobs Þ

(2)

The non-thermal effect includes all physical and biochemical processes that may alter CO2 concentration in the marine environment, such as mixing among water masses, airesea CO2 exchange, freshwater advection, primary production and respiration. As the Takahashi model was developed for oceanic waters (Takahashi et al., 1993), its applicability to the coastal waters of the GoT was tested. The effect of temperature on pCO2 at constant DIC and the effect of DIC variation at a constant temperature were calculated, both with Takahashi equations and the carbon chemistry equations of the CO2SYS program. Within the variability of the available dataset, results were in good agreement (avg. pCO2 difference <2.2 matm). Therefore, thermal and non-thermal effects on pCO2 (pCO2T and pCO2NT) were calculated for each month at the mean annual SST ¼ 16.8  C and at the mean annual surface pCO2 ¼ 366 matm. The airesea fluxes of CO2 (FCO2) were computed from the difference between the partial pressure of carbon dioxide at sea surface (pCO2 sea) and the overlying atmosphere (pCO2 air) according to:

FCO2 ¼ k$K0 $½pCO2 sea  pCO2 air  were k is the gas transfer velocity of CO2 and K0 is the solubility coefficient of CO2 (Weiss, 1974). We used the k parameterisation as a function of quadratic wind speed given by Wanninkhof (1992), which is suitable for short-term or “steady winds”. Negative fluxes are directed from the atmosphere to the sea. Experimental data of atmospheric CO2 molar fraction (XCO2 air) were available for the Northern Adriatic in January 2008, showing an average value of 399.8  4.1 ppm scarcely variable at the regional scale (S. Piacentino, pers. comm.). The amplitude of seasonal oscillations of XCO2 air in the time series of Mediterranean remote site in Lampedusa is also scarce (8e9 ppm; Artuso et al., 2009), tenfold smaller than the seasonal changes of pCO2 calculated for surface waters in this study (z110 matm). For these reasons, the XCO2 air values used for CO2 fluxes estimate at airesea interface here was assumed as constant, although the consequences of this approximation have been discussed. The partial pressure of CO2 at sea surface (pCO2 air) was derived from the value of XCO2 air according to Dickson et al. (2007), using experimental data of surface temperature and salinity collected during the cruises and the average daily atmospheric pressure measured at the CNR-ISMAR building in Trieste. 3. Results and discussion 3.1. Ambient conditions and hydrology During 2008, the monthly average air temperature at PALOMA station ranged from 6.8  C in February to 24.8  C in August, while during 2009 it ranged from 7.2  C in January to 25.9  C in August

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(Fig. 2a). The near-surface sea temperature in the Gulf of Trieste was largely influenced by the atmospheric forcing as shown by its tight coupling with the oscillations of air temperature (Fig. 2a and b) and by previous studies in this area (Raicich and Crisciani, 1999). During the investigated period, the Isonzo River freshwater inflows were characterised by an average flow rate of 43  208 m3 s1 and by pronounced variability (Fig. 2c). A period of drought was observed from August to October in both years, whereas the highest discharge was recorded from November to December 2008 (1.27 109 m3), when it reached about one fourth of the annual water load. Several freshets also occurred in the first months of 2009 (up to 2000 m3 s1, on 30 March), unlike what happened in spring 2008. The hydrological properties of the water column in the study site were also strongly modulated by seasonal warming and freshwater advection. From January to March, the homogeneous cooling of seawater led to the lowest annual temperatures (8.0  C in March 2008, 8.8  C in February 2009). However, in January 2008 and February 2009 an episodic spreading of the Isonzo River plume in the centre of the gulf increased stratification of the upper layer, setting up a thin layer of low-salinity water (Fig. 2d). Wintry water bodies with density anomaly greater than 29.2 kg m3 were observed in JanuaryeMarch 2008 and 2009. These dense waters

can spread on the Northern Adriatic continental shelf and contribute to the formation of NAdDW (Mala ci c and Petelin, 2001). In April and May of both years, the stratification of the water column increased over the area due to the warming of the upper layer and to the riverine inflows. Under such conditions, different circulations in upper and deeper layers are often induced by density gradients and meteorological forcings (Malacic and Petelin, 2001). The highest stability of the water column was reached in summer, as a result of strong thermal (up to 10.1  C in June 2008) and haline (3.0 psu in July 2009) gradients, causing a more pronounced isolation of the waters in the deeper layer. In autumn, the cooling of the atmosphere induced a decrease of seawater temperature in the whole water column, while the greatest variability of the runoff significantly altered the salinity profiles (Fig. 2b, c and d). During this phase, the timing of disruption of the pycnocline and the enhancement of the circulation of the water masses are often triggered by short wind events. On the whole, these features indicated that the investigated area can be basically considered as a two-layer pelagic system, with an upper layer characterised by highly variable hydrological conditions and a more stable deeper layer influenced by the circulation at the gulf level and by its prolonged isolation during summer. 3.2. Chemical characteristics of the pelagic environment The dynamics of inorganic nutrients and dissolved oxygen in the site clearly showed the role of the major hydrological and biological processes driving the biogeochemistry of this coastal zone (Fig. 3).

Fig. 2. (A) Daily average air temperature measured at PALOMA meteorological station. (B) Contour plots of seawater temperature and (D) salinity at PALOMA station, from January 2008 to November 2009 based on monthly samplings. (C) Median daily flow rates of Isonzo River (black stars indicate the days of samplings).

Fig. 3. Contour plots of seawater concentration (mmol kg1) of (A) nitrate, (B) ammonium, (C) reactive phosphorus and (D) AOU at PALOMA site, based on monthly samplings.

C. Cantoni et al. / Estuarine, Coastal and Shelf Science 115 (2012) 51e62

The frequent advection of low-salinity light waters characterised by high concentrations of NO3 (up to 29.97 mmol N kg1) pointed out that rivers plumes may reach the central area of the gulf during all seasons. After the advection of river waters, the occurrence of a strong vertical mixing, such as the one recorded in December 2008, may increase the depth of the nutriclines and cause the spreading of land-borne nutrients in almost all the water column. Unlike the major continental origin of NO3, the concentration of NH4 and PO4 showed higher values in the deeper layer during summer and autumn (10.11 mmol N kg1, in August 2008; 0.35 mmol P kg1 in September 2009), as a result of a strong remineralisation of organic matter. In such cases, the excess of nutrients was actually associated to the strong decrease of dissolved oxygen concentration indicated by the high values of apparent oxygen utilisation (AOU) in Fig. 3d (AOU up to 139.2 m mmol O2 kg1 in August 2008). Considering the oxygenation of the coastal waters of this marine environment, three different periods can be observed: (a) from January to March the water column was generally well oxygenated (AOU from 10.0 to þ10.0 mmol O2 kg1), indicating that autotrophic and heterotrophic processes were close to the balance; (b) from April to August, an increased oxygenation of seawater (AOU < 10.0 mmol O2 kg1) indicated the prevailing effect of primary production down to 15e20 m of depth. In these periods, high NO3 concentrations persisted in the upper layer only in case of a recent advection of continental waters (Cantoni et al., 2003). On the contrary, oxygen consumption mostly prevailed in the deeper layer through all the period;

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(c) from September to December the water column was mostly undersaturated with oxygen, due to the prevalence of respiration both in the upper and deeper layers. Strong oxygen consumption leading to hypoxic conditions has been frequently observed in this coastal zone. Their occurrence is connected to the prolonged water column stratification and to high sedimentation rates, but it is also favoured by the deepening of pycnocline reducing the bottom layer volume (Faganeli et al., 1991). This process was partially observed during March 2008 and October 2009 even if, due to the shallowness of the GoT, nutrients and oxygen dynamics in the bottom waters also reflect the biogeochemical fluxes with the benthic compartment (Faganeli and Ogrinc, 2009). 3.3. Carbonate system variability pHT at in situ temperature exhibited a clear seasonal cycle over the studied period (Fig. 4a). Values were high from January to March along the water column (up to 8.200 in January 2008), whereas they constantly decreased through spring and summer reaching the lowest value within the bottom layer, under the occurrence of strong remineralisation processes (down to 7.732 in August 2008). The same seasonal cycle of pHT was detectable also by the analysis of averaged depth integrated values (Table 1). Carbon dioxide partial pressure (pCO2) showed opposite cycle with respect to pHT (Fig. 4a and c, Table 1). The lowest pCO2 values were reached from January to March in the whole water column (down to 291 matm), whereas in August 2008 pCO2 increased up to 1043 matm close to the bottom.

Fig. 4. Average values in upper (0e14 m) and deeper (15e24 m) layers of (A) pHT, (B) pHT at 16.8  C, (C) pCO2, (D) DIC, (E) CO¼ 3 , and (F) UAr.

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Table 1 The represented data are averaged monthly depth integrated water column values of temperature, salinity, AOU and inorganic carbon parameters at PALOMA site. T ( C)

J F M A M J J A S O N D

J F M A M J J A S O N D

S

AOU (mmol kg1)

pHT

pHT

pCO2 (matm)

16.8

2008

2009

2008

2009

2008

2009

2008

2009

2008

2009

2008

2009

8.81 9.14 8.50 10.48 13.53 17.27 21.36 22.13 22.31 18.32 17.57 12.40

9.74 8.89 9.09 11.25 13.02 18.79 20.18 22.30 20.92 21.05 15.62

37.63 37.56 37.84 37.53 36.54 36.46 37.31 37.37 37.75 37.74 37.14 36.89

37.85 37.26 37.60 36.42 37.28 36.31 36.58 36.73 37.13 36.74 37.20

5.0 3.9 4.0 14.1 2.9 10.5 32.8 24.7 15.4 6.7 22.8 11.21

0.2 4.6 4.2 3.9 14.2 28.8 21.0 13.2 48.0 25.4 18.1

8.197 8.143 8.132 8.156 8.102 8.086 8.072 7.924 7.974 8.104 8.013 8.085

8.176 8.168 8.174 8.111 8.153 8.110 8.064 7.988 7.977 8.062 8.110

8.071 8.022 8.002 8.057 8.051 8.093 8.141 8.004 8.057 8.127 8.024 8.016

8.064 8.043 8.053 8.025 8.094 8.140 8.116 8.071 8.039 8.126 8.092

297 345 353 332 391 410 417 663 549 384 497 409

316 325 318 391 339 383 434 536 548 446 377

AT (mmol kg1)

DIC (mmol kg1)

1 CO¼ 3 (mmol kg )

CO¼ 3

2008

2009

2008

2009

2008

2009

2008

2009

2008

2009

2008

2009

2619 2620 2615 2620 2636 2638 2624 2642 2626 2636 2631 2645

2636 2638 2635 2663 2639 2646 2641 2634 2622 2641 2621

2330 2360 2366 2341 2366 2339 2293 2384 2344 2310 2372 2391

2348 2367 2356 2407 2338 2322 2330 2348 2354 2320 2323

207 189 182 202 200 220 242 196 210 237 194 188

207 196 201 190 219 238 230 214 201 236 217

211 193 186 205 201 220 240 194 208 236 193 190

211 201 205 193 221 237 228 211 199 234 218

4.83 4.41 4.24 4.73 4.70 5.21 5.68 4.62 4.92 5.52 4.54 4.41

4.83 4.60 4.70 4.46 5.12 5.63 5.43 5.06 4.73 5.57 5.09

3.08 2.81 2.70 3.02 3.02 3.38 3.72 3.04 3.23 3.59 2.94 2.82

3.08 2.93 3.00 2.85 3.29 3.66 3.54 3.32 3.09 3.64 3.28

DIC concentrations in the upper layer showed a 50 mmol kg1 seasonal decrease from winter to spring and summer (Fig. 4d). By contrast, the lower layer was mostly characterised by DIC peaks during the stratified period (up to 2501 mmol kg1, August 2008). These opposite trends of DIC during summer were often balanced, determining overall smoother variations characterised by the absence of clear temporal trends (Table 1). Carbonate ion concentration exhibited a strong seasonal cycle in the upper layer (Fig. 4e) with lower values in winter (down to 186 mmol kg1 in January 2009) and values up to 267 mmol kg1 in summer (June 2008). In deeper waters, CO¼ 3 concentrations were generally lower without a clear seasonal trend and with minimum values during the stratified period (down to 125 mmol kg1, August 2008). Calcium carbonate saturation states (UCa, UAr) indicate the saturation degree of calcite and aragonite, the two main calcium carbonate minerals, in seawater. Values >1, indicate that seawater is oversaturated and CaCO3 precipitation is thermodynamically favoured. Values <1 indicate an undersaturation: under this environmental condition the dissolution of carbonate minerals slowly occurs. Calcite and aragonite saturation states are mainly driven by ¼ the availability in seawater of CO¼ 3 ; consequently, UAr and CO3 showed the same temporal patterns (Fig. 4e and f). The waters of the Gulf of Trieste were supersaturated both in calcite and aragonite in all the sampling months (Table 1). The aragonite saturation state (UAr) in the upper layer ranged from 2.8 to 4.2, with low values from December to March and higher values in spring and summer (Fig. 4f). In deep waters, UAr reached the minimum value of 1.8 in August 2008, under the occurrence of strong remineralisation processes. The calcite saturation state (UCa) was approximately 50% higher than UAr and showed similar seasonal and vertical variability (Table 1). AT concentrations in the upper waters varied from 2859 mmol kg1 in April 2009e2615 mmol kg1 in September 2009. In deep waters, AT was less variable with an average value of 2635  10 mmol kg1, that lies in the upper range of AT measured

16:8

(mmol kg1)

UCa

UAr

in the Mediterranean basin (Schneider et al., 2007; Rivaro et al., 2010). AT was significantly related to salinity changes in the upper layer waters (Fig. 5) with a strongly positive intercept (AT ¼ 20.06S þ 3379; n ¼ 41, r2 ¼ 0.89 p < 104) while a weaker correlation was found in deeper waters (r2 ¼ 0.14, p ¼ 0.01). 3.4. Physical and biogeochemical drivers of carbonate system dynamics The seasonal variability of carbonate system parameters in the continental shelf regions is driven by the interaction of several factors, such as the large seasonal amplitude of SST, the CO2 uptake and release due to biological processes and the variable river inputs. In the following paragraphs, different approaches will be used to analyse the dataset and better understand the role played by these three main factors. 3.4.1. Temperature vs. biological effects The interaction between biological processes and carbon chemistry parameters in marine environments is not straightforward (Soetaert et al., 2007; Wolf-Gladrow et al., 2007). For the purpose of this data analysis, only the main processes occurring in an oxic water column will be considered. During photosynthesis CO2 or HCO 3 are taken up to form particulate organic matter. Primary production processes lead to a decrease in DIC, combined with a decrease in pCO2 and HCO 3 and an increase in pH and CO¼ 3 concentration, which consequently make saturation states of carbonate minerals higher (Zeebe and Wolf-Gladrow, 2001). The respiration release CO2 in seawater causing the occurrence of opposite processes. Changes in temperature also have great influence on gas solubility in seawater and on carbon chemistry equilibria. In a parcel of seawater with constant chemical composition, a temperature increase leads to an increase in CO2 concentration and partial pressure, as well as a consequent pHT decrease. This thermodynamic effect determines an increase in carbonate ion concentration

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Fig. 5. (a) AT-S plot at PALOMA site in upper and deeper layers, linear regression in the upper layer, (b) AT-S plot of surface values at the other stations, linear regression for March (AT ¼ 4455e48.5 S; r2 ¼ 0.94; p < 104) and May 2009 (AT ¼ 3342e18.8 S; r2 ¼ 0.97; p < 104).

and calcite and aragonite saturation states, although the temperature effect does not influence all inorganic carbon parameters with the same intensity (Zeebe and Wolf-Gladrow, 2001). Over the two years considered, the amplitude of the SST annual cycle was 18.0  C in 2008 and 16.9  C in 2009, while it was 13.2  C in 2008 and 11.0  C in 2009 in the bottom waters. In order to distinguish the roles played by temperature and by biogeochemical processes, pHT was calculated at the fixed temperature of 16.8  C (pHT 16.8), which is the average value for SST (Fig. 4 b). The trends of these temperature-normalized pHT data was thus the result of only the changes in bulk chemical composition. In the upper part of water column, the lowest pHT 16.8 values were reached from January to March whereas the highest values from June to August (up to 8.244 in surface water in June 2008), under prevailing primary production (AOU < 0) conditions. This seasonal trend was opposite to that of pHT, which was characterised by high values in winter and low values in summer (Fig. 4a). Such different behaviour put in evidence that the effect of temperature on chemical equilibria was able to reverse the changes due to the bulk chemical composition of seawater. However, it was also pointed out that the strong temperature cycle typical of the GoT does not influence all carbon chemistry parameters in the same way. The seasonal trend of CO¼ 3 concentration in the upper layer calculated at the mean annual temperature was as similar as that calculated at the temperature in situ, with the highest value recorded in June 2008 (267 mmol kg1) and the lowest value recorded in April 2009 (178 mmol kg1). In this case, the scarce effect of temperature was also well evidenced by the comparison of in situ and temperature normalised concentrations, which showed a maximum difference of 4 mmol kg1 corresponding to less than 10% of the variability in situ (Table 1). As a consequence, UCa and UAr were also weakly influenced by temperature variations, being their maximum differences limited to 0.13 and 0.17, respectively (August 2008; Fig. 4b, e and f). These results indicate that the annual cycle of such parameters was primarily controlled by changes of the bulk seawater chemical composition which, in turn, is mostly determined by the biological processes. These different thermodynamic effects on the inorganic carbon system also lead to the paradox that winter surface waters had the highest pHT values and, at the same time, they were the most undersaturated by calcite and aragonite and, consequently, the most “acidic” with regard to carbonate saturation state and to the

concentration of CO¼ 3 . This condition has to be carefully taken into account when other potential effects of the ocean acidification are studied, like the consequences on trace metal solubility and availability (Millero et al., 2009). The method proposed by Takahashi et al. (2002) was also applied in this coastal zone in order to better understand the relative importance of temperature changes and of CO2 production/ utilization on the variations of surface pCO2 (Section 2.3, Eqs. (1) and (2)). The “thermal” effect (pCO2T) and the “non thermal” effect (pCO2NT) on sea surface pCO2 were calculated for each month, showing the results as difference with respect to the values of January 2009 (pCO2T ¼ 251 matm, pCO2NT ¼ 423 matm and pCO2 ¼ 315 matm). These normalised values (dpCO2, dpCO2T and dpCO2NT) are shown in Fig. 6. January 2009 was chosen as a reference because, within the dataset, it well represented a condition of low temperature (9.8  C), minimum biological activity (AOU ¼ 1.8 mmol O2 kg1) and negligible advection of river waters (S ¼ 37.8). dpCO2T reached the highest values in August of both years (up to 256 matm). This means that in the presence of a constant bulk chemical composition, the increase of temperature would have caused an increase of pCO2 of 256 matm in August 2008 and of 242 matm in August 2009. However, this thermodynamic increase was damped by the biological removal of CO2 as indicated by the decrease of dpCO2NT during summer months. Similarly, if the surface water would have maintained the same temperature as in January 2009, the biological uptake acting from winter to summer would have determined a decrease of pCO2 of 150 and 129 matm in 2008 and 2009, respectively. On the whole, the seasonal increase of surface dpCO2 indicated that the thermodynamic effect was still prevailing on the biological forcings in this marine environment. Considering the whole water column, dpCO2 summer values sharply increased up to 346 matm and reached values more than three times higher than surface dpCO2 values (Fig. 6b). This behaviour indicated the importance of the remineralisation processes occurring in the deeper layer to generate an excess of CO2 with respect to the assimilation of inorganic carbon in the upper layer, even when the thermal contribution was included. The relative importance of biology and temperature effects on the average annual surface pCO2 cycle can be evaluated by estimating the average annual pCO2NT (B ¼ pCO2NT max  pCO2NT min) and pCO2T (T ¼ pCO2T max  pCO2T min) cycles (Takahashi et al., 2002). The months of April 2009 and December 2008, characterised by high river inputs, were excluded from this calculations.

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Fig. 6. (a) Surface values of dpCO2, dpCO2T and dpCO2NT at PALOMA site, from January 2008 to November 2009. Arrows indicate the presence of river waters in the upper layer. (b) Average values of dpCO2, dpCO2T and dpCO2NT in the whole water column.

The relative importance of biology and temperature effect can be expressed by the ratio (T/B). In marine areas where the effect of temperature changes on surface water pCO2 exceeds the biological effect, the T/B ratio is greater than 1, whereas in areas where the biological effect exceeds the temperature effect the T/B ratio varies from 0 to 1. At PALOMA station B and T were 190 and 257 matm respectively, both values being among the maxima estimated for oceanic regions (Takahashi et al., 2002). The resulting T/B and was 1.35 and indicated strong temperature control. Ribas-Ribas et al. (2011) reported similar values for the Bay of Cadiz (T/B ¼ 1.3), whereas the T/B ratio of 0.74 calculated for the Southern Bight of the North Sea indicated the prevalence of biological control (Schiettecatte et al., 2007). An analysis of DIC/AOU relationship can provide complementary information on the importance of interior biological processes to regulate inorganic carbon dynamics in the GoT, in comparison to the other allochthonous sources of DIC like river loads, water mass inflow and airesea CO2 exchanges. Assuming that photosynthesis and remineralisation follow the Redfield’s stoichiometry (106 CO2: 138 O2), any biological change of CO2 or HCO 3 causes DIC concentration to vary with the same stoichiometry. The Redfield’s stoichiometry determines the slope of theoretical DIC/AOU relationship (Fig. 8a), whereas the average DIC value (2348 mmol kg1) measured on January 2009 (the reference month) was chosen as DIC concentration corresponding to AOU ¼ 0. This linear model fits well with the experimental data of DIC and AOU in the deeper layer, where they showed a highly significant linear relationship (r2 ¼ 0.62, p < 0.0001; Fig. 7a). On the contrary, there was no correlation in the upper layer (p ¼ 0.10), more influenced by riverine loads and gas exchange with the atmosphere. The remaining parameters of the carbonate system influenced by CO2 uptake/release showed trends similar to those of DIC. pHT 16.8 (Fig. 7b), pCO2NT, UCa16.8 and UAr16.8 were significantly correlated with AOU in the deeper layer (0.62 < r2 < 0.69, p < 104), but four groups of data, poorly described by biological variations of CO2 alone, were also noticed (Fig. 7).

Fig. 7. Plot of (a) DIC vs. AOU and of (b) pHT 16.8 vs. AOU. Data from PALOMA site for 0e14 m depth (dots) and for 15e24 m depth (crosses). Linear regression for 15e24 m data (dashed line) in comparison to the theoretical CO2 vs. AOU Redfield’s relationship (solid line). Letters indicate the four groups of data discussed in the text.

Group A included surface data with high DIC content and low salinity (May 2008 and April 2009), strongly influenced by riverine DIC inputs. Group B included upper layer data characterised by negative AOU, and DIC concentrations lower than those expected on the basis of oxygen consumption. As they were referred to summer, this anomaly was likely due to a faster equilibration with the atmosphere of dissolved oxygen compared to CO2. In fact, a higher amount of oxygen can be released into the atmosphere when the

Fig. 8. (a) Daily averaged wind speed during the days of sampling measured at PALOMA meteorological station, at 10 m above sea level (u10). (b) pCO2 in the surface waters and airesea CO2 fluxes (FCO2) during the days of sampling.

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early phases of intense blooms determine its oversaturation in the surface waters. This decoupling between carbon and oxygen stoichiometry may result in an overconsumption of DIC and in pHT 16.8 values higher than those expected. Group C included data in the range of 0e25 mmol kg1 AOU showing a higher dispersion with respect to the theoretical line. These data were mostly collected in autumn, during periods characterised by the breakup of thermal stratification and by the increase of circulation in the gulf. The weak DIC-AOU correlations supported here the hypothesis that the area of study was affected by the circulation of new water bodies, with different DIC characteristics, flowing into the GoT. In the bottom layer, group D data also showed an excess of DIC. They were measured in August 2008, when strong remineralisation processes led to a hypoxic event (DO down to 86.9 mmol kg1). This event was also characterised by the decrease of pHT (down to 7.732) and an increase in AT (up to 2667 mmol kg1), with a concentration more than 30 mmol kg1 higher than in the upper layer. This excess of alkalinity could be ascribed both to alkalinity accumulation due to nitrogen remineralisation and NH4 production (up to 10.1 mmol N l1; Wolf-Gladrow et al., 2007), as well as to the release of AT and DIC from sediments, due to an enhanced dissolution of carbonate in acidic sediments (Faganeli and Ogrinc, 2009). Moreover, in coastal areas with high dissolved organic matter content, organic bases can also account for a small fraction of the titrated alkalinity (Hernández-Ayon et al., 2007) that complicates data interpretation even further and does not explain the causes of the DIC increase observed. Besides production/respiration, also the calcification process exerts an important effect on carbonate chemistry. As a matter of fact, it determines a strong AT and DIC decrease, weaker decreases in carbonate and bicarbonate concentrations as well as an increase in pCO2 and a consequent drop in pH (Zeebe and Wolf-Gladrow, 2001). The analysis of this time series did not show clear signs of high pelagic calcification rates. This result is in line with previous studies on phytoplankton species in the area (Mozeti c et al., 1998) that reported low abundance of coccolithophores, the most important calcifying phytoplankton group, that is present only occasionally during winter. 3.4.2. The role of river loads Isonzo, Timavo and other minor rivers of the area flow through carbonate-dominated terrain and are influenced by complex karst hydrology (Szramek et al., 2011). Although data on the lower Isonzo course are scarce, studies carried out on its watershed indicate variable alkalinity values higher than 2.50 mM, high DIC concentrations and CO2 oversaturation (Kanduc et al., 2008; Szramek et al., 2011). In spring 2009, AT was also measured at the surface in a few stations of the GoT (Fig. 1) to better characterise river inputs of AT. Data collected showed two different AT-S correlations (Fig. 5), with AT values extrapolated at S ¼ 0 equal to 4455 and 3346 mmol kg1 in March and May, respectively. These two distinct relationships are consistent with the previous literature data on the Isonzo waterc et al., 2008), and they shed (Idrica river, AT: 3.09e4.66 mM; Kandu also indicate a possible high temporal variability of riverine AT inputs. When river water mixes up with seawater two main processes contribute to the decrease of pCO2: the ventilation of CO2 to the atmosphere and the biological uptake by concomitant phytoplankton blooms triggered by the loads of continental nutrients: the “fertilization effect”. CO2 ventilation in river plumes is deeply influenced by the residence time of freshwater in the inner estuary: longer residence times increase the loss of CO2 toward the atmosphere within the estuary, and the contribution to pCO2 in the plume will be lower (Borges et al., 2006).

59

During the field activity, eight samplings were carried out under recent and consistent inputs of river waters in the gulf that generated low-salinity and high-nitrate surface waters: January, May, June and December 2008 and February, April, June and July 2009 (Fig. 6a). According to the normalisation proposed by Takahashi et al. (2002), dpCO2NT peaks in May and December 2008, February and April 2009 can be ascribed to direct CO2 loads (Fig. 6a): in these months the CO2 riverine input prevailed over the “fertilisation effect” thus determining high surface pCO2 values even at PALOMA station. In contrast, the riverine pCO2 contribution appeared not detectable on the basis of dpCO2NT values in June 2008, June and July 2009. This observation suggested that, during these months both the biological CO2 uptake and the CO2 fluxes to the atmosphere were able to remove river CO2 loads more quickly. The river inputs effect on the other inorganic carbon system parameters was clearly detectable only in April 2009. On that month surface waters become more acidic (pHT ¼ 7.997), and appeared characterised by lower carbonate concentrations (154 mmol kg1) therefore were less saturated in calcite and aragonite (UCa ¼ 3.84, UAr, ¼ 2.42) than the underlying water column (Fig. 4) despite high AT values (AT ¼ 2859 mmol kg1).

3.5. pCO2 annual cycle and airesea CO2 fluxes During the two years of study, the combination between physical and biological forcings determined significant trends of pCO2 in the surface waters that, in turn, was the main driver of airesea CO2 fluxes. Fig. 8 shows CO2 fluxes calculated on the sampling days based on average daily wind speeds and assuming a constant atmospheric CO2 concentration, equal to wintry open-sea values. Considering the dynamics of CO2 exchanges with the atmosphere, three periods were identified: a) from January to March, low water temperature and a weak biological activity led to low pCO2 values in both years (down to 316 matm in January 2009). In these conditions, the water column resulted undersaturated in CO2 (down to 83 matm) with CO2 fluxes clearly directed toward the sea. Since fluxes were calculated only one day a month, they have a too low temporal resolution to be considered fully representative of the whole period. However, they are considered to lie in the lower range of the period for two reasons. Firstly, coastal areas are more influenced by episodic inputs of anthropogenic CO2 at regional scale, and an increase in pCO2 air would have eventually increased undersaturation and favoured CO2 dissolution. Secondly, sampling days were usually characterised by weak winds (1.5e5.2 m s1) as sampling during strong wind events has not been possible; b) from July to November, surface pCO2 was oversaturated or nearly to the equilibrium (up to þ73 matm in September 2008) determining more variable fluxes that, however, were characterised by high episodic releases of CO2 toward the atmosphere (up to 3.9 mmol m2 d1 in September 2008). In addition, in this area, strong wind events cause a significant mixing of the whole water column, potentially enhancing the release of CO2 as deeper waters were enriched in CO2. c) from April to June, the combined effects of seawater warming, phytoplankton blooms and riverine CO2 loads resulted in very variable surface pCO2 and airesea fluxes. The spreading of river plumes produced the highest surface pCO2 values of the whole period (606 matm in April 2009) and large positive CO2 fluxes (3.0 mmol m2 d1).

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On annual basis, the average cycle of surface pCO2 (143 matm) in the GoT was characterised by winter minima and spring maxima determined by river inputs. If April 2009 was excluded from this calculation, the average cycle of pCO2 would have shown the same magnitude (116 matm) and seasonal evolution of that reported in the NW Mediterranean for the Bay of Angels (Borges et al., 2006), the only coastal site with a published annual time series of data in the Mediterranean. Other European shelf areas, such as the Southern Bight of the North Sea, the English Channel and the Gulf of Biscay were characterised by springtime decreases of pCO2 determined by biological CO2 uptake, if the sea temperature was not warm enough to prevail on the biological effects (Borges et al., 2006). At PALOMA station, collected data suggested that CO2 decrease during spring blooms was often counterbalanced by direct river CO2 loads leading to variable and supersaturated surface pCO2 values. Turk et al. (2010) presented high-frequency sea surface pCO2 data collected at Vida station, near the Slovenian coast, during 4 deployments from April 2007 to December 2008 (Fig. 1). Surface pCO2 values down to 200 matm were measured in April 2008, while spring drops of pCO2 were often associated to river inputs and chlorophyll increases. In April 2008, our data also showed a decrease of pCO2, although characterised by higher values (324 matm; Fig. 8). This difference could be explained by two factors. On the one side, our data were collected only once per month and they might have missed the most productive phase of the spring bloom. On the other, Vida station is more distant from the Isonzo mouth than PALOMA station, and the importance of river CO2 loads could have been lower than the biological CO2 uptake stimulated by riverine nutrients. In autumn 2008, data of both stations were in agreement, as clearly shown by the increase of pCO2NT in both, following the breaking of seasonal stratification and the ingression of new surface waters in the GoT.

Table 2 Carbonate parameters during three Bora events. Length of the Bora events (n. days), average wind speed at 10 m above sea level (u10), average pCO2 seaeair difference (pCO2 sea  pCO2 air), average airesea CO2 fluxes (FCO2), negative fluxes are directed from the air to the sea. Changes of average water column values of carbonate parameters determined by the calculated dissolution of CO2 during the event: (f) and (i) indicate respectively final and initial values.

3.6. Bora events and potential acidification of dense waters

4. Conclusions

The strong and cold Bora wind blowing in winter is one of the main responsible for the formation of dense waters in the continental shelf area of the GoT. Moreover, this wind can favour a higher dissolution of CO2 in seawater when surface CO2 undersaturation occurs, because it enhances the gas transfer through airesea interface. In order to better understand the potential effect of Bora wind on the characteristics of carbonate system in the dense waters, three strong wind events occurred in February and March 2008 and in January 2009 were considered. They lasted from 7 to 14 days, with average wind speeds up to 9.9 m s1 (Table 2), in the presence of a water column characterised by an average density anomaly >29.2 kg m3. The surface pCO2 measured in these cases was considered to be representative of the whole wind events, as no peaks of river waters were observed and the water column was homogeneous and undersaturated in CO2 in all these periods (Table 2). Using a simplified model, the average daily CO2 flux was calculated for each day of the event on the basis of SST, S and pCO2 sea values measured during sampling and the average wind speed daily measured at PALOMA station. A wind mixing effect through the whole water column, without water renewal, was also assumed. The amount of CO2 absorbed during each event was calculated per square metre and redistributed over the whole water column. Afterwards, the consequent shift in the carbonate system was calculated as a DIC increase at constant AT (Lewis and Wallace, 1998; Table 2). Model results indicated that the amount of absorbed CO2 can led to a small change in DIC concentration (<3%), with changes of

This study was focused on the analysis of the major biogeochemical drivers that modulate the seasonal cycles of pCO2, pHT and carbon mineral saturation states in the shallow northern Adriatic continental shelf area. Experimental data and model results indicated that the annual cycle of surface pCO2 was mainly controlled by the strong seasonal variability of seawater temperature. During winter, surface seawater was undersaturated with CO2, acting as a sink of atmospheric CO2 in particular during strong wind events. In summer, the increase of seawater temperature led to higher pCO2 values. This thermodynamic effect was dampened by the effect of CO2 removal from the upper layer due to primary production. Overall, the balance between these two contrasting processes made often the surface waters as weak source of CO2 for the atmosphere. In the upper layer, the inorganic carbon system was also modulated by the riverine inflows. The advection of continental waters caused direct inputs of riverine CO2, which can be partially counterbalanced by a higher biological uptake of CO2 sustained by the supply of continental nutrients. The relative importance of these two forcings was probably highly variable in space and time, as suggested by the observations at Vida station near the Slovenian coast (Turk et al., 2010). By contrast, intense remineralisation of organic carbon in the deeper waters releases CO2, reinforcing the temperature-driven pCO2 increase and leading to values up to 1043 matm during hypoxic periods. Concerning the vulnerability of this region to the processes related to the ocean acidification, two critical elements were observed: the condition of wintry dense waters and the combined

Bora event

7e13/02/2008

2e8/03/2008

5e18/01/2009

n. days avg u10 (ms1) avg pCO2 sea  pCO2 air (matm) avg FCO2 (mmol m2 d1) DIC(f)  DIC(i) (mmol kg1) pHT(f)  pHT(i) pCO2sea(f)  pCO2sea(i) (matm) ¼ 1 CO¼ 3 (f)  CO3 (i) (mmol kg ) UCa(f)  UCa(i) UAr(f)  UAr(i)

7 8.5 56 11.6 3.2 0.006 5.4 2.0 0.05 0.03

7 9.9 32 11.1 3.0 0.006 5.3 1.9 0.04 0.03

14 6.6 82 11.9 6.5 0.011 9.7 4.2 0.10 0.06

airesea pCO2 difference lower than 12%. Nevertheless, the effect of wind caused a measurable acidification in the whole water column with a consequent shift of all carbon chemistry parameters. The estimated pHT decrease (0.006 pHT) was 3e6 times higher than the measured acidification of the North Atlantic ocean over one year (0.001e0.002 pHT units y1, Santana-Casiano et al., 2007; Bates, 2007). This acidification also caused a measurable decrease of carbonate mineral saturation states (Table 2). These findings clearly indicate that ambient conditions favouring the formation of dense waters in the GoT can also favour CO2 absorption. Combined to the shallowness of this coastal zone, this process can lead to measurable pHT decreases. This result confirms the susceptibility to the acidification of North Adriatic dense water (Luchetta et al., 2010) and its importance as source of anthropogenic carbon for the deeper waters of the Eastern Mediterranean.

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effect of acidification and respiration during summer. Low temperatures, strong winds and a scarce runoff in the gulf favour the local formation of dense waters that, afterwards, contribute to the spreading of NAdDW on the shelf. This study shows that these ambient conditions also favour the absorption of CO2 and their consequent acidification. Similar oceanographic properties in the GoT and in the North Adriatic (Malacic and Petelin, 2001) indicate that NAdDW is probably enriched in atmospheric CO2 at sub-basin scale, too. Consequently, the circulation of this water mass is a way for the transfer anthropogenic carbon in the deeper thermohaline cell of the Eastern Mediterranean. The waters of the GoT were well saturated both with calcite and aragonite but the Mediterranean is expected to adsorb higher amounts of CO2 than the oceans (Touratier and Goyet, 2011; Krasakopoulou et al., 2011). Even if UCa and UAr will likely remain above 1 during most of the year, a drop in their values by the end of the century is likely expected. Calcareous organisms usually require seawater U-values much higher than 1 to achieve optimal growth (Guinotte et al., 2003; Miller et al., 2009) and their rapid decrease will probably influence the ecosystem of this coastal area. During summer, bottom waters will likely experience the most acute impact because of the combined effects of acidification, respiration and warming. Additional studies are needed to better characterize the spatial variability of CO2 system, with particular emphasis on the effect and variability of riverine inflows. At PALOMA site the main physical and biogeochemical forcing modulating inorganic carbon chemistry were clearly detectable and hence it represents a suitable site to study and monitory acidification process in the Northern Adriatic. Monthly hydrological and biogeochemical monitoring are still going on and the station has been recently implemented with high-frequencies automated sea surface pCO2 measurements. Acknowledgements This work was partially supported by the VECTOR Project, funded by the Ministry of Education, University and Research with an Integrated Special Fund for Research (FISR). We are grateful to the Regional Environmental Protection Agency (ARPA) of Friuli Venezia Giulia and to the crew of M/N EFFEVIGI for their support in field work, to OSMER-ARPA for meteorological data and to S. Piacentino (ENEA- ACS-CLIMOSS) for the atmospheric CO2 analysis. We are also grateful to three anonymous reviewers for their helpful comments. References Artegiani, A., Bregant, D., Paschini, E., Pinardi, N., Raicich, F., Russo, A., 1997. The Adriatic Sea general circulation. Part I: air-sea interactions and water mass structure. Journal of Physical Oceanography 27, 1492e1514. Artuso, F., Chamard, P., Piacentino, S., Sferlazzo, D.M., De Silvestri, L., di Sarra, A., Meloni, D., Monteleone, F., 2009. Influence of transport and trends in atmospheric CO2 at Lampedusa. Atmospheric Environment 43 (19), 3044e3051. Bates, N.R., 2007. Interannual variability of CO2 sink in the subtropical gyre of the North Atlantic Ocean over the last 2 decades. Journal of Geophysical Research 112, C09013. Borges, A.V., Gypens, N., 2010. Carbonate chemistry in the coastal zone responds more strongly to eutrophication than ocean acidification. Limnology Oceanography 55 (1), 346e353. Borges, A.V., Schiettecatte, L.-S., Abril, G., Delille, B., Gazeau, F., 2006. Carbon dioxide in European coastal waters. Estuarine, Coastal and Shelf Science 70 (3), 375e387. Cantoni, C., Cozzi, S., Pecchiar, I., Cabrini, M., Mozeti c, P., Catalano, G., Fonda Umani, S., 2003. Short-term variability of primary production and inorganic nitrogen uptake related to the environmental conditions in a shallow costal area (Gulf of Trieste, northern Adriatic Sea). Oceanologica Acta 26, 565e575. Clayton, T., Byrne, R.H., 1993. Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results. Deep Sea Research Part I 40, 2115e2129.

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