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Distribution of dissolved inorganic carbon and related parameters in the Thermaikos Gulf (Eastern Mediterranean)

Data on the distribution of dissolved inorganic carbon (measured as TCO 2 ) and related parameters in the Thermaikos Gulf were obtained during May 1997. High TCO 2 concentrations were recorded close to the bottom, especially in the northern part of the gulf, as a result of organic matter remineralisation. The positive relatively good correlation between TCO 2 and both apparent oxygen utilisation (AOU) and phosphate at the last sampling depth con ﬁ rmed the regenerative origin of a large proportion of TCO 2 . The comparatively conservative behaviour of alkalinity, together with the relatively low value of the homogenous buffer factor β ( β = ∂ lnfCO 2 / ∂ lnTCO 2 ) revealed that calci ﬁ cation or carbonate dissolution takes place on a very small scale, simultaneously with the organic carbon production. The correlations between fCO 2 and chlorophyll α , as well as AOU and the surface temperature, revealed that the carbon dioxide ﬁ xation through the biological activity is the principal factor that modulates the variability of fCO 2 . A rough ﬁ rst estimate of the magnitude of the air-sea CO 2 exchange and the potential role of the Thermaikos Gulf in the transfer of atmospheric CO 2 was also obtained. The results showed that during May 1997, the Thermaikos Gulf acted as a weak sink for atmospheric CO 2 at a rate of -0.60 - -1.43 mmol m -2 d -1 , depending on which formula for the gas transfer velocity was used, and in accordance to recent reports regarding other temperate continental shelves. Extensive study of the dissolved inorganic carbon and related parameters, and continuous shipboard measurements of fCO 2a and fCO 2w during all seasons are necessary to safely quantify the role of the Thermaikos Gulf in the context of the coastal margins CO 2 dynamics.


Introduction
Continental shelves and slopes have frequently been considered as a small area of the ocean and therefore, unimportant in terms of the global carbon cycle. Although ocean margins comprise less than 10% of the surface area of the world's oceans, they account for up to one-third of global marine primary production (ALONGI 1998), ~80% of organic matter burial, ~90% of sedimentary mineralisation and ca 50% of calcium carbonate deposition (PERNETTA & MIL-LIMAN, 1995). They also exchange nutrients and inorganic and organic carbon with the open ocean intensively across marginal boundaries, thus constituting the major link between the terrestrial and the open ocean environments and consequently play a key role in the global carbon cycle by linking the terrestrial, oceanic and atmospheric reservoirs. However, nowadays there is a great uncertainty about the overall role of coastal ecosystems in global carbon cycling and the question of whether shelf seas act as a sink or source for carbon dioxide still remains open.
Coastal waters are characterised by large spatial and temporal variations of the parameters of the CO 2 /carbonate system, associated with intensive physical, chemical and biological processes occurring in these sites. The role of the shelves in the inorganic carbon cycle is uncertain because it results from the integration of production/remineralization/export of organic carbon, burial/dissolution of carbonates in the shallow sediment and input of inorganic carbon from rivers and coastal upwelling. One major challenge in studying the carbon cycling of coastal areas arises from the complexity and heterogeneity of the systems. Field data on the distribution of carbonate system species are needed in order to provide information on the carbon cycling and the biogeochemical processes in the coastal zone, as well as to assess the effective role of the coastal zone in the global carbon cycle.
In this work we attempt to study, for the fi rst time, the distribution of the inorganic carbon species and their relationships with other properties such as salinity, temperature, dissolved oxygen and chlorophyll a in an eastern Mediterranean regime. In addition, the air-sea CO 2 fl ux is estimated using an indirect method, in order to assess the potential role of the Thermaikos Gulf as a source or sink of atmospheric CO 2 during late spring of 1997.

Study area
The Thermaikos Gulf is an elongated basin characterised by an extensive shelf (depths <200m) located in the NW Aegean Sea. The northern part of the gulf (inner Thermaikos Gulf) comprises the Bay of Thessaloniki, where the city of Thessaloniki (population ca 1.2 million) is found, and the Gulf of Thessaloniki (Fig.1). Three major rivers Axios, Aliakmon, Pinios, and two smaller ones, Loudias and Gallikos, discharge along its northern and western boundaries and transport signifi cant nutrient loads, which eventually enter the coastal ecosystem, mostly through episodic fl ooding events (KARAGEORGIS et al., 2005). In addition to the riverine nutrients inputs, the gulf receives the urban effl uents from the city of Thessaloniki and the adjacent industrial zone. Due to the increased infl ux of inorganic and organic material at the shelf area, the inner part of the gulf exhibits higher plankton biomass and productivity than the oligotrophic Aegean Sea and the trophic status of the area is characterised as a eutrophic one (GOTSIS-SKRETAS & FRILI- GOS, 1990;BALOPOULOS & FRILIGOS 1993;PAGOU et al., 2000;PAGOU, 2005).
Considerable production of carbon dioxide, particularly in the bottom layer, is to be expected from the decomposition of the external imported organic matter (mainly from the rivers and the sewage pipes) and of that produced in situ in the Thermaikos Gulf.
The forcing mechanisms that control shelf circulation in the Gulf of Thermaikos are air-sea interaction (wind and surface heating/cooling), land-sea interaction (buoyancy due to river discharge), and coastal deep-sea interaction (intrusion of Aegean waters onto the continental shelf). Topography also plays an important role due to the presence of both gradual and steep shelf slopes (KON-TOYIANNIS et al., 2003). Generally, saline waters from the Aegean enter the gulf from its central and eastern part, fl ow northwards and as they turn towards the southwest, they mix with freshwater originating from the rivers and fl ow southwards along the western coastline (ROBLES et al., 1983;POULOS et al., 2000;HYDER et al., 2002).

Sampling
Sampling was conducted during May 1997 with R/V Aegaeo within the framework of the EU Metro-Med project (Dynamics of matter transfer and biogeochemical cycles: Their modelling in coastal systems of the Mediterranean Sea). Twenty-four stations were sampled in the Thermaikos Gulf, covering the areas of the Thessaloniki Bay and the Thessaloniki Gulf (TP stations) and the continental shelf (M stations) (Fig. 1). Seawater samples were collected with a General Oceanics rosette equipped with 12 Niskin bottles of 10 L mounted on a SeaBird SBE-911+ conductivity-temperature-depth (CTD) profi ler. Two depths (surface and bottom) were sampled in most stations and three to four depths in the stations TP5, TP25, M24; a total of 52 samples were analysed. The seawater samples were drawn into 500-1000 mL borosilicate glass bottles sealed with glass stoppers according to the collection methods of DOE (1994). The samples were poisoned with 1mL/L of saturated HgCl 2 to prevent further biological activity, stored at room temperature away from light and analysed on shore the as soon as possible. The apparent oxygen utilisation (AOU) was calculated according to the difference of the measured oxygen values to the oxygen saturation given by the algorithm of MILLERO (1986).

Determination of TCO 2 and TA
The carbon dioxide system in the oceans can be characterized using any two of the four measurable parameters, pH, total alkalinity (TA), partial pressure (pCO 2 ) or fugacity (fCO 2 ) of carbon dioxide and the total inorganic carbon (TCO 2 ), providing that constants are available for the other acid/base species in seawater. Dissolved CO 2 constitutes less than 1% of the dissolved inorganic carbon content of seawater, which otherwise consists of bicarbonate, carbonate and a negligible amount of carbonic acid: TCO 2 = [CO 2(aq) ] + [H 2 CO 3 ] + [HCO 3 -] + [CO 3 2-]. Dissociation constants of carbonic acid are needed to calculate the components of the CO 2 system from these measurements.
Dissolved Inorganic Carbon (TCO 2 ) and Total Alkalinity (TA) were measured simultaneously with a precise potentiometric method, based on the analytical and calculation procedure described by DYRSSEN & SILLEN (1967), EDMOND (1970) and BRADSHAW et al. (1980& 1981. The seawater is considered as a weak base and is titrated by hydrochloric acid 0.1 N. The progress of titration is monitored using a glass electrode/reference electrode pH cell. The TA corresponds to the carbonic acid endpoint and the TCO 2 concentration is computed by the difference between the two endpoints. The two endpoints of the titration are calculated using a modifi ed Gran approach, a procedure recognised by DOE (1994). The analytical precision on replicate seawater samples was about 0.3%. The results are expressed in μmol/kg for the TCO 2 and μeq/kg for the TA. The system was calibrated using certifi ed reference material (CRM) supplied by Dr A.G. Dickson of Scripps Institution of Oceanography, La Jolla, CA. The fugacity of carbon dioxide (fCO 2 ) was calculated from TA and TCO 2 measurements and the dissociation constants of carbonic acid of MEHR-BACH et al., (1973) and the solubility constants of WEISS & PRICE (1980) using the software of LEWIS & WALLACE (1998). The fCO 2 at 1 atm total pressure is 0.3-0.4% lower than its partial pressure (pCO 2 ) due to the non-ideal behaviour of carbon dioxide.

Spatial trends in TCO 2 concentrations
Dissolved inorganic carbon concentrations in the Thermaikos Gulf ranged from 2351 μmol/kg at the surface of TP11 to 2706 μmol/kg close to the bottom of TP5 (Thessaloniki Bay); the bottom samples of the same station also showed the lowest values of dissolved oxygen and the highest of nutrients (O 2 <3.0ml/l, NO 3~0 .04 μmol/L, SiO 4~5 .9 μmol/L and PO 4~0 .15 μmol/L). The surface values of TCO 2 are lower than those of the last depth ( Fig. 2) refl ecting the consumption of inorganic carbon by photosynthesis. In particular, the difference between the mean surface and the mean bottom values of TCO 2 , 2451 and 2517 μmol/kg, respectively, is statistically signifi cant at the 99% confidence level (n=24, t-test value=4.38). Lower surface TCO 2 concentrations appear near the north-western shore in response to the infl ow of riverine waters. On the other hand, the increase of TCO 2 concentrations in the deeper layer, especially in the inner part of the gulf, implies that there is a formation of TCO 2 species in the near bottom waters. In addition, in the northern part of the gulf the bottom distribution of TCO 2 fi ts inversely with that of oxygen ( Fig. 2), namely, the maximum TCO 2 values correspond to the minimum oxygen concentrations, implying a probable relationship between the two parameters.
Generally the concentrations of the TCO 2 found in the Thermaikos Gulf are significantly lower than those reported in the literature for the extreme environment of the Black Sea (GOYET et al., 1991), in the same order of magnitude as those measured in the shallow northern Adriatic Sea (SOUVERM-EZOGLOU & KRASAKOPOULOU, 1999) and higher than those reported for the central part of the NW Mediterranean Sea (COPIN- MONTEGUT & BEGOVIC, 2002).

Relationships of TCO 2 and TA with salinity and temperature
The variation of TCO 2 and TA with salinity has been described by different investigators using linear regressions (e.g. COPIN-MONTEGUT, 1993;BAKKER et al., 1996;IANSON et al., 2003;BOZEC et al., 2005). If these relationships are truly linear then they suggest that both TCO 2 and TA concentrations are controlled by dilution (evaporation, precipitation, river runoff). Biological processes (uptake/release of TCO 2 during photosynthesis/respiration, CaCO 3 dissolution/formation) that occur at different water depths (and thus salinities) as well as surface TCO 2 exchange, which is temperature dependent, cause non linear variations of TCO 2 with salinity. In contrast to other variables of the CO 2 /carbonate system (i.e. TCO 2 , pH, fCO 2 ), biological processes such as photosynthesis and oxidation of organic matter have a rather small, negligible infl uence on the alkalinity (MILLERO et al., 1998;BOEHME et al., 1998). In the process of calcium carbonate dissolution/formation, TA would change with TCO 2 at a ratio of 2 to 1, due to the chemical equilibrium involved in solid carbonate production (i.e. Ca 2+ + 2HCO 3 -⇔ CaCO 3 + CO 2 +H 2 O). In anoxic environments, TA change would be signifi cant due to consumption of H + during anaerobic respiration. Since the waters of the inner Thermaikos Gulf are not anoxic, anaerobic respiration should be negligible.
Linear regressions (Model II) between our TCO 2 and TA data and salinity were calculated (Figs. 3a and b) in order to investigate the TCO 2 and TA characteristics of the water masses and to discuss deviations from linear-ity. In this fi gure we can distinguish the large difference between the TCO 2 and TA values per salinity of the TP-stations (circles) and those of the M-stations (crosses). Salinity ranged from 31.854 in front of the Aliakmon river estuary to 38.434 at 68m depth in the easternmost part of the study area (Station M30). Elevated values of both parameters appear at higher salinities; however, the data appear linear only when the TP-stations are considered. The lower values of the correlation coeffi cient which are obtained when the data of both the TP-and M-stations are used (0.273 for TCO 2 and 0.463 for TA) are probably due to the narrower variation of salinity of the M-stations. The M-stations are mainly infl uenced by the saline Aegean waters and exhibit higher salinity than the TP-stations, which are also infl uenced by the riverine water inputs. The weaker linear relationship between TCO 2 and salinity than is the case for TA is related to the larger modifi cation of TCO 2 by biological processes and to the airsea exchange of CO 2 . In addition, the stronger correlation of TA with salinity relative to TCO 2 , indicates that calcium carbonate dissolution/precipitation processes are likely to be negligible and that dilution exerted the major control over TA in the Thermaikos Gulf during May 1997.
In order to remove the effects of changes in salinity due to mixing, fresh water runoff, precipitation and evaporation, the TCO 2 and TA values were normalized to a salinity of 37 representing the average salinity during the cruise; thus normalized data = measured data × 37/measured salinity. Consequently, changes in the normalized TCO 2 and TA can be attributed to the production and oxidation of plant material and formation and precipitation of CaCO 3 . The salinity normalized data of both parameters (NTCO 2 and NTA) are inversely proportional to salinity (Figs. 3c and d). The strong linear relationship of NTA with salinity (r 2 =0.917) confi rms the almost conservative character of alkalinity that results from the mixing of the saline Aegean waters with the fresh waters of the inner gulf. The scatter around the line observed in the case of NTCO 2 gives evidence that additional processes occur and cause the variations of the TCO 2 concentrations in the study area during May 1997.

Relationships of TCO 2 with oxygen and nutrients
The enrichment of the bottom waters in TCO 2 is probably caused by the oxidation of the sinking organic matter. The organic material which is transported by the rivers and/ or is produced in situ, falls into the deeper layer and undergoes remineralisation by consuming oxygen and producing equivalent amounts of carbon dioxide and of regenerated nutrients. According to the simplest stoichiometric formula of organic matter in the ocean [(CH 2 O) 106 (NH 3 ) 16 H 3 PO 4 ], an increase of 106 moles of carbon dioxide and a consumption of 138 moles of dissolved oxygen occurs, when this molecule is completely oxidised (AOU:C:N:P = -138:106:16:1; REDFIELD et al., 1963). The positive linear correlation between TCO 2 and Apparent Oxygen Utilisation (AOU) for the samples collected at the last depth of the stations (Fig.  4a) confi rms that carbon dioxide is released proportionately to the dissolved oxygen utilisation. The experimental ratio of carbon regeneration, i.e. the TCO 2 to AOU slope, is 1.63, almost double the predicted value from simple theory, 0.768 (stripped line, Fig.4a), but close to the value observed in the Jabuka Pit in the middle of the Adriatic Sea (KRAS-AKOPOULOU et al., 2005). The TCO 2 data from the last sampling depth were also plotted against phosphate (Fig. 4b). Phosphate was chosen rather than nitrite+nitrate because these waters have the potential complication of denitrifi cation in the water column (MARTY et al., 2001), as well as in the sedi- ments, and which can affect the nitrogen to carbon balance. The relatively good correlation between TCO 2 and phosphate (r 2 =0.591) indicates that in the close to the bottom layer of the Thermaikos Gulf a large proportion of both parameters originates from the remineralisation of organic matter. However, there is more carbon and less phosphorus regenerated than the amounts predicted from the oceanic model (almost 10 times). The composition of the organic matter that is remineralised -particularly rich in carbon in relation to phosphorus-could be a possible reason for the value of the ratio.
During May 1997, both the estimated oxygen consumption and phosphate and carbon regeneration rates in the Thermaikos Gulf are considerably higher in relation to the open ocean estimates, indicating an excess in TCO 2 , and/or a defi cit in AOU and phosphate and should probably be attributed to the proximity of the study area to the coastal zone and/or to the quantity and quality of the organic matter that is remineralised.

The fugacity of CO 2 in seawater (fCO 2 )
The main factors controlling the variation of sea surface fCO 2 in the open ocean are the warming and cooling of surface waters (temperature), and the balance between photosynthetic carbon fi xation and release of CO 2 by respiration or oxidation of organic matter. In coastal seas the same processes take place, yet with more intensity and variability. One common aspect of the coastal CO 2 studies is the large variability of fCO 2 measured in surface waters, signifi cantly greater in both space and time than typically observed in open ocean environments.
In the Thermaikos Gulf during May 1997 the surface fCO 2 varied between 273 μatm (TP01) and 383 μatm (TP16), while the surface temperature ranged between 13.9 (TP13) and 18.3 °C (TP01). In the greater part of the gulf the fCO 2 values (Fig. 5) are lower (average: 332 μatm) than the present atmospheric annual mean value of about 365 μatm (CHAMARD et al., 2001), the surface waters of the Thermaikos appear undersaturated with respect to the atmosphere and the region could act as a sink for atmospheric CO 2 .
Temperature affects the equilibrium constants of inorganic carbon and particularly the solubility coeffi cient of CO 2 ; heating decreases the solubility of CO 2 and increases the fugacity of CO 2 in seawater (e.g. for a temperature increase of 1 °C, pCO 2 rises ~4%), which may induce CO 2 oversaturation in many cases and a CO 2 source to the atmosphere. The fCO 2 data are normalised to a fi xed temperature of 16 °C (fCO 2 T ), representing the mean surface water temperature during the cruise, in order to eliminate the thermodynamic effect on fCO 2 . Model II regressions were calculated between the temperature-normalised fugacity data and chlorophyll α, temperature and AOU in order to analyse the factors behind the fCO 2 variability.
The correlation between chlorophyll α and fCO 2 T is negative (Fig. 6a); the maximum fCO 2 T values were recorded in the southern part of the gulf where the phytoplanktonic biomass was low. The minimum fCO 2 T coincides with the maximum concentration of chlorophyll α in the innermost part of the gulf, where biological activity leads to the draw down of CO 2 and thus strongly reduces fCO 2 T levels. Moreover, the dynamics of inorganic carbon resulting from photosynthesis are highly dependent upon the types of organisms present and whether they are calcifying or not. Photosynthesis and calcifi cation have opposing effects on the fCO 2 and the fi nal result of their concomitance depends on their relative importance. A convenient parameter to study the importance of calcifi cation, the homogenous buffer factor β, which is defi ned as the relative variation of the fCO 2 to that of TCO 2 (β = ∂lnfCO 2 /∂lnTCO 2 ), was also employed. The factor β ranges from ~12, when no calcifi cation occurs during organic matter production to about -7, when precipitation/ dissolution of CaCO 3 is the only occurring process (FRANKIGNOULLE, 1994). The calculated β value of 5.31 (Fig. 6b) indicates that calcifi cation or carbonate dissolution takes place on a very small scale, simulta-neously with organic carbon production, in agreement with the comparatively conservative character of TA (section 3.2).
In addition, the fCO 2 T drastically decreases instead of increasing as expected from the temperature increase and shows a moderate negative correlation with temperature (Fig.  6c). This fact arises from a correlation between warm waters of low fCO 2 with cold waters of high fCO 2 levels and suggests that the spatial variability of fCO 2 T is mainly driven by photosynthetic activity, which seems to overwhelm the expected variability due to temperature differences.
The negative AOU values (i.e. oxygen production) over the whole study area and the positive correlation between AOU and fCO 2 T (Fig. 6d) strengthens the view that the principal factor that modulates fCO 2 T variability is primary productivity.

Air-sea CO 2 exchanges
A concentration difference of carbon di- oxide (CO 2 ) across the air-water interface drives the net gas exchange. A common assumption is that a concentration gradient of CO 2 exists only near the air-water interface and that the gas is well mixed in bulk air and bulk water. Air-sea exchange slowly brings the concentration of dissolved CO 2 in surface water towards an equilibrium with the atmospheric CO 2 content. More rapid physical and biological disturbances of the concentration of CO 2 in water generally prevent this equilibrium from being established.
The net fl ux of CO 2 through the air-sea interface (F CO2 , mmol m -2 d -1 ) is described as: where K w (cm h -1 ) is the gas transfer coeffi cient, S (mol L -1 atm -1 ) is the solubility of CO 2 in seawater, and fCO 2 w and fCO 2 a (μatm) are the fugacity of CO 2 in seawater and air, respectively. Several parameterizations of K w have been proposed based on laboratory and fi eld studies, taking into account a wide variety of factors affecting air-sea exchange: wind, bubbles, turbulence, temperature, atmospheric boundary layer stability, and drag coeffi cients (SMETHIE et al., 1985;LISS & MERLIVAT, 1986;WOOLF, 1997;TANS et al., 1990;WANNINKHOF, 1992;WAN-NINKHOF & MCGILLIS, 1999;Mc GILLIS et al., 2001). In our calculations, we use the K w -wind speed algorithms proposed by LISS & MERLIVAT (1986), WANNINKHOF (1992, WANNINKHOF & Mc GILLIS (1999) and Mc GILLIS et al., (2001), hereafter referred to as LM86, W92, WMG99 and MG01, respectively. In fact, it is diffi cult to choose one relationship because even the latest experiments using the most recent tracer techniques could not reliably distinguish which was the more favourable relationship. In addition, the fl uxes reported in the literature are usually computed using only one of the above mentioned relationships and thus, for comparative reasons, it is useful to have the values calculated using the various relationships. We, therefore, chose to report the results computed with the different formulations.
To evaluate the air-sea CO 2 exchanges, the fCO 2 a must be known. However, during this study atmospheric measurements were not performed. The spatio-temporal variability of the atmospheric molar fraction of CO 2 (xCO 2 a ) is much smaller than that of seawater (xCO 2 w ), mainly because of the high lateral atmospheric mixing rates, which tend to rapidly homogenize xCO 2 a . Therefore, we can compute the fl ux of CO 2 across the air-sea interface, assuming a constant atmospheric fCO 2 during the cruise, using the values of atmospheric fCO 2 a from Lampedusa Island in the central Mediterranean Sea, which is the nearest marine site of atmospheric CO 2 monitoring to the Thermaikos Gulf (CHAMARD et al., 2001).
Wind speed data for the period of our observations were obtained from the EC-MWF MARS database and correspond to the available measurements from the meteorological station located at Thessaloniki airport (40.517N 22.967E); however, wind speed measurements of a higher resolution, both temporally and spatially, are likely to be necessary in order to estimate more realistically the air-sea CO 2 exchanges. The data from an onboard meteorological station would be most appropriate to calculate the gas transfer coeffi cient; nevertheless, through the use of the available data, both of fCO 2 a and wind speed, a rough fi rst assessment of the role of the Thermaikos Gulf in air-sea CO 2 exchange can be obtained.
This simplistic approach to calculating the air-sea CO 2 fl ux involves several major assumptions, but the approximate magnitude and the relative role of the Thermaikos Gulf in the uptake or release of atmospheric CO 2 can be determined.
The results of the four formulations show that during May 1997 the net fl ux of CO 2 through the air-sea interface is negative and the whole Thermaikos Gulf absorbs atmos-pheric CO 2 at a rate between -0.60 and -1.43 mmol m -2 d -1 , depending on which gas transfer coeffi cient is used (Table 1). The obtained results are in agreement with the direction of the CO 2 fl uxes reported in other temperate continental shelf areas. Existing data on air-sea CO 2 fl uxes suggest that temperate marginal seas act as sinks for atmospheric CO 2 (e.g. East China Sea, TSUNOGAI et al., 1999;Gulf of Biscay, FRANKIGNOULLE & BORGES, 2001;US Middle Atlantic Bight, DEGRANDPRE et al., 2002;RIA DE VIGO, NW Spain, GAGO et al., 2003;North Sea, BOZEC et al., 2005). Comparing the magnitude of the fl uxes to the data from other coastal seas during spring-summer (Table 1), our study shows that the Thermaikos Gulf acted as a weak sink for atmospheric CO 2 during May 1997.
During winter, higher gas transfer veloci-ties are expected due to the usual increase in wind force; this higher wintertime transfer of carbon dioxide is also provoked by the temperature decrease of surface waters, which drives a decrease of fCO 2 w values and enhances the penetration of atmospheric CO 2 . In addition, the dense water formed due to the water cooling in the shallow northern part of the gulf (KONTOYIANNIS et al., 2003) and the subsequent slow spreading and mixing transports this enriched in dissolved inorganic carbon water into the deeper layer of the outer gulf. Therefore, it is quite possible that during winter the Thermaikos Gulf could be a more effective CO 2 sink than in late spring. This scenario is likened to the 'continental shelf pump' that has been employed recently by TSUNOGAI et al., (1999) to explain the observed large net absorption of CO 2 in the East China Sea during winter. Table 1 Air-sea CO 2 fl uxes calculated using different gas transfer coeffi cients for the Thermaikos Gulf during May 1997 and other representative temperate continental seas during spring-summer.

Conclusions
The present study enabled us to describe the distribution of dissolved inorganic carbon and related parameters in the Thermaikos Gulf waters during May 1997. Surface concentrations of TCO 2 were lower than those recorded close to the bottom. The decomposition of the organic material, imported directly from external sources (rivers, sewage) and produced in situ in the Thermaikos, resulted in an increase of the concentration of dissolved inorganic carbon, particularly in bottom layer. The positive relatively good correlation between TCO 2 and both AOU and phosphate at the last sampling depth confi rmed the regenerative origin of a large proportion of TCO 2 . The correlations between surface fCO 2 , calculated from the TA and TCO 2 measurements, and chlorophyll α as well as AOU revealed that the carbon dioxide fi xation through biological activity was the principal factor modulating the variability of fCO 2 .
Moreover, a simplistic fi rst estimate of the role of the Thermaikos Gulf in air-sea CO 2 exchange was obtained. The results showed that during May 1997, the Thermaikos Gulf acted as a weak sink for the atmospheric CO 2 at a rate of -0.60 --1.43 mmol m -2 d -1 , depending on which formula for the gas transfer velocity was used.
Extensive study of the dissolved inorganic carbon and related parameters, continuous shipboard measurements of fCO 2 a and fCO 2 w during all seasons would be necessary to safely quantify the role of the Thermaikos Gulf in the context of the coastal margins CO 2 dynamics. Simultaneous measurements of the in situ processes affecting the dissolved inorganic carbon system may yield a better understanding of the mechanisms involved.

Aknowledgements
This work was supported by the EU MAST-III ELOISE Project METRO-MED (Dynamics of Matter Transfer and Biogeochemical Cycles: Their Modelling in Coastal Systems of the Mediterranean Sea), contract MAS3-CT96-0049. We wish to thank Dr A. Papadopoulos for providing the wind data from the EMCWF database and Dr K.Pagou and G. Assimakopoulou for the chlorophyll α data.