The seasonal variations of dimethyl sulphide and carbon disulphide in surface waters of the Venice lagoon

Marine Chemistry 71 Ž2000. 283–295 www.elsevier.nlrlocatermarchem

The seasonal variations of dimethyl sulphide and carbon disulphide in surface waters of the Venice lagoon Ivo Moret a,b,) , Andrea Gambaro a,b, Rossano Piazza a , Carlo Barbante a,b, Carlo Andreoli c , Fabiana Corami b, Giuseppe Scarponi d b

a Dipartimento di Scienze Ambientali, UniÕersita` Ca’ Foscari, Dorsoduro 2137, I-30123 Venezia, Italy C.N.R., Centro di Studio sulla Chimica e le Tecnologie per l’Ambiente c r o Dipartimento di Scienze Ambientali, UniÕersita` Ca’ Foscari, Dorsoduro 2137, I-30123 Venezia, Italy c Dipartimento di Biologia, UniÕersita` di PadoÕa, Via Bassi 58 r B, I-35121 PadoÕa, Italy d Istituto di Scienze del Mare, UniÕersita` di Ancona, Via Brecce Bianche, I-60131 Ancona, Italy

Received 23 February 1999; received in revised form 5 January 2000; accepted 4 May 2000

Abstract The seasonal variation in the concentration of dimethyl sulphide ŽDMS. and carbon disulphide ŽCS 2 . in the surface water of the Venice lagoon was determined at two stations in the period from 3 June 1996 Ž3 March 1997 for CS 2 . to 26 November 1997. Other parameters such as chlorophyll a, water temperature, and wind speed were also measured. DMS concentration Žranges 0.85–15.0 nmol S ly1 at Stn. 1 and 0.40–16.3 nmol S ly1 at Stn. 2. showed two relative maxima in the spring–summer period, probably connected to phytoplanktonic and macro-algal blooms. Low and constant values were observed in autumn, whereas the maximum concentration was observed in the late winter period. The CS 2 concentration Žranges 0.17–2.8 nmol S ly1 at Stn. 1 and 0.08–2.0 nmol S ly1 at Stn. 2. increased in the spring, was maximal in summer and then decreased in autumn, with a different trend from that of DMS, suggesting that the production and the fate of CS 2 and DMS in water are different. Interpretation of the seasonal trends, based on current knowledge of possible formation and transformation processes, is reported. The mean flux of DMS to the atmosphere is estimated to be 0.34 mmol S ŽDMS. my2 dayy1, which is about one order of magnitude lower than that observed in the open sea, due mainly to lower wind speed. The CS 2 flux ŽMarch to November average 0.086 mmol SŽCS 2 . my2 dayy1 . may represent, at least in lagoon environments, an appreciable fraction of the total natural reduced sulphur emitted to the atmosphere. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Seasonal variations; Dimethyl sulphide; Carbon disulphide

1. Introduction Oceanic reduced sulphur is considered to play a key role in climate control ŽCharlson et al., 1987; for ) Corresponding author. Tel.: q39-41-2578549; fax: q39-412578565. E-mail address: [email protected] ŽI. Moret..

a recent review, see also Andreae and Crutzen, 1997.. Recent estimates of global sulphur emissions to the atmosphere show that 0.47–2.2 Tmol S yeary1 ŽRodhe, 1999. originate from natural sources, while 1.9–2.7 Tmol S yeary1 are of anthropogenic origin ŽRodhe, 1999.. About 2r3 of natural sulphur emissions are accounted for by oceanic dimethyl sulphide ŽDMS., with a range 0.3–1.6 Tmol S yeary1 ŽRodhe,

0304-4203r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 Ž 0 0 . 0 0 0 5 6 - 6

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1999., while global oceanic emissions of carbon disulphide ŽCS 2 ., ; 0.007 Tmol S yeary1 ŽKim and Andreae, 1987a., are negligible compared with the DMS flux at a global level. In the atmosphere, the oxidation of DMS contributes to the formation of sulphate aerosols and cloud condensation nuclei Žsee, e.g. Andreae and Crutzen, 1997. with direct Žbackscattering solar radiation. and indirect Žcloud albedo. influence on climate ŽShaw, 1987; Charlson et al., 1987.. Moreover, since oxidation products are mainly acidic ŽSO 2rH 2 SO4 and methane sulfonic acid., DMS influences the pH of aerosols and rain in remote areas, e.g. Antarctica and North Pacific ŽWagenback et al., 1988; Savoie and Prospero, 1989., and in lightly industrialised regions, e.g. Scandinavia and Ireland ŽFletcher, 1989; Turner et al., 1989.. The oxidation of CS 2 in the atmosphere produces COS and SO 2 either by reaction with OH radicals and oxygen atoms or by spontaneous photodissociation ŽLogan et al., 1979; Jones et al., 1982; Wine et al., 1981.. The relatively stable COS molecule contributes to the stratospheric sulphate concentration and thereby influences the earth’s radiation budget, climate and the stratospheric ozone concentration ŽCrutzen, 1976.. DMS is produced within the water column by enzymatic cleavage of b-dimethylsulphoniopropionate ŽDMSP. ŽCantoni and Anderson, 1956., a compound which is present in a variety of marine phytoplankton as an osmolyte and cryo-protectant ŽVairavamurthy et al., 1985; Kirst et al., 1991.. DMS may be excreted directly from phytoplankton cells ŽVairavamurthy et al., 1985. or released when phytoplankton are subjected to zooplankton grazing or bacterial attack ŽDacey and Wakeham, 1986.. DMS can also be produced in the sediment from degradation of detritus settled on the bottom ŽAndreae, 1985.. The loss factors for DMS from surface waters are bacterial oxidation ŽKiene, 1992., photolysis ŽBrimblecombe and Shooter, 1986. and efflux to the atmosphere ŽKieber et al., 1996.. The presence of CS 2 in sea water was first observed by Lovelock Ž1974.. This important reduced sulphur compound could be produced by anaerobic fermentation and by the reaction of H 2 S with organic matter in pore-water of sediments ŽAndreae, 1986., and by «pulp-mill»-type reactions of terrige-

nous plant matter with dissolved polysulphides originating from dissimilatory sulphate reduction ŽAndreae, 1990.. DMS in seawater has been studied extensively, although only a limited number of works report on temporal variations. In contrast, CS 2 has received relatively little attention. No information is available in the literature for either compound in lagoon ecosystems. The lagoon of Venice ŽItaly. is a shallow coastal basin, 8–13 km wide, with a surface of 549 km2 , and an average water depth of about 1.2 m ŽFig. 1.. From a hydrological viewpoint, the lagoon is separated by natural divides into three basins ŽLido, Malamocco, Chioggia. each one communicating with the Adriatic Sea through an inlet in the coastal strip and with practically independent hydraulic behaviour. Several channels lead from the inlets and spread out among the marshes, islands, and mudflats. The water exchange with the sea depends essentially on tidal currents. The historic city of Venice and some other small islands are located in the central area of the lagoon. About 1,500,000 inhabitants live in the drainage basin, while 200,000 reside in lagoon centres, mainly located in the central area, with 70,000 in Venice Žcommuters and tourists not considered.. These centres discharge their municipal loading in this area. Immediately inland of the central area is a vast industrial zone, which consists mainly of steelworks, chemical factories, oil refineries and a power station. The city of Venice is very famous for its many marble and limestone faced monuments and palaces the decay of which may be caused, in part, by acid precipitation. The chief contribution to acid precipitation in Venice probably comes from anthropogenic SO 2 emissions, mainly from the nearby industrial area. Nevertheless, until now, no data have been available on the possible contribution from biogenic sulphur emissions. In a previous paper, we reported on the determination of DMS in surface water in the Venice lagoon considering its temporal trends at two stations from June 1996 to November 1996 ŽMoret et al., 1998.. In this work, we discuss the set-up of CS 2 determination Žsimultaneously with DMS. and report on a study of the temporal changes of DMS and CS 2 concentrations in the lagoon water carried out with

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Fig. 1. The Venice lagoon showing locations of the two stations considered for the study, the University laboratory and the Cavanis Institute.

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the aim of assessing the seasonal trends of the two sulphur species. The period investigated spans from June 1996 ŽMarch 1997 for CS 2 . to November 1997 and data are interpreted according to the current knowledge of the formation and transformation processes of the two compounds. The data are also used to estimate the natural sulphur flux from the lagoon water to the atmosphere and its seasonal variations and to compare them with local, known anthropogenic emissions.

2. Experimental

surface, using a 250 ml polyethylene container. The sampling bottle was completely filled with water and closed without leaving headspace. Samples were stored in the dark, at about 48C and analysed within 1 h. The suitability of the plastic containers for storing DMS and CS 2 water solutions had previously been tested as follows: aliquots of lagoon water Žpreviously purged by gas stripping. and ultrapure water ŽMilli-Q water, Millipore, Bedford, MA, USA. were stored at 48C and subsequently analysed. The gas chromatograms obtained according to the methodology described below revealed no peaks during 6 and 12 h storage.

2.1. Sampling stations 2.3. Determination of DMS and CS2 Two stations, located near our laboratory, were selected for the study Žsee Fig. 1.. The first ŽRio S. Nicolo, ` Stn. 1. is an internal Venetian canal Žrio.; it is shallow Žaverage depth 1.0 m. and subject to urban waste discharge. It is thus representative of the urban area of the lagoon. The second ŽCanale della Scomenzera, Stn. 2. is a wide canal with an average depth of 2.2 m; this canal is in close communication with an area of the lagoon that exchanges water directly with the Adriatic Sea. It is thus more representative of the lagoon water of the central basin. 2.2. Sampling The samples of lagoon water analysed in this research were collected at the above mentioned two stations between 2 December 1996 Ž3 March 1997 for CS 2 . and 26 November 1997. Different frequencies were adopted in different periods Žfrom once every 15 days to five times per week.. Sampling was interrupted in Rio S. Nicolo` ŽStn. 1. between 6 December 1996 and 3 March 1997 due to the inaccessibility of the site. The data obtained were joined with those for DMS between 3 June 1996 and 28 November 1996 ŽMoret et al., 1998.. Altogether, 132 and 159 determinations were carried out for DMS at Stns. 1 and 2, respectively, while 51 and 59 CS 2 determinations were obtained at the same two stations. On each sampling day, lagoon water was collected at 10:00 am, approximately 0.5 m below the

The determination of DMS and CS 2 was carried out on unfiltered samples by a method described in a previous work ŽMoret et al., 1998. and previously used only for DMS determination. The method is based on a purge and trap technique followed by gas chromatographic quantification. Samples of 40 or 100 ml, depending on the DMS and CS 2 concentration levels, were purged with helium Ž100 ml miny1 . for 15 min at 808C by a «Dynamic Thermal Stripper» ŽSupelco, Bellefonte, CA, USA. and the compounds trapped in a multibed sorption tube, such as Carbotrap 200 ŽSupelco., homefilled with graphitized carbon black ŽCarbopack B, Supelco. and carbon molecular sieve ŽCarbosieve S-III, Supelco. and kept at 858C. After stripping, the sorption tube was flushed with helium at 50 ml miny1 for 5 min. DMS and CS 2 were desorbed from the sorption tube by a Thermal Desorption Unit ŽSupelco. coupled to a gas chromatograph ŽCarlo Erba, model 5160, Rodano, Italy., equipped with a 30 m megabore capillary column ŽGS-Q, J & W Scientific, Folson, CA, USA. and a flame photometric detector. The following operating conditions were used: oven temperature 708C for 7 min, 28C miny1 to 1208C, 1 min, 208C miny1 to 1808C, 1 min; detector temperature 1808C; helium flow rate 7 ml miny1 . Quantitative determinations were carried out using the external standard method and two calibration plots were prepared for high concentrations Žproce-

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dure with 40 ml. and low concentrations Žprocedure with 100 ml.. For details of the procedure, see the description for CS 2 determination, below. As regards DMS, it was demonstrated in our previous work ŽMoret et al., 1998. that logarithmic calibration plots obtained with standard solutions prepared using ultrapure water and gas stripped, lagoon water are both linear and practically coincident Žwithin the experimental error.. Thus, standard solutions prepared with ultrapure water were used throughout this work. As for repeatability of measurements, it was also shown that the relative standard deviation obtained from 16 repetitions of a freshly-prepared standard solution Ž9.5 nmol S ly1 . was 14%. Moreover, accuracy was evaluated analysing DMSP standard solutions in ultrapure water ŽDMSP Ø HCl from Research Plus, Addison, USA, assay 95%. after quantitative transformation to DMS ŽpH 13, NaOH, 12 h.. From four measurements, the mean difference between experimental and theoretical values Žthe latter corrected for reagent impurity. was y7%. Considering CS 2 , the calibration curves to be used for the two concentration ranges were prepared as follows. Again either ultrapure water ŽMilli-Q. or lagoon water previously purged by gas stripping was

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used. A parent standard solution of CS 2 in methanol Ž24.62 mmol S ly1 . was prepared from CS 2 reagent at assay 99.7% ŽBDH, Poole, England.. From the parent solution, five stock standard solutions of CS 2 in methanol were prepared with concentrations of 0.023, 0.035, 0.074, 0.115 and 1.186 mmol S ly1 . The parent solution and the five stock standard solutions were stored at 48C in glass vials closed with Mininert Valves ŽSupelco.. Immediately before use, the final standard solutions were prepared by adding 2 ml of the stock standard solution to 40 or 100 ml of ultrapure water or gas stripped lagoon water, obtaining concentrations ranging from 1.16 to 59.30 nmol S ly1 , and from 0.46 to 23.72 nmol S ly1 , respectively. Fig. 2 shows the calibration curves obtained by linear regression analysis in the logarithmic plot. As for DMS determination ŽMoret et al., 1998., no significant differences can be observed in results obtained with the two matrices. Consequently, ultrapure water was used for the preparation of the standard solutions. Considering that the temporal stability of aqueous CS 2 solutions was unknown, the usefulness of the calibration curves during the long-term experiment, together with the repeatability of measurements, were tested as follows. A standard solution in ultrapure

Fig. 2. Logarithmic calibration plots obtained with CS 2 standard solutions in ultrapure water ŽMilli-Q. Ž –`. and lagoon water Ž –v .. Peak area in counts, CS 2 concentration in nmol S ly1 .

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water at a concentration of 2.65 nmol S ly1 was periodically prepared from the appropriate stock standard solution and analysed before the measurement of real samples. From 38 repetitions of the procedure with this standard, carried out over 137 days, a mean CS 2 concentration of 2.67 nmol S ly1 was obtained with 6.7% RSD. Except for the laboratory experiment which used standard solutions as described above, the accuracy of measurements in natural aqueous matrix could not be assessed due to the lack of certified reference materials. Though the samples of lagoon water were analysed within 1 h of sampling, stability during storage at 48C in the dark was tested by analysing a stored sample every 65 min Žthe time required for one analysis. for 6 h. The results of the six consecutive analyses did not show any trend and the relative standard deviation was 6.6% for DMS ŽMoret et al., 1998. and 6.1% for CS 2 . This experiment proved that under the storage condition DMS and CS 2 concentrations remain stable for at least 6 h. Similar findings were reported for CS 2 by Kim and Andreae Ž1987b.. 2.4. Other measurements

where F s flux, mmol S my2 dayy1 ; DC s concentration gradient across the sea–air interface; the atmospheric concentration is practically negligible ŽSimo` and Grimalt, 1998; Turner et al., 1996. and thus, the DC used coincides to seawater concentration in mmol S my3 ; K s transfer coefficient, m dayy1 . According to Liss and Merlivat Ž1986., K can be computed from the wind speed at 10 m height Ž u, m sy1 . and the Schmidt number ŽSc. as follows: K s a = 0.0408 = u

for

u F 3.6 m sy1

K s 0.24 b Ž 2.85 = u y 10.26 . q 0.612 = a for 3.6 - u F 13 m sy1 K s 0.24 b Ž 5.90 = u y 49.91 . q 0.612 = a u ) 13 m sy1 where a s Ž600rSc. 2r3 ; b s Ž600rSc.1r2 ; and the Schmidt number is a function of the surface water temperature Ž t, 8C.. For DMS ŽSaltzman et al., 1993.; for

Sc s 2674.0 y 147.12 = t q 3.726 = t 2 y 0.038 = t 3 . For CS 2 ŽXie and Moore, 1999. Sc s 3377.8 y 221.71 = t q 6.9370 = t 2

Surface water temperature was measured by dipping a mercury thermometer directly in the lagoon water at a depth of ; 0.5 m. The determination of chlorophyll a in the presence of pheophytin a was carried out according to the spectrophotometric method introduced by Lorenzen, 1967 Žsee also, e.g. Clesceri et al., 1998.. Chlorophyll a was measured only at Stn. 1 from 3 March 1997 to 26 November 1997. Daily average data of wind speed Ž20 m above sea level. were obtained from the meteorological observatory of the Cavanis Institute in Venice which stands within 500 m of the two stations Žsee Fig. 1.. 2.5. Sea–air flux of DMS and CS2 The sea-to-air fluxes of DMS and CS 2 were assessed utilising the general equation ŽLiss and Slater, 1974.: F s K = DC

y 0.08751 = t 3 . The wind speed measured at a height of 20 m probably differs by a negligible amount from that at the standard height of 10 m, and any errors introduced by use of the 20 m wind speed data would not have a significant impact on our conclusions.

3. Results and discussion 3.1. Chlorophyll, temperature, and wind speed A time series of chlorophyll a, surface water temperature, and wind speed is shown in Fig. 3. Note that chlorophyll data are not available for Stn. 2. The observed temporal trends will be considered below in connection with the discussion of the seasonal variations of sulphur species and their sea-to-air fluxes.

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Surprisingly, the absolute maximum of DMS concentration Ž16.3 nmol S ly1 . was observed in late winter Ž13 Feb. 97, Stn. 2.. Unfortunately, DMS data for Stn. 1 during the same period are unavailable. After the maximum, a generally decreasing trend was observed during spring–summer seasons, with secondary maxima superimposed approximately in June and August. In spring–summer, values declined from about 10 nmol S ly1 ŽStn. 1. or 7 nmol S ly1 ŽStn. 2. to about 2 nmol S ly1 in September. Finally, stable low values were observed in autumn–winter periods. As far as we know, no other time series are available for DMS in the Venice lagoon. A similar temporal trend to the one we observed was found by Kwint and Kramer Ž1996., who studied the Marsdiep

Fig. 3. Time series of chlorophyll a ŽStn. 1., temperature of surface water Ž –`, Stn. 1; –v, Stn. 2. and wind speed ŽCavanis Institute..

3.2. DMS and relationship to chlorophyll The temporal trend of the DMS concentration in the two stations for the period considered is shown in Fig. 4 Žoriginal data. and in Fig. 5 Žmonthly average.. In general, it can be observed that DMS concentration in the surface water of the lagoon of Venice are between ; 0.4 and ; 16 nmol S ly1 . The overall means and ranges are Žnmol S ly1 .: Stn. 1, 5.4 Ž0.85–15.0.; Stn. 2, 3.7 Ž0.40–16.3.. These values are comparable with those reported by other authors for surface marine waters Žsee Table 1.. The two time series show that very similar temporal trends appear at the two stations, with marked, characteristic seasonal variations.

Fig. 4. Temporal trend of DMS concentration in lagoon water observed in the period from 3 June 1996 to 26 November 1997. Ža. Rio San Nicolo` ŽStn. 1. Ž –`.. Žb. Canale della Scomenzera ŽStn. 2. Ž –v ..

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Fig. 5. Monthly average of DMS concentration in lagoon water observed in the period from 3 June 1996 to 26 November 1997. Ž –`. Rio San Nicolo` ŽStn. 1.. Ž –v . Canale della Scomenzera ŽStn. 2..

Channel Ža tidal inlet of the Dutch Wadden Sea.. In this case, the major production of DMS appeared to be limited to a period of only 2 months ŽMarch and April., and this production was connected to phytoplankton blooms. We found that the concentration trend of chlorophyll a at Stn. 1 ŽFig. 3. also shows maxima in the

Table 1 DMS concentrations Žnmol S ly1 . from the literature Region

Reference

North Pacific eastern Seawater around mainland Britain

Watanabe et al., 1995

Turner et al., 1988 winter summer Mediterranean Simo´ et al., 1997 Sea western Continental shelf Open sea Ligurian, coastal Boniforti et al., 1993 Ligurian Sea Belviso et al., 1993 Baltic Sea Leck et al., 1990 Northeast Atlantic Malin et al., 1993 Venice lagoon Stn. 1 Žthis study. Stn. 2 Žthis study.

Mean Range 4.10 1.54–10.82 0.03–34.31 0.12 6.86 4.9 1.8 16.2 4.6 12 5.4 3.7

0.0–19.3 0.1–4.3

0.06–6.24 1.06–93.8 0.85–15.0 0.40–16.3

March–April period, indicative of phytoplankton blooms. Indeed, other researchers working in the central lagoon of Venice have observed phytoplankton blooms in the spring–summer period ŽSfriso et al., 1989; Sfriso, 1999. and a biomass peak of macrophyte algae in June ŽSfriso and Marcomini, 1999.. Thus, the spring–summer DMS secondary maxima may be related to phytoplanktonic and algal blooms as found by several authors Žsee, e.g. Kwint and Kramer, 1996.. DMS can be produced directly by several phytoplankton and algal species ŽVairavamurthy et al., 1985., or by bacterial transformation of algal-derived DMSP ŽCantoni and Anderson, 1956., or during phytoplankton grazing and digestion by zooplankton ŽDacey and Wakeham, 1986.. For the winter period Žsee Fig. 4., the absolute maximum DMS concentration was found in February, i.e. much earlier than the period of March–April reported in the study quoted above ŽKwint and Kramer, 1996.. We have no direct chlorophyll data, but the experience of other authors working in the Venice lagoon shows that no phytoplankton blooms occur in February ŽSfriso, 1999.. At present, no

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general conclusion can be reached about the origin of the late winter DMS maximum. 3.3. CS2 and relationship to temperature The trend of CS 2 concentration in the period considered is shown in Fig. 6 and as monthly averages in Fig. 7. In general, it can be observed that CS 2 concentration is only a little higher at Stn. 1 than at Stn. 2 and that the temporal trend was very similar for the two stations. There was a general increase during springtime, almost steadily high values in summer, and a decrease in autumn. Seasonal averages are Žin nmol S ly1 .: spring, 0.97 at Stn. 1 and 0.78 at Stn. 2; summer, 1.46 at Stn. 1 and 1.14 at Stn. 2; autumn, 0.50 at Stn. 1 and 0.42 at Stn. 2. Maxima are found in July at Stn. 1 Ž2.8 nmol S ly1 . and in early September at Stn. 2 Ž2.0 nmol S ly1 .; minima are found in October at Stn. 1 Ž0.17 nmol S ly1 . and in November at Stn. 2 Ž0.08 nmol S ly1 .. The overall means and ranges are Žnmol S ly1 .: Stn. 1, 1.11 Ž0.17–2.8.; Stn. 2, 0.92 Ž0.08–2.0.. The greater concentrations found at Stn. 1 are probably

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linked to the greater influence of urban wastewater around this station. It is important to note that CS 2 concentration Žin molar units referred to sulphur. is always lower by a factor of 4–5 than that found for DMS. Average values, however, are higher than those found by Kim and Andreae Ž1992. in estuarine water Ž0.118 nmol S ly1 ., in shelf water Ž0.051 nmol S ly1 . and in ocean water Ž0.028 nmol S ly1 .. Comparison of the observed seasonal trend with literature data cannot be made since as far as we know, no papers dealing with temporal variation of CS 2 are available. Comparison of Fig. 6 with the temperature data shown in Fig. 3 shows that the temporal trend of CS 2 concentration is quite similar to that of water temperature. Moreover, it can be seen that the CS 2 temporal trend is different from those of DMS and chlorophyll a. These observations suggest that the mechanisms controlling the quantity of CS 2 present in bulk water are different from those controlling the quantity of DMS. According to current knowledge, CS 2 could be produced in sediments by bacterial activity through fermentation reaction or by reaction of H 2 S with

Fig. 6. Temporal trend of CS 2 concentration in lagoon water observed in the period from 3 March 1997 to 26 November 1997. Ž –`. Rio San Nicolo` ŽStn. 1.. Ž –v . Canale della Scomenzera ŽStn. 2..

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Fig. 7. Monthly average of CS 2 concentration in lagoon water observed in the period from 3 March 1997 to 26 November 1997. Ž –`. Rio San Nicolo` ŽStn. 1.. Ž –v . Canale della Scomenzera ŽStn. 2..

organic matter ŽAndreae, 1986., i.e. processes whose rates increase with temperature. These mechanisms are in agreement with the seasonal trend of the sulphate-reduction rate in the Venice lagoon sediments, showing summer maxima ŽIzzo et al., 1997., and with the seasonal variation of reduced sulphur species concentrations in mud-flat pore-waters of the Venice lagoon, which are low in winter and high in summer ŽBertolin et al., 1997.. Another mechanism that could be responsible for CS 2 accumulation is photochemical production ŽXie et al., 1998..

3.4. Sea–air flux of DMS and CS2 Fig. 8 shows the temporal trends of the DMS and the CS 2 fluxes to the atmosphere for Stn. 2. The seasonal average values are reported in Table 2. As regards DMS, it can be noted that the seasonal trend of the flux is quite similar to that of the concentration except in late winter, when there is no peak in the flux to correspond to the peak of DMS concentration Žcompare Fig. 8 with Fig. 4.. This fact is due to the combined effects of low wind speed and

of low temperature in winter Žsee Fig. 3.. However, it is important to note that in this study, the winter flux is quantitatively similar to that of the other seasons. Considering all the available data Žsee Table 2., the mean flux to the atmosphere estimated for Stn. 2 is 0.34 mmol S ŽDMS. my2 dayy1. As far as we know, no data of DMS flux are available for any other lagoon ecosystems. Comparison of the present results is only possible with reference to marine environments. For instance, the following Žmean annual. fluxes Žin mmol S ŽDMS. my2 dayy1 . have been reported: Mediterranean Sea, 11.2 and 2.5 for shelf water and open sea, respectively ŽSimo´ and Grimalt, 1998.; Northeast Atlantic, 17.3 ŽMalin et al., 1993.; North Pacific, 7.2 ŽWatanabe et al., 1995.. The comparison shows that mainly due to low wind speed, the sea–air DMS flux for the Venice lagoon is lower than those observed in marine environments by a factor of 7–50. It is worth noting that winter DMS flux reported in the literature has often been obtained from summer values using a summer to winter ratio of 2.0–2.5 ŽBates et al., 1987; Erickson et al., 1990; Nguyen et al., 1990; Bates et al., 1992; Simo´ and Grimalt, 1998.. In particular, Simo´ and Grimalt Ž1998. used a

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Fig. 8. Trend of the DMS Ž –v . and CS 2 Ž –`. fluxes to the atmosphere from lagoon water observed in the period from 3 June 1996 to 26 November 1997 at Stn. 2.

2.3 summer to winter flux ratio to obtain fluxes for wintertime in the North-western Mediterranean Sea. In contrast, the present study clearly demonstrates that, at least for the Venice lagoon, the winter flux is quantitatively similar to that of other seasons. Now, it is important to know if this result is characteristic of the Venetian lagoon ecosystem or whether it is valid for other environments.

Table 2 Seasonal average values of DMS and CS 2 fluxes Žmmol S my2 dayy1 . for Stn. 2 Season

DMS

CS 2

Spring 1996 Summer 1996 Autumn 1996 Winter 1996–1997 Spring 1997 Summer 1997 Autumn 1997 Meana

0.50 0.37 0.16 0.34 0.69 0.25 0.09 0.34

0.112 0.083 0.026 0.086

a

Computed from all the available data.

Although we have no winter data, the seasonal trend of CS 2 flux was similar to that of CS 2 concentration, with high values during the summer. The average sea-to-air flux of CS 2 in terms of sulphur moles Žmmol S ŽCS 2 . my2 dayy1 . was 0.086 which appears comparable with values reported by Kim and Andreae Ž1992. for estuarine, coastal and oceanic regions, i.e. 0.326, 0.140, and 0.074, respectively, even if these authors used a different way to calculate the gas transfer coefficients. Surprisingly, the CS 2 flux detected in the Venice lagoon is not negligible but as high as ; 20% of the DMS flux. Finally, if one adds the DMS and CS 2 mean fluxes Ž0.43 mmol S my2 dayy1 . and, in a very crude approximation, assumes that the value obtained is representative of all the lagoon area, the total emissions of biogenic volatile sulphur compounds to the atmosphere from the lagoon Žsurface area excluding land surface 522.8 km2 . amount to 82 kmol S yeary1 . This value appears negligible with respect to the anthropogenic SO 2 emissions only from the power plant of the industrial area, which account for 378 = 10 3 kmol S yeary1 ŽProvincia di Venezia, 1998..

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4. Conclusions DMS and CS 2 concentrations in the surface water of the Venice lagoon show different characteristic seasonal trends, with the maximum concentration in late winter for DMS and in summer for CS 2 . For DMS, two secondary maxima are also observed in the spring–summer period. These trends suggest that the formation and transformation processes of CS 2 are different from those of DMS. DMS concentrations are comparable with those reported by other authors for the Mediterranean Sea and other marine environments, while for CS 2 , our values are ; 10 times higher than those reported in the literature for estuarine waters, and ; 20 times higher than shelf and ocean waters. This result shows the remarkable importance of CS 2 in lagoon ecosystems and possibly in other coastal environments. The DMS flux to the atmosphere, although lower than that observed in the open sea Žby about one order of magnitude., is quantitatively similar in all seasons while the CS 2 flux is about 20% of DMS flux, which is an unusually high value with respect to marine environments. The approximate sea-to-air flux of biogenic sulphur compounds in the lagoon is negligible with respect to anthropogenic SO 2 emissions. Acknowledgements Financial support for the «Sistema Lagunare Veneziano» project of the Italian Ministry of the University and Scientific and Technological Research ŽMURST. is gratefully acknowledged. The authors thank I. Ongaro and W. Zampieri for sampling support. References Andreae, M.O., 1985. Dimethylsulfide in the water column and the sediment porewaters of the Peru` upwelling area. Limnol. Oceanogr. 30, 1208–1218. Andreae, M.O., 1986. The ocean as a source of atmospheric sulfur compounds. In: Buat-Menard, P. ŽEd.., The Role of Air–sea Exchange in Geochemical Cycling. Reidel, Dordrecht, The Netherlands, pp. 331–362. Andreae, M.O., 1990. Ocean–atmosphere interactions in the global biogeochemical sulfur cycle. Mar. Chem. 30, 1–29. Andreae, M.O., Crutzen, P.J., 1997. Atmospheric aerosols: bio-

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