Monsoon-Driven Vertical Fluxes of Organic Pollutants in the Western

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Environ. Sci. Technol. 1999, 33, 3949-3956

Monsoon-Driven Vertical Fluxes of Organic Pollutants in the Western Arabian Sea J O R D I D A C H S , †,§ J O S E P M . B A Y O N A , * ,† VENUGOPALAN ITTEKKOT,‡ AND J O A N A L B A I G EÄ S † Department of Environmental Chemistry, IIQAB-CID-CSIC, Jordi Girona 18-26, E-08034, Barcelona, Catalunya, Spain, Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, Bundesstrasse 55, D-20146 Hamburg, FRG, and Department of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, New Jersey 08901-8551

A time series of sinking particles from the western Arabian Sea was analyzed for aliphatic and polycyclic aromatic hydrocarbons, polychlorinated biphenyls, 4,4′DDT and 4,4′-DDE, to assess the role of monsoons on their vertical flux in the Indian Ocean. Concurrently, molecular markers such as sterols and linear and branched alkanes were analyzed enabling the characterization of the biogenic sources and biogeochemical processes occurring during the sampling period. Hierarchical cluster analysis (HCA) of the data set of concentrations and fluxes of these compounds confirmed a seasonal variability driven by the SW and NE monsoons. Moreover, the influence of different air masses is evidenced by the occurrence of higher concentrations of DDT, PCBs, and pyrolytic PAHs during the NE monsoon and of fossil hydrocarbons during the SW monsoon. Total annual fluxes to the deep Arabian Sea represent an important removal contribution of persistent organic pollutants, thus not being available for the global distillation process (volatilization and atmospheric transport from low or mid latitudes to cold areas). Therefore, monsoons may play a significant role on the global cycle of organic pollutants.

Introduction The study of the transport and fate of semivolatile persistent organic pollutants (POPs) in remote areas has become a major issue over the past decade from the standpoint of a better understanding of the global cycle of these compounds (1, 2). It starts to be widely accepted that POPs tend to mobilize from low latitudes to mid and high latitudes by global distillation and fractionation (3). However, most of the studies have been carried out in mid latitude areas such as Europe and North America, very few in equatorial and tropical environments and, particularly, on the marine compartment that may play an important role in this respect. The long-range atmospheric transport, deposition (dry + wet), and air-water exchange are key processes regulating the global distribution of semivolatile POPs (2, 4), and a major * Corresponding author phone: +34 93 400 6119; fax: +34 93 204 5904; e-mail: [email protected]. † IIQAB-CID-CSIC. ‡ University of Hamburg. § Rutgers University. 10.1021/es990200e CCC: $18.00 Published on Web 10/02/1999

 1999 American Chemical Society

concern has largely been expressed on the function of the oceans as a sink for these compounds (5). The biogeochemical cycle of POPs in the marine environment is linked to the organic carbon cycle, because they exhibit moderate to high hydrophobicities (6, 7). Effectively, vertical sinking of particulate matter is the major removal process of both organic carbon and organic pollutants from surface waters (7-9). Extensive studies carried out in the Indian Ocean using sediment trap deployments have shown that Asian monsoons play an important role on the removal of organic carbon from the sea surface down to the water column (10). Indeed, the vertical flux of particulate matter to the interior of the Arabian Sea shows a strong seasonal and interannual variability related to monsoons (10, 11), thus suggesting this may also be an important removal mechanism of POPs at low latitudes. On the other hand, Rixen et al. (12) have shown, in the western Arabian Sea, that deep ocean vertical fluxes of particles are related, during the SW monsoon, to ocean surface processes such as upwelling and to ocean-atmosphere interactions such as monsoon-driven cooling of the surface waters. It is well-known that diffusive air-water exchange plays a major role on the exchange of pollutants between aquatic environments and the atmosphere (13, 14), and it may support and even control the concentrations found in phytoplankton in remote aquatic environments (15, 16). Moreover, air-water exchange of POPs has a strong dependence on wind speed and temperature (13), so that monsoon driven variability may exert a strong influence on the inputs of pollutants in the open Arabian Sea. The aim of this paper is the assessment of the role of monsoons on the vertical fluxes of POPs and other organic compounds in the western Arabian Sea. This study is based on concentrations and fluxes obtained from a time-series of sediment traps deployed at the Arabian Sea. Sinking particles were characterized by measuring organic carbon and molecular markers such as alkanes, isoprenoids, and sterols in order to determine the sources and variability of organic matter. Organic pollutants, namely, aliphatic and polycylic aromatic hydrocarbons, polychlorinated biphenyls (PCBs), and other organochlorine compounds (OCs) such as 4,4′DDT and 4,4′-DDE were also studied. Vertical fluxes of organic pollutants will be discussed in terms of the atmospheric inputs and the biogeochemical processes occurring in the surficial waters. A better understanding of the removal processes of POPs in the equatorial Indian Ocean will certainly contribute to determine the influence of monsoons on the global cycle of POPs.

Site Description and Experimental Section Site Description. The area of study is shown in Figure 1. The physical and biological processes in the Arabian Sea exhibit a characteristic seasonal pattern deeply mediated by the regional monsoons that sweep, SW and NE alternatively, an area almost parallel to the Arabian peninsula coast of 200400 km wide (17). The Arabian Sea is strongly influenced by the southwest (SW, June to September) and, moderately, by the northeast (NE, December to February) monsoons causing a reversal of the surface currents from a clockwise to an anticlockwise direction. Intermonsoon transitions occur from October to November and from March to May. Monsoonal upwelling appears in summer, along the Arabian and Somalian coasts, following the variation of the wind field with the opening of the SW monsoon. Furthermore, there is a coupling of the monsoon-related surface processes with VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Arabian Sea map showing the sampling site location. the deep ocean sinking particle fluxes, which show a bimodal seasonal cycle (12). The wide extension of highly productive areas from the Arabian Peninsula into the open Arabian Sea has been attributed to the horizontal advection of upwelled water and to wind-driven open sea upwelling. Wind-induced vertical mixing leads to mixed layer deepening of monsoons. In addition to temporal variations within the basin, spatial variability and gradients of biogeochemical processes in the pelagic zone have been observed due to asymmetric effects of the monsoons in the Arabian Sea (17). The Arabian Sea receives river inputs from the Indus, Narmada, and Tapti Rivers (18, 19) and a supply of aeolian particles mainly in its southwestern parts from the Arabian and Somalian deserts achieving relatively high concentrations of dust during the SW monsoon (20, 21). Aeolian particles are also supplied from India and Pakistan during the NE monsoon (22), but little information on the significance of this source is available. Sampling. Samples were collected as part of an ongoing sediment trap experiment in the Arabian Sea, details of which are given by Haake et al. (11). Briefly, three moorings consisting of two sediment traps (12) at approximately 1000 m above seafloor and 1000 m below sea surface were deployed in the western, central, and eastern Arabian Sea. The present study (Figure 1) was carried out on samples from the deeper trap (3000 m below sea surface) in the western Arabian Sea (WAST) (16°20′N; 60°32′E; water depth 4017 m, trap depth 3039 m) collected at intervals of 25 days from June 6 to October 5, 1988 and from December 30, 1990 to October 26, 1991 (Table 1). Analytical Methods. Aliquots of freeze-dried particulate material (40-60 mg) were spiked with octachloronaphthalene, perdeuterated pyrene, and cholestane as analyte surrogates and extracted (3 times) with 5 mL of dichloromethane-methanol (2:1) by sonication. Organic extracts 3950

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were fractionated by column chromatography (5 mm id × 20 mm length) using 2 g of activated (120 °C) alumina. Three fractions were separated and collected: (I) 6 mL of n-hexane (aliphatic hydrocarbons and PCBs), (II) 5 mL of 2:1 dichloromethane-n-hexane (organochlorine, PAHs), and (III) 5 mL of 1:1 dichloromethane-methanol (sterols). Elemental sulfur was removed from fractions I and II by an activated powdered copper treatment. Fractions I and II were analyzed for organochlorine compounds in a 5890 Hewlett-Packard GC (Palo Alto, CA) equipped with an ECD and fraction I for aliphatic hydrocarbons in a Mega 5000 series GC-FID (Fisons Instruments, Milan, Italy). Samples were injected in the splitless injection mode at 280 °C, and the FID and ECD temperatures were held at 330 and 310 °C. A fused silica capillary column of 30 m × 0.25 mm i.d. coated with 0.25 µm of DB5 was used. Helium was the carrier gas supplied at 30 cm s-1. PAHs were analyzed by GC-MS in a MD 800 apparatus (Fisons) using a SIM acquisition program under electron impact ionization (70 eV energy) as described elsewhere (7). Other analytical conditions were similar to those described above for the GCECD analysis. The identification of aliphatic hydrocarbons and sterols was accomplished by GC-MS in the full-scan mode (50-500 m/z), while the quantitation was done by GC-FID with column and conditions similar to those described above for PAHs. Total organic carbon was determined as reported elsewhere (11). Quantification was performed by the external standard procedure. The calibration mixture contained the analyte surrogates spiked in the sample and the target analytes, which were 14 PAHs included in the U.S. EPA priority pollutant list, 9 PCB congeners (IUPAC 28, 52, 70, 101, 118, 153, 138, 180, 170), 4,4′-DDT, 4,4′-DDE, n-alkanes (with carbon numbers 14, 15, 16, 22, 23, 28, 32, 36), pristane, and sterols. In the analytical conditions used, the following PCB congeners

TABLE 1. Sample Interval, Monsoon Period, Organic Carbon Content, Total Fluxes and Concentrations (ng/g) of Molecular Markers, and Pollutants in the Sediment Trap Samples Analyzed

sample

sample interval

ASI ASII ASIII AS2 AS3 AS4 AS5 AS6 AS7 AS8 AS9 AS10 AS11 AS12 AS13

21.06-17.07.88 17.07-08.09.88 08.09-05.10.88 30.12-24.01.91 24.01-18.02.91 18.02-15.03.91 15.03-09.04.91 09.04-04.05.91 04.05-29.05.91 29.05-23.06.91 23.06-18.07.91 18.07-12.08.91 12.08-06.09.91 06.09-01.10.91 01.10-26.10.91

a

total flux OC flux monsoon (mg m-2 (mg m-2 alkanes CPI pristane phytane UCM sterols PCBs DDT DDE PAHs perioda d-1) d-1) (µg/g) (C23-C36) (µg/g) (µg/g) (µg/g) (µg/g) (ng/g) (ng/g) (ng/g) (ng/g) SW SW SW NE NE NE NE IM IM IM IM SW SW SW SW

164 351 191 158 119 169 119 94.4 50.2 89.8 90.4 196 239 203 188

8.5 22 7.8 9.9 6.9 13 10 7.6 4.0 6.0 2.9 7.6 13 11 13

78.9 49.5 73.9 451 22.3 31.6 22.1 23.9 115 155 34.2 261 822 675 476

2.87 1.85 3.71 1.04 1.60 1.61 2.00 2.19 1.19 1.13 1.11 1.12 1.02 1.03 1.02

7.4 2.2 9.9 11 2.3 2.7 1.6 2.4 6.2 1.9 0.5 1.1 58 116 71

0.9 0.1 0.5 1.0 1.1 1.0 0.6 1.3 6.7 1.6 0.5 0.5 2.2 4.4 2.5

60.5 77.9 88.8 450 199 119 164 186 594 811 284 336 1701 2391 1720

220 194 724 365 93.8 373 426 104 670 261 93.3 297 373 210 275

25.4 2.70 23.7 79.2 79.4 91.8 102 60.7 77.6 632 116 50.6 16.2 nd 17.7

1.96 0.54 1.27 2.09 0.27 4.66 3.76 1.17 0.58 nd 0.11 3.68 2.55 2.02 1.81

0.33 nd 0.29 3.17 1.86 5.55 2.54 3.59 4.94 6.47 3.11 2.06 0.16 n.d. 0.19

124 103 109 396 351 397 261 332 425 719 102 135 192 153 245

Estimated monsoon cycle at the sampling depth (delayed 2-3 weeks with respect to surface) (IM: intermonsoon period) (nd: not determined).

coeluted 28-31, 118-149, 153-132. Recoveries of spiked samples according to the standard addition procedure were higher than 80%, and the RSDs were below 15% (N ) 3). The unresolved complex mixture (UCM) was quantified using the response factor of the n-alkane eluting in the zone of maximum response (C23 for most of the samples). Procedural blanks and control samples were processed in the same manner as real samples, and they were below 5% of the abundance of analytes. Results were corrected for recoveries. Specific ratios were calculated for the characterization of biogeochemical processes, such as the carbon preference index (CPI) which is defined as the ratio of n-alkanes with odd carbon number to even carbon number in the range of C23-C36. A ratio indicating the relative predominance of fossil over pyrolytic PAHs was calculated as the addition of methylphenanthrenes, dimethylphenanthrenes, and chrysene concentrations over that of fluoranthene, benz[a]anthracene, benzofluoranthenes, benzo[e]pyrene, benzo[a]pyrene, dibenz[ah]anthracene, benzo[ghi]perylene, and indeno[1,2,3-cd]pyrene concentrations. Statistical Analysis. The hierarchical cluster analysis (HCA) was done using the multivariate data analysis software “Pirouettte” (Infometrix Inc., Washington, U.S.A.). A centroidal approach called incremental link clustering was used for HCA. HCA was carried out without any data pretreatment. The variables included were the concentrations of the following: organic carbon, n-alkanes with carbon numbers ranging from 14 to 36, pristane, phytane, the PCB congeners 28, 52, 47, 44, 70, 101, 118, 153, 138, 187, 128, 180, and 170, phenanthrene, methyphenanthrenes, dimethylphenanthrenes, anthracene, fluoranthene, pyrene, chrysene, benzofluoranthenes, benzo[e]pyrene, benzo[a]pyrene, perylene, dibenz[ah]anthracene, benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, cholesta-5,22(E)-dien-3β-ol, 5R(H)-cholest-22(E)-en3β-ol, cholest-5-en-3β-ol, 5R(H)-cholestan-3β-ol, 24-methylcholesta-5,22-dien-3β-ol, 24-methyl-5R(H)-cholest-22(E)en-3β-ol, cholest-en-3-one, 24-methylcholest-5-en-3β-ol, n-C28-ol, 24-ethylcholesta-5,22(E)-dien-3β-ol, 24-ethylcholest5-en-3β-ol, and 4R,23,24-trimethyl-5R(H)-cholest-22(E)-en3β-ol.

Results and Discussion Statistical Analyses. HCA was carried out with the aim of confirming the compositional seasonal trends of the samples. The dendogram (Figure 2) indicates that samples AS11, AS12, and AS13, corresponding to the 1991 SW monsoon, exhibited a close similarity (ca. 60%) and formed a different group

FIGURE 2. Dendogram showing the results of the hierarchical cluster analysis (HCA). Sample codes are identified in Table 1. from the rest of samples. Another cluster was formed by samples AS7 and AS8 (ca. 70% similarity), corresponding to a north air mass event in June 1991. These two samples exhibited lower similarity with the remaining samples (ca. 65%) which formed a very coherent cluster (>80% similarity). It is noteworthy that the 1988 SW monsoon samples are grouped together with the samples corresponding to interand NE 1991 monsoon and not with the 1991 SW monsoon. The different grouping behavior between both SW monsoons (1989 and 1991) will be discussed in terms of source compositions and physicochemical processes taking place in the surface water. Sample Characterization and Biogenic Processes. Vertical Fluxes of Bulk Parameters. The downward flux of total particles in the Arabian Sea exhibited a strong seasonality characterized by maxima at the monsoon periods (119-169 mg m-2 d-1 during the NE monsoon and 164-351 mg m-2 d-1 during the SW monsoon) followed by minima at the intermonsoons (50-94 mg m-2 d-1) (Table 1). Organic carbon followed a similar seasonal trend reflecting the biogenic nature of the particles that contribute to enhance the vertical fluxes during the monsoon periods (10, 11). These seasonal cycles are driven by atmospheric forcing characterized by high wind speeds and relatively cooler water temperatures during the NE and SW monsoons, particularly remarkable during the latter (12, 23). High wind speeds during the SW monsoon result in a more intense upwelling, enhancing particle fluxes into the deep sea which are among the highest recorded worldwide (11, 12). On the other hand, the different ocean-circulation patterns during the different seasons of the year also affect the biogeochemical processes in the area. VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Individual distribution of (A) n-alkanes and (B) sterols of samples AS4, AS7, and AS11. n-Alkanes are labeled according to carbon number. The identification of sterols is as following: (S1) cholesta-5,22(E)-dien-3β-ol, (S2) 5r(H)-cholest-22(E)-en-3β-ol, (S3) cholest-5-en-3β-ol, (S4) 5r(H)-cholestan-3β-ol, (S5) 24-methylcholesta-5,22-dien-3β-ol, (S6) 24-methyl-5r(H)-cholest-22(E)-en-3βol, (S7) cholest-en-3-one, (S8) 24-methylcholest-5-en-3β-ol, (S9) n-C28ol, (S10) 24-ethylcholesta-5,22(E)-dien-3β-ol, (S11) 24-ethylcholest5-en-3β-ol, and (S12) 4r,23,24-trimethyl-5r(H)-cholest-22(E)-en-3βol. Aliphatic Hydrocarbons. n-Alkane concentrations were 1 order of magnitude higher during the 1991 SW monsoon than during the other seasons (Table 1). These quantitative differences were accompanied by significant differences in the n-alkane profiles (Figure 3A), as also discerned by HCA. Thus, during the NE monsoon they exhibited maxima at n-C17 or n-C18, whereas during the 1991 SW monsoon a monomodal distribution peaking at n-C25 or n-C26 dominated the profile. The intermonsoon samples exhibited a composite of the distribution patterns found in the NE and SW monsoon samples. The n-C17 is of planktonic origin, while n-C16 and n-C18 may derive from either bacterial reworking of the organic matter in the water column or inputs of cyanobacterial lipids (24). A moderate odd-to-even CPI (1.1-2.2, see Table 1) is observed in the n-C23-n-C36 range during the NE monsoon, indicating allochthonous inputs from terrigenous sources related to deposition of long-range transported aerosols. However, the 1991 SW monsoon n-alkane distributions, with CPI close to unity, are indicative of a predominant fossil source. Overlapping with petrogenic inputs, the high pristane-to-phytane ratios found during the SW monsoons (i.e. 8-27.8) indicate the importance of zooplanktonic grazing (Figure 4B). Similar patterns have been reported in the Black Sea (25) and from oil spills after the Gulf War (26). In contrast, the pristane/phytane ratio of 2.5 during NE monsoon is consistent with a lower input of planktonic sources. The contribution of n-alkanes during the SW monsoon (samples 10-13) accounted for 96% of the total 1991 n-alkane flux (Figure 4A), clearly showing the strong relationship of the monsoon episodes and their associated biogenic fluxes, with the seasonal variability of the aliphatic hydrocarbon 3952

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FIGURE 4. Time series of (A) n-alkanes and UCM fluxes and (B) sterol fluxes and pristane/phytane ratios. Code samples are indicated in Table 1. concentrations and fluxes. An unusually abundant source of n-alkanes between June and September 1991 should account for these figures. An enhanced input of petrogenic hydrocarbons is supported by the co-occurrence of a similar temporal trend in the UCM of aliphatic hydrocarbons. Indeed, UCM fluxes during the 1991 SW monsoons (462-689 µg m-2 d-1) were much higher than those found from May to June 1991 (23-54 µg m-2 d-1) and earlier in 1988 (9-20 µg m-2 d-1). Although it has been suggested that the oil spills and burning, in early 1991, during the Gulf War, could have had some influence on soot deposition in the region (27, 28), it has also been recognized that the fast degradation of the oil spills strongly reduced the environmental effects of the War (29, 30). Therefore, an accidental oil spill cannot be ruled out, taking into account the particular n-alkane distribution found in these samples, but the relative importance of this source is difficult to discern in the present case due to the lack of a consistent weathering index for estimating either the distance from the source or the degradation of the aliphatic fraction. Sterols. The sterols, as autochthonous constituents of the settling particles, exhibit a more uniform distribution of concentrations than any other organic marker (Table 1). However, some differences in the individual contributions can be noticed (Figure 3B). During the NE and SW monsoons, sterols from phytoplanktonic origin predominated, such as cholesta-5,22(E)-dien-3β-ol (diatoms), 24-methylcholesta5,22-dien-3β-ol (diatoms and pelagic coccolithophorids), 4R,23,24-trimethyl-5R-cholest-22-en-3β-ol (dinoflagellates), 24-ethlylcholesta-5,22(E)-dien-3β-ol, and 24-ethylcholest-5en-3β-ol (unicellular algae) (31, 32). On the other hand, the intermonsoon samples exhibited an important contribution of cholesterol (64% of total sterols in sample 7) (Figure 3B), suggesting a major role of zooplankton packaging of organics in fecal pellets. It is well-known that fecal pellets are an efficient carrier in removing pollutants from surface waters

FIGURE 5. Representative distributions of (A) PCB congeners of samples AS4, AS7, and AS10, and (B) PAHs of samples AS4, AS7, and AS12. PCBs are identified by their IUPAC nos. and PAHs as follows: Ph, phenanthrene; MPh, methylphenanthrenes; DMPh, dimethylphenanthrenes; Ant, anthracene; Fl, fluroanthene; Py, pyrene; BaA, benz[a]anthracene; Cr, chrysene; BFl, benzofluoranthenes; BeP, benzo[e]pyrene; BaP, benzo[a]pyrene; Per, perylene; DB(ah)A, dibenz[ah]anthracene; B(ghi)P, benzo[ghi]perylene; and InP, indeno[1,2,3cd]pyrene. (7, 8). Conversely, during the high productivity seasons (SW and NE monsoons), bacteria may play a more important role than zooplankton as grazers (33). The temporal variability of the sterol fluxes followed a similar trend than those found for organic carbon and total particles (Figure 4B). The maximum flux occurred during the monsoon periods, particularly during the SW monsoon, in agreement with the contribution of the upwelling to the downward particle flux. Polychlorinated Biphenyls (PCBs). The complex mixture of PCBs has been examined on the basis of the most abundant congeners, including those six considered as target compounds in marine studies (34). The overall concentrations are given in Table 1 and those of individual compounds in representative samples in Figure 5A. Despite the variability of the observed profiles, they are rather similar to those exhibited by different marine settling and suspended particles, such as in the Mediterranean (7, 35), the Baltic (37), and the NW Atlantic (37). The predominance of tri- to pentachlorinated species (IUPC nos. 28-118) in these samples is consistent with long-range transported inputs and subsequent diffusive air-water exchange (13, 14). In fact, a chlorinated dependent fractionation process occurs during seaward transport, so that a depletion of the higher chlorinated congeners in both, marine suspended particles and sediments, with increasing distance from the continental source is usually observed (38, 39). Time series of PCB fluxes are depicted in Figure 6A. Values of 6-15 ng m-2 d-1 for the whole series of congeners analyzed (0.5-1.5 and 0.4-0.9 ng m-2 d-1 for congeners 52 and 128, respectively) were found during the NE monsoon, whereas during the SW monsoons they were below 5 ng m-2 d-1. Although it is difficult to compare PCB fluxes on a basis of

FIGURE 6. Time series of (A) PCBs and DDTs (4,4′-DDT+4,4′-DDE) fluxes and (B) total PAH fluxes and fosil to pyrolytic PAH ratios.

global values that include different components, it appears that, based on data for congener 52, they are comparable to those reported in other pelagic regions surrounded by industrialized countries (0.23 ng m-2 d-1 in the Sargasso Sea at a depth of 3200 m; 0.4-0.8 in the Alboran Sea at 250-270 m water depth; and 0.6 in the NW Atlantic at the surface) (7, 38, 40). The coherence of these data may indicate baseline values for the open seawaters and contribute to improve our budget estimates. The values reported for coastal waters are consistently much higher (6-11 ng m-2 d-1 for congener 52) (38, 41) and, in addition, more time dependent, so more difficult to compare. Higher vertical fluxes of PCBs in samples corresponding to high productivity conditions in the surficial waters (SW monsoon) would be expected due to an enhanced uptake of organic compounds by phytoplankton supported by airwater exchange (15, 16). However, moderate fluxes were found only during the early stages of the SW monsoon (samples 9 and 10). This would suggest an efficient removal of PCBs from the surficial waters during the beginning of the phytoplankton bloom, followed by a depletion in the water column or, most probably, a lack of PCB inputs when the wind was blowing from the SW direction. Lower fluxes during the SW monsoon may well reflect less intensive usage or remobilization of PCB from the African desert region. On the other hand, the higher concentrations and fluxes observed during the NE monsoon are consistent with a scenario in which PCBs are atmospherically transported from Pakistan and India, followed by a diffusive air-water exchange flux. The predominance of low chlorinated congeners in the samples, which exist to a large extent in the gas phase and, therefore, may be transported to longer distances, supports this hypothesis. This process is modeled by the following equation (13)

FA-W ) KOL(CW - CA/H′) VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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where FA-W is the diffusive air-water exchange flux (ng m-2 d-1), CA (ng m-3) and CW (ng m-3) are the concentrations of the chemical in the gas phase and in the water (dissolved phase), respectively; H′ is the dimensionless Henry’s law constant corrected by temperature; and Kol (m s-1) is the air-water mass transfer rate coefficient, which is compound dependent and both wind speed and temperature have an important influence on its value (13). Even though in the present study we do not have the actual values of CA and CW, we can use eq 1 in a qualitative basis in order to explain the variability of the air-water flux which depends on wind speed and temperature through H′ and Kol. Furthermore, air-water flux may be related to sinking flux in the sense that consistent estimates reported elsewhere indicate a net absorption of organochlorine compounds in the Arabian Sea (42). Indeed, relatively high wind speeds during the NE monsoon (12) can explain an enhanced input of PCBs during this season. This would increase the bioavailable dissolved phase concentration leading to higher concentrations in phytoplankton and zooplankton, which contribute significantly to the production of sinking particles (8). The importance of phytoplankton in linking the cycle of PCBs with that of the organic carbon is supported by the concurrent variability of organic carbon and PCB fluxes during NE and intermonsoon periods (r 2 ) 0.88, p < 0.01). However, this is basically due to the predominance of the tri- to pentachlorinated congeners in the samples, because the phytoplankton uptake is, on one hand, much faster for these than for the higher chlorinated congeners, and, on the other, air-water exchange is the limiting step for the latter (16, 43). In fact, it has been recently demonstrated that the current partitioning models between water and planktonic organisms do not explain the accumulation of highly chlorinated PCBs, particularly under bloom conditions (44). Besides this seasonal pattern, a significant increase of the concentration and, subsequently, of the PCB flux was found in sample AS8 (June 1991), which could be attributed to an incidental input of PCBs and other pollutants (see PAHs below). The northern winds occurring during that season may be responsible for such strong short-term input. Moreover, the importance of zooplanktonic fecal pellets suggested by the predominance of cholesterol in the sterol fraction may have increased the concentrations and export efficiency of organic carbon and associated PCBs (8, 45). Organochlorinated Pesticides (OC). 4,4′-DDT and its main metabolite, 4,4′-DDE were identified throughout the sampling period. Usually, the parent compound predominated, and the concentrations were slightly higher during the NE monsoon (Table 1), reflecting the input of DDT recently produced and used in low latitude countries such as India (46). Hexachlorobenzene and R- and γ-hexaclorocyclohexanes were also occasionally found, at concentrations below 1 ng/g, particularly during the NE monsoon. The range of DDT concentrations and fluxes found in the Arabian Sea (0.2-1.8 ng m-2 d-1) are comparable to those reported recently (7) or even much earlier (48) in the western Mediterranean Sea (1-2 ng m-2 d-1). Furthermore, the temporal trends of 4,4′-DDT and 4,4′-DDE fluxes were similar to those found for PCBs (Figure 6A), with a seasonal pattern characterized by a maximum during the NE monsoon, and a secondary maximum during the initial stages (June to July) of the SW monsoon. These trends are consistent with different patterns of usage of these pesticides in the region and reflect the association of monsoon air masses with the downward fluxes of POPs along the water column. Polycyclic Aromatic Hydrocarbons (PAHs). PAH distributions were generally dominated by the low-to-medium molecular weight PAHs (Figure 5B), namely phenanthrene (Ph) and its methyl and dimethyl derivatives, fluoranthene (Fl) and pyrene (Py). These distributions are similar to those 3954

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found in traps deployed in other open sea areas such as the western Mediterranean Sea (7, 48), although the relative abundance of the dominant PAHs in these deep water samples is lower than those usually found in surficial or intermediate suspended particles. The ratio between the three and four ring aromatics (phenanthrene vs chrysene) is in the range of 3-6 in the present samples and higher than 10 in the pelagic Mediterranean samples (7, 48). This is probably the result of the degradation and/or recycling of the more soluble PAHs during their transport down the water column (35, 48). Beyond this general feature, the individual composition exhibits slight differences among samples that can be associated to the contribution of different PAH sources. In this respect, the fluoranthene/pyrene, benz[a]anthracene/ chrysene, and indeno[1,2,3-cd]pyrene/benzo[ghi]perylene ratios indicate the signature of a combustion source in the NE and intermonsoon samples. In addition, the ratio of alkylphenantherenes to phenantherene increases from the NE (0.7) to the SW (1.9) monsoon samples, suggesting a higher contribution of fossil PAHs during the latter event, consistently with the relatively higher concentration of the UCM of aliphatic hydrocarbons during this period. Figure 6B shows the increase of the ratio of fossil to pyrolytic PAHs during the SW monsoons (1.5-2.7) with respect to the NE monsoon (0.3-1.1). The particulate PAH concentrations given in Table 1 are higher during the NE and intermediate monsoon periods. Concurrently, the PAH fluxes exhibited maximum values at the monsoon seasons, generally higher during the NE than during the SW events, when the contribution of pyrolytic PAHs is apparently more evident (Figure 6B). Total PAH fluxes also followed the temporal variability of the total mass and organic carbon fluxes (r 2 ) 0.62, p < 0.01). These observations can be explained by taking into account the different air masses driving different types and concentrations of PAHs as well as the processes occurring in surface waters. Temperature is one of the variables with an important influence on air-water exchange, because it modifies Henry’s law constant that affects equilibrium concentrations between air and water and the mass transfer coefficient Kol appearing in eq 1. Wind speed is another variable with an important effect on air-water exchange of semivolatile organic compounds. High wind speeds will increase Kol in a nonlinear way (13, 14). Water temperature during the 1991 NE monsoon was almost 6 °C above that of during the SW. On the other hand, wind speed is usually higher during SW than during NE monsoons, the 1991 SW monsoon being an event with particularly high values (14 m s-1) (12). Higher wind speeds and lower temperatures during the SW monsoon may induce an increase in the PAH fluxes in 1991 due to an enhanced net absorption of these compounds through the air-water interface. Furthermore, the fact that PAH distributions during the 1991 SW monsoon exhibited higher contributions of pyrolytic PAHs than during the 1988 SW monsoon may be explained by a more efficient transport and deposition of aerosol associated components from land based sources during the former. In addition to the air-water exchange and biogeochemical processes taking place in the surficial waters, air masses enriched in PAHs could also account for the higher fluxes observed during the NE 1991 monsoon. A possible candidate for such a source of PAHs is the “Gulf War” of early 1991, in agreement with the higher contributions of pyrolytic PAHs in these samples. Model simulations have suggested a slightly enhanced deposition of soot particles in the region, at distances of 2000 km (26, 27). However, the influence of the Gulf War on PAH occurrence, if it existed, was small as has been observed along the Kuwait and Saudi Arabian coasts

and in the Northern Arabian Sea, a few months after the war (29, 30). Moreover, the relatively fast photodegradation of the atmospheric PAHs together with further degradation during their transport down the water column could also have contributed to dilute the Gulf War signal. In any case, the relative importance of this source for PAH fluxes cannot be decided from the available data. The high flux corresponding to sample AS8 (June 1991), similar to that found for PCBs, may be related to time limited inputs of PAHs associated to northern winds with an efficient vertical flux driven by zooplankton fecal material. The comparison of these PAH fluxes with those of other marine regions is difficult by the limited information available, and the lack of coherence of the series of compounds analyzed. Therefore, an accurate assessment can only be carried out on the basis of specific components. In this respect, the range of fluxes found in the Arabian Sea (9-67 ng m-2 d-1), slightly lower to those found in mid and high latitude open seawaters (7, 48, 49), includes a mean of 5.6, 4.3, 1.5, and 0.7 ng m-2 d-1 of, respectively, phenantherene, pyrene, chrysene, and benzo[a]pyrene. In the Alboran Sea, the following values have been reported at depths of 250-750 m, 43, 10, 6, and 2 ng m-2 d-1 (7). Fluxes of 10 and 0.2 ng m-2 d-1 of pyrene and benzo[a]pyrene, respectively, have been quoted for surficial waters of the NW Atlantic (49). The geographical site location and the water depth may account for these differences. Obviously, values several orders of magnitude higher are found in coastal waters (49-51). The Role of Monsoons on the Global Cycle of Organic Pollutants. The dependence of vertical fluxes of POPs on monsoon dynamics has been demonstrated for the first time. The higher pollutant fluxes due to high wind periods during the monsoons and with an enhanced vertical flux driven by an increase of primary productivity during the SW monsoon suggest that monsoons may have a major influence on the transport and fate of organic pollutants in the Indian Ocean. Particulate organic fluxes in the Arabian Sea show a rapid attenuation with depth due to dissolution and biological remineralization. The highest rates of flux decrease occur on top and bottom boundaries of the water column, decreasing then slightly between 300 and 3000 m depth (46). As only 4-8% of the carbon fixed by primary producers is exported below the photic zone to the deep waters, POP fluxes may also decrease by 1 order of magnitude between surface waters and the 3000 m water depth. Therefore, the estimates reported above for POP annual vertical fluxes may represent a lower limit of the actual sequestration of POPs from surface waters. In addition, if we consider all the Indian Ocean under the influence of monsoons and not only the Arabian Sea, the potential of monsoon driven vertical fluxes as a major sink of POPs is even larger. The actual extent of this sequestration process from surface waters cannot be accurately determined with the existing information. However, based on the limited data set reported here, a preliminary estimate of POPs sinking in the Arabian Sea can be intended. Assuming a surface area of 5 × 106 km2 and the average concentrations of pollutants found during the SW, NE, and intermonsoon periods, at 3000 m below the surface (Table 1), the following estimates of the total annual inputs for the whole basin have been obtained (Table 2): 193 500 tons of aliphatic hydrocarbons (UCM), 61 of PAHs (15 components), 14 of PCB (13 congeners), and 1.1 of DDTs (4,4′-isomers). It is interesting to notice that about 50% of the PAH, PCB, and DDT inputs correspond to the NE monsoon period, whereas during the SW monsoon the inputs of UCM of aliphatic hydrocarbons were much more significant. To compare the budget of these pollutants in the Arabian Sea with other oceanic regions, independently of the size of the basins, the values for a number of individual species

TABLE 2. Estimation of Vertical Deposition (ton) of UCM of Aliphatic Hydrocarbons, PAHs, PCBs, and DDTs (DDT + DDE) in the Arabian Sea, during the NE, SW, and Intermonsoon (IM) Seasons av fluxes (ng/m2 d) monsoon period UCM PAHs (Σ15) phenanthrene fluoranthene pyrene b[a]anthracene chrysene b.fluoranthenes b[a]pyrene b[ghi]perylene PCBs (Σ13) 28 52 101 128 180 DDTs

NE 33 000 50.6 9.3 7.4 7.4 1.3 2.3 2.1 1.1 1.35 12.4 1.5 1.5 1.3 0.6 0.8 0.9

IM 24 000 20.3 3.6 2.3 2.4 0.3 0.8 0.9 0.6 1.3 6.7 0.5 0.4 0.8 0.2 0.4 0.3

SW 320 000 37.3 8.3 5.8 4.4 0.4 1.5 0.9 0.7 0.2 4.3 0.8 0.2 0.4 0.1 0.1 0.6

vertical seasonal deposition (ton) NE 18 500 28 5.1 4.1 4.1 0.7 1.3 1.2 0.6 0.7 6.8 0.8 0.8 0.7 0.3 0.4 0.5

IM 18 000 15 2.6 1.7 1.8 0.2 0.6 0.7 0.4 1 4.9 0.4 0.3 0.6 0.2 0.3 0.2

SW 157 000 18 4.1 2.8 2.2 0.2 0.7 0.5 0.4 0.1 2.1 0.4 0.1 0.2 0.04 0.1 0.3

have been considered. As an example, annual inputs of 1.2 and 0.5 tons of PCB congeners 52 and 128, respectively, and 8.6 and 1.4 tons of pyrene and benzo[a]pyrene, respectively, have been estimated for the Arabian Sea (5 × 106 km2). In the open NW Atlantic (1.3 × 107 km2) the depositions, at the surface, are of 2.6 and 0.3 tons per year of PCB congeners 52 and 128, respectively (38), and 36 and 0.8 tons per year of pyrene and benzo[a]pyrene (49). Finally, in the Alboran Sea (SW Mediterranean) (5.4 × 104 km2) the inputs at mid depth are of 1.2 and 0.9 tons per year of PCB congeners 52 and 128 and of 2.2 and 0.5 tons of pyrene and benzo[a]pyrene, respectively (7). The Indian Ocean accounts for an important fraction of the world oceans, thus the enhanced sedimentation during monsoons may remove a significant and important fraction of the organic pollutants released at low latitudes, therefore not being available for global distillation. The present study points out the need for an extensive study of the sources of POPs and biogeochemical processes affecting their transport in the Indian Ocean and its watershed. This research would allow a better understanding of the global cycle of POPs and would help to assess the impact of POPs in this highly populated region of the world.

Acknowledgments J.D. acknowledges predoctoral and postdoctoral fellowships from the Spanish Research Council and the Spanish Ministry of Education and Culture, respectively. Financial support was obtained from the CICYT (AMB96-0926). The comments of two anonymous reviewers are greatly appreciated.

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Received for review February 19, 1999. Revised manuscript received August 5, 1999. Accepted August 10, 1999. ES990200E