Petroleum hydrocarbons in the Mediterranean Sea: A mass balance

Marine Chemistry, 20 (1986) 141-157

141

Elsevier Science Pubhshers B.V, Amsterdam - - Printed m The Netherlands

P E T R O L E U M HYDROCARBONS IN THE M E D I T E R R A N E A N SEA: A MASS B A L A N C E

KATHRYN A. BURNS

Bermuda Bmlogwal Statmn, 1-15 Ferry Reach (Bermuda) ALAIN SALIOT

Laboratoire de Physique et Ch~m~e Marines, UA CNRS 353, Univers~td Pierre et Mane Curie, 4 Place Juss~eu, 75230 Paris Cedex 05 (France) (Received J a n u a r y 7, 1986, revmlon accepted June 2, 1986)

ABSTRACT Burns, K.A. and Sahot, A., 1986. Petroleum hydrocarbons m the Mediterranean Sea: a mass balance. Mar. Chem., 20: 141-157. Over three quarters of a mllhon tonnes of oil were estimated to be introduced annually into the Mechterranean Sea from land-based and open-sea discharges. This paper is a critical assessment of data available through 1983 on the distribution of petroleum-derived hydrocarbon remdues and the blogeochemmal processes controlling the transport and fate of orgamc contaminants m thin regional sea ecosystem. Inputs, outputs and ecosystem partitioning or inventories are computed and a complete mass balance model is proposed. The approach raises several lmphcations with respect to strategies for the samphng and analysis of orgamc contaminants in ocean ecosystems. The report also provides a basis on which to evaluate the effectiveness of recent discharge regulations m reducing polluhon loads m the Mediterranean. The agreement between calculated fluxes, inventories and input time scales demonstrates the usefulness of orgamc contaminants as markers for the development of global and ocean flux models. INTRODUCTION

The Mediterranean is a useful model for regional sea calculations of fluxes and mass balances of both natural and anthropogenically derived materials because of a large available data base and because of unique hydrographic features t h a t simplify the parameters t h a t must be considered. It is an enclosed sea with deep basins and the large-scale layered circulation of an ocean system; yet is has negligible tidal movements and local circulation patterns are predominantly wind-driven (Hopkins, 1978). The evaporation rate is high so that surface circulation results in a net influx of surface waters from the North Atlantic and the Black Se~. Bethoux (1980) computed a salt balance for the Mediterranean. The mass balance approach is useful for estimating and confirming the rates of important biogeochemical process which control the transport, distribution and fate of constituents in marine ecosystems. Like the artificialradionuclides, organic contaminants are useful markers since they have been introduced in

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142 the global environment over a brief and known time scale. Burns and Villeneuve (1986) computed a partial budget for the polychlorinated biphenyls (PCBs) in the open Mediterranean. Petroleum hydrocarbons (PHCs) as a group of organic contaminants are an extremely complex assemblage of chemicals from varied sources. Processes such as equilibrium partitioning of different individual components between dissolved and particulate phases in seawater, photooxidation and biological metabolism modify hydrocarbons discharged to the sea making analytical quantification and data interpretation difficult. Three major groups of pollutant hydrocarbons for which discussion is possible are (a) 'volatiles', the low molecular weight hydrocarbons excluding methane, (b) 'non-volatile' petroleum-derived hydrocarbons usually defined by gas chromatographic analytical techniques as eluting in the dodecane (C12) to the pentriacontane (C35) and greater n-alkane range, and (c) the polynuclear aromatic hydrocarbons (PAH) from combustion products. Recently biosynthesized or 'biogenic' hydrocarbons are present in all marine systems and these are usually distinguishable from petroleum-derived products when analytical methods are of high enough resolution to quantify individual hydrocarbons (Wakeham and Farrington, 1980; Saliot, 1981). The biogenics are excluded from this discussion of contaminant hydrocarbons. To calculate the PHC budget we critically evaluated the large and diverse data base published from traditional inventory-type monitoring efforts in the Mediterranean and combined this with results of process-oriented studies in other seas and enclosed ecosystem experiments and with our own recently published efforts to estimate atmospheric inputs and the rates of sedimentation and in situ degradation of PHC at coastal and deep-sea stations in the western basin. After tabulating inputs, outputs and ecosystem partitioning or inventories we further evaluate the data base and recommend strategies for more effective monitoring of organic contaminants in ocean systems with the aim of contributing to global ocean-flux models. INPUTS A review article by Le Lourd (1977) estimated that total petroleum input from all sources into the Mediterranean averaged between 0.5 x 106 and 1.0 × 108 tonnes per year (T/y) with half discharged from the coast and half in the open sea. Le Lourd's estimate was based on local tanker practices and probably remained a reasonable range through 1985. He stressed that chronic pollution is more important than accidents, and he assumed that operational and land-based discharges from tanker operations were equal. Tanker practices remained similar up to 1985 with tank and ballast water discharges permitted is specific areas. Recent international agreements (IMCO, 1981) may be acting to reduce this source of spilled oil, but increased industrialization and urbanization in bordering countries is likely to increase inputs from other sources. Le Lourd's total estimated input range was refined as follows. Land-based industrial discharges of mineral oils were summarized by Hel-

143 mer (1977). Per capita discharges from urban and rural populations were calculated by using the estimates of Eganhouse and Kaplan (1981) of 1014 g/y per person for urban populations and 398 g/y per person from rural populations as river discharges. These per capita rates are in reasonable agreement with estimates published by NRC (1975) and with recent measurements in other urbanized estuaries in North America (Barrick, 1982; Hoffman et al., 1983, 1984) and France (Marchand, 1985; Tronczynski, 1985). Applying these rates to the urban and rural population estimates tabulated by Henry (1977), with the exclusion of portions bordering other seas, gives the figures shown in Table I. The values are minimum estimates due to population growth since 1977 and changing land usage. To estimate PHC inputs from the atmosphere, we assumed t h a t the sea is a sink for combustion-derived (pyrogenic) PAHs but is a source of volatiles from spilled oil to the atmosphere. Other PHCs would be delivered to the sea surface from air masses traversing industrial zones. Prahl and Carpenter (1979) demonstrated by a mass balance calculation t h a t the combusion products are extremely stable in coastal ecosystems and t h a t inputs are balanced by sedimentation rates. Burns and Villeneuve (1983) estimated the deposition rate of PAH from a sediment core taken 2 km off the coast of Monaco as 0.69 mg/m2/y. H5 (1982) and Saliot et al. (1985) reported deep-water vertical fluxes of PAH based on particles collected in nets and a particle flux model as 0.76 mg/m2/y at a mid-basin station off Corsica and 1.19 mg/m2 /y at a station near the Gulf of Lion. The agreement in these flux estimates calculated by two different methods is very encouraging. Applying these PAH fluxes to the total area of the eastern and western basins (2.5 × 1012m 2, Sverdrup et al., 1942) would indicate an input range of 1.7 × 103 to 3.0 × 103 T/y of PAH combustion products. Estimating the net flux of total PHC from the atmosphere into the sea is complicated by recycling processes at the air/sea interface. H5 et al. (1983) and Saliot and Marty (1986) estimated t h a t the flux of total hydrocarbons based on atmospheric samples from two shipboard transects in the western basin ranged from 2.6 to 26mg/m2/y and 1.3 to 13mg/m2/y. Assuming all n-alkanes were TABLE I Inputs a (T/y)

Spilled oil from tankers: deballasting and loading operations, bilge and tank washings Land-based mdustrml discharges Per capita discharges: Urban Rural Atmospheric deposition Annual total (1970s to early 1980s)

600 x 103 120 × 103 101 x 103 51 x 103 Ii × 103 883 × 103

a Estimated inputs of petroleum hydrocarbons to the Mediterranean Sea. See text for source materials.

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biogenic yields a conservative estimate for PHC. The average of the mid-points of these transects applied to the total area yields a flux of 11 x 103T/y (Table I). This value is an average of a very wide range from data generated over a limited time span. Total atmospheric inputs will only be confirmed by further analysis of wet and dry deposition at coastal and open-sea stations over an expanded period of time. Total calculated inputs (Table I) fall within the range estimated by Le Lourd (1977). Inputs from adjacent seas were assumed to be negligible compared with petroleum discharges within the Mediterranean. Updated estimates of PHC inputs to the world's oceans were published by NRC (1985). Compared with these figures the Mediterranean receives between 10 and 34% of the estimated range for the world total or 24% of the NAS 'best reasonable estimate'. In the Mediterranean, input patterns are different from global patterns. For example, the ratio of atmospheric to total inputs is 0.08 global and only 0.01 for the Mediterranean; while the ratios of transportation to total inputs are 0.42 and 0.68, respectively. Since transportation is the single most important input, continued surveillance will be required to determine the impact of discharge restrictions on the PHC budget of the Mediterranean. For the discussion of outputs we assume that all of the PHC inputs are to surface waters. OUTPUTS

Volatilization Although there is variation in the percentage of oil evaporated from a spill related to its chemical composition and wind conditions (Stiver and MacKay, 1984), a general estimate of 30% volatilization followed by subsequent photochemical and biological oxidation is suggested by studies of oil slick dispersion (NRC, 1975; 1985) and by laboratory weathering experiments (Atlas and Bartha, 1972). Studies of the behavior of volatile hydrocarbons in seawater have shown that sea/air exchange is the removal process of greatest importance compared with other possible mechanisms, such as particle adsorption and sedimentation (Gschwend et al., 1982). The result of volatilization is to remove a large portion of the acutely toxic low molecular weight aromatic hydrocarbons from the sea surface, leaving a highly modified residue of non-volatiles. Brown and Huffman (1979), Saliot et al. (1985) and Burns and Villeneuve (1986) remark on the low percentage of low molecular weight aromatic hydrocarbons in the PHCs measured in surface waters of the Mediterranean.

Tar formation From the residual fractions followed by stranding on shore, tar formation was estimated by Girotti (1968) and cited in NRC (1975) to occur at approximately 30% of spilled oil in the Mediterranean. This is higher than expected in open-ocean conditions because of the increased probability of suspended tar

145 contacting shores in an enclosed sea. Degradation and other removal processes appear to procede fast enough that long-term build up of tar on beaches is not reported; only the continuous supply and associated nuisance. Beach tar data have been compiled under the auspices of marine pollution monitoring programs coordinated by United Nations Agencies (IOC/UNESCO/UNEP). Sedimentation

Sedimentation of organic residues occurs by their adsorption onto particles suspended in seawater and by their incorporation into large fast-settling particles such as animal feces and marine snow. These processes have been discussed by numerous authors to explain the vertical transport of materials through ocean systems. We calculated the sedimentation rate of PHC in the Mediterranean from several independent data sets. From 27 sediment trap samples collected between May 1980 and February 1982 at a station 2 km off the Monaco coast, Burns et al. (1985) reported t h a t PHC fluxes ranged from 8 to 233 mg/m2/y and averaged 90 + 65 mg/m2/y. The rates of settling varied by an order of magnitude throughout the year and roughly followed the cycles of productivity in surface waters. This rate for PHC should be applicable to coastal waters outside the immediate influence of point sources. Saliot et al. (1985) estimated the sedimentation rate of hydrocarbons at four stations in the western basin by analyzing particles greater than 63 ~m collected in nets set below the photic zone and by applying the particle settling model of McCave (1975). The flux estimates for total hydrocarbons minus n-alkanes for samples collected in 1981 ranged from 3 to 77mg/m2/y and averaged 35 + 33 mg/m2/y. Most values were within the range measured by the sediment trap work. Burns and Villeneuve (1986) used a third tactic to estimate the deposition rate of organic contaminants in the open Mediterranean. Since the sedimentation rate was reported to be approximately 15 cm/1000 years in the deep basins (Ryan et al., 1970), all organic contaminants should be in the top few mm of surface sediments if no disturbance has altered their distribution. A core collected from the western basin under 2900 m of water was sectioned into 1 cm thick slices, and the surface particles t h a t had been resuspended during the coring operation were carefully siphoned off for separate analysis. The core was 21 cm in diameter and the sediment surface area sampled was thus 346 cm 2. The water overlying the core contained a total of 6.3 g of dry sediment. By assuming a sediment density of 2.6 g/cm 3, a water content of 55% and the sedimentation rate of 0.15 mm/y, it can be calculated t h a t approximately 0.2 mm or 1.4 years worth of sediment had been resuspended. The resuspended flocculent particles had a concentration of hydrocarbons (total minus n-alkanes) of l167#g/g, which would indicate an average sedimentation rate of 152mgPHC/m2/y. This value again lies within the range measured in the sediment trap work. If the predominant mechanism of rapid sedimentation is in zooplankton

146 feces, then sedimentation rates based on excretion and feeding models provide further independent assessments. Burns et al. (1985) estimated the vertical flux of PHC by analysis of freshly collected zooplankton feces as 82 + 10mg/ m2/y (n = 5). For comparison with another ocean area we cite Sleeter and Butler (1982) who estimated an annual yearly flux of 23mg oil/m2/y in the Sargasso Sea using similar methods. Conover (1971) observed the rapid settling ofoil from a spill of Bunker C fuel by zooplankton feeding and egestion. Under spill conditions 0.7% of zooplankton food was oil. Generalization of Conover's measured defecation rates over a year would indicate a maximum sedimentation due to plankton feeding of 360 mg/m2/y in this coastal ecosystem. We estimated an average zooplankton biomass in the Mediterranean of 1 g/m 2 based on measurements reported by Razouls and Thiriot (1973), Franqueville (1975) and Nival et al. (1975). Generalization of this average to the area for both basins is justified by the work of Stirn (1973), showing similar standing stocks of zooplankton for a series of samples collected from a single cruise through both basins. Parsons et al. (1966) showed that zooplankton can consume greater than 15% of their own organic weight in particulate matter per day. Childress and Nyaard (1974) reported that the organic carbon (OC) content of zooplankton averages 40% of dry weight. Thus, based on the average zooplankton biomass in the Mediterranean, we calculated the maximum sedimentation rate of PHC to be 153 mg/ m2/y. For the mass balance we used the long-term average measured by the sediment trap experiment of 90mg/m2/y and estimated that approximately 225 × 103T/y PHC or 25% of the total yearly input is removed from surface waters and delivered to the surface sediments on rapidly settling, large particles. Desorption of hydrocarbons from slowly settling particles as they sink through the water column transports residues to deep waters. As the residues move through the ecosystem they are continually modified by biological and chemical oxidation processes. Obviously the accumulation of contaminants in the system depends on the relative rates of transport and degradation.

Degradation rates From the three-year sediment trap experiment, Burns et al. (1985) showed that approximately ten times more PHC fell through the water column than was eventually incorporated into sediments. This quantitative discrepancy as well as the observed changes in composition of the PHC residues between surface and deep samples (Burns and Villeneuve, 1983) implied a rapid rate of oxidation of most of the PHC at the sea/sediment interface. Applying the diagenesis model of Suess (1980) and Reimers and Suess (1983), Burns (1986) calculated that 86% of the PHC flux to the sediments is consumed at the interface and another 13% is consumed by diagenesis processes within the sediments. Thus, in coastal areas with relatively slow burial rates, in situ oxidation removes as much as 99% of the flux of PHC to the sediments leaving a highly modified and stable residue for burial into the sediments. The yearly

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in situ degradation rate can be equated with the amount consumed preburial or approximately 83mg/m2/y. Burns (1986) compared the levels of organic contaminants in resuspended flocculent particles from the sea/sediment interface with the compacted sediments in a core from 1087 m depth in the Gulf of Lion. Using the more refractory PCBs for comparison in a semi-quantitative model to calculate sediment dilution by deeper uncontaminated layers, it was estimated t h a t 99% of the PHC residues transported to depth were oxidized before compaction into the sediments. The concentration and type of PHCs and the presence of biogenic hydrocarbons typical of algal inputs indicated that the flocculent particles had recently been delivered to the sediment interface from surface waters. The PHC composition was similar to that seen in fresh zooplankton feces and in sediment trap material collected at the Monaco trap station. The sediments, in contrast, contained higher boiling, exceedingly degraded petroleum residues. Chromatograms were published in Burns (1986). These observations were repeated in the core collected from 2900 m depth at the mid-basin station (Burns and Villeneuve, 1986). Using the calculated sedimentation rate of PHC as the flux and the sediment core data, the diagenesis model can be used to calculate the amount of PHC consumed before burial. At this open-sea station, approximately 140mg/m2/y or 93% of the estimated flux was consumed at the interface. This evidence for rapid in situ oxidation is consistent with recent studies in semi-tropical estuaries (Van Vleet and Reinhardt, 1983). The Mediterranean is a warm sea with bottom water temperatures above 12°C even in winter (Lacombe and Tchernia, 1972). Our field evidence suggests t h a t the sea/sediment interface contains extremely high levels of PHC due to rapid vertical transport, but there is little evidence of large-scale accumulation of the relatively labile PHC in Mediterranean sediments, except in areas where burial rates are high and petroleum inputs are large. These observations are supported by laboratory degradation experiments in which the half-life of petroleum residues left after evaporation of 30% as volatile fractions, was approximately 65 days at temperatures of 15 and 20°C (Atlas and Bartha, 1972). Lee and Anderson (1977) and Lee et al. (1978) measured the degradation rate of naphthalene on suspended particles in the CEPEX experiments as 5% per day. Studies using large enclosed coastal ecosystems with oil additions showed an exponential decay rate of oil residues with a half-life of 58 days for labile components in surface sediments (Gearing et al., 1980; Wade and Quinn, 1980). These authors also discussed the distribution patterns of residues between various sized particles. Boehm et al. (1982) provided a clear example of the modification of oil residues due to environmental weathering processes. To complete the output computations we assumed that exchange with adjacent seas is insignificant. We then calculated that about 86% of the flux to sediments would be degraded on a yearly basis and 14% left for burial and diagenesis processes. We then forced a mass balance to calculate the amount left in the water column and assumed a similar proportion of labile and residual components (Table II).

148 TABLE

H

Outputs a (T/y) Volatilization with subsequent degradation in the atmosphere Tar formation, stranding on shores with subsequent degradation Sedimentatlon wlth subsequent: Degradation of lablle fractlons Burial of residual fractions Biodegradation within the water column Accumulation of residuals in deep waters Exchange with other seas

193.5 × 103 31.5 × 103 256 x 103 42 x 103 negligible

Annual total

883 x 103

180 x 103 180 x 103

a Estimated output terms based on mass balance calculations. See text for details.

ECOSYSTEM PARTITIONING AND INVENTORIES A large and diverse data base on the distribution of PHCs in various portions of the Mediterranean ecosystem has been generated in regional monitoring programs. Analytical methods range from semiquantitative and nonspecific t e c h n i q u e s s u c h a s u l t r a v i o l e t f l u r o r e s c e n c e ( U V F ) a n d i n f r a r e d (IR) s p e c t r o s copy to more quantitative and exact techniques based on gas chromatography (GC), o f t e n c o u p l e d w i t h m a s s s p e c t r o m e t r y ( G C / M S ) . W e h a v e n o t a t t e m p t e d to c a t a l o g u e all p u b l i s h e d analyses. R a t h e r , we p r e s e n t our a r g u m e n t s for the

TABLE III Inventories (tonnes) Annual

Cumulative

Atmosphere Beach tar Floating tar Surface oil films Surface microlayers Sediment flocculent layers Coastal sediments Open Sea sediments B,omass Seawater Near Surface Subsurface

191 x 10a 180 x 103 9 x 103 5 × 103 negligible 225 x 103

125 × 10a 147 x 103

2.5 x 10e

Total

883 x 103

2.8 x 10e

234 x 103 117 x 103 1 x 103

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most probable average concentrations of PHCs in each compartment based on our definitions and experience. The area of the eastern plus western basins is approximately 2.5 x 1012m2, with a volume estimated at 3.7 x 1015ms (Sverdrup et al., 1942). Modern hydrographic charts show that in the western basin the 1000 m contour covers 360£ of the area. If only the northern, most industrialized coastal zone is considered, the 'coastal' area is 26% of the total. We assumed a similar percentage for the eastern basin. The calculations were made by forcing inventories to equal inputs on a yearly basis plus what remains of residual components from previous years. We assumed a steady-state input so that residues are continuously replace in each reservoir as they are lost by output processes. The inventory is summarized in Table III.

Atmosphere As estimated by volatilization rates, the atmosphere should receive approximately 180 x 103 T/y. Added to this would be the input of pyrogenic PAH and other PHCs from coastal sources, estimated as 11 × 103T/y. No long-term accumulation would be expected in the atmosphere due to photooxidation processes.

Beach tar Assuming 300£ of spilled oil strands on shore, yields a standing stock of 180 × 108 T/y. Beach tar monitoring data tabulated by UNEP (1978) report a stranding rate of 3 to 350 g/m2 in 15 days. Since shores vary greatly in their slopes it is not possible to calculate total standing stocks from data reported in this manner. An integrated measure of tar between low and high tide marks would provide further means of evaluating the estimated inventory.

Floating tar Measurements of tar collected in neuston nets were reported by Horn et al. (1970), Morris (1974), Ros and Faraco (1978) and Levy et al. (1981). Values ranged from 0.1 to 20 mg/m 2 of sea surface. An overall average from these works is approximately 3.6mg/m 2 which, integrated over the total area, yields an inventory of 9 x 103 T/y. This estimate is only 10£ of the total yearly inventory. Thus we do not think it necessary to comment on the inherent errors in the measurements of floating tar.

Surface oil slicks Morris (1974) reported surface oil films ranging from 40 to 230mglipid/m 2 with an average of 151 mg/m 2. Gas chromatographic analysis of the surface film showed t h a t the lipid extract was 75°£ hydrocarbons and of these 65% were unresolved. A conservative calculation of PHCs in Morris' data would thus average approximately 73 mg/m 2. Since his measurements were made in the

150 eastern basin where t a n k e r discharges were permitted, we presumed this value to be an overestimate for the entire Mediterranean. Recent remote sensing data put an upper limit of 10% of the surface area at only one time covered by oil slicks. To estimate this portion of the i nvent ory we used an average concentration of one-fourth t hat calculated from Morris' 1974 data integrated over 10% of the area or about 5 × 108T/y.

Surface microlayers Sea areas not covered with visible slicks generally have microlayers enriched in biogenic and pollutant lipids compared to the seawater underneath. Values for PHCs in surface microlayers collected by the method of G a r r e t t (1965) were published by M a r t y and Saliot (1976), H5 et al. (1982), Burns and Villeneuve (1983), Burns et al. (1985) and Sicre et al. (1985). Samples collected by Saliot's group were t a ke n with a stainless steel screen, 0.48 m 2 in area, which collected 0.1271 per dip. Burns' group used a screen with an area of 0.375m 2 which collected 0.0441 per dip. Thus, from the PHC c o n c e n t r a t i o n data reported as pg/1, we calculated pg PHC/m 2. The data ranged from 0.1 to 13.3 #g/m 2 and averaged about 5 + 2 pg/m 2 (n = 21). Applying this average to the remaining 90% of the surface area yields an estimate of 11 T/y. Quantitatively this is a very minor portion of the total inventory, but high concentrations at this interface are important for toxicity to organisms and for air/sea exchange and photooxidation processes.

Sediments Integrating the estimated sedimentation rate of 90mg/m2/y over the total area yields an in vent or y of 225 × 103T/y which should be contained in the surface flocculent layers. A percentage of this yearly input is preserved in the sediment column, especially in coastal zones where inputs are large and burial rates are relatively rapid. Elevated levels of PHCs in sediments under commercial ports and bordering industrial zones have been reported (Mestres et al., 1975; Aloisi et al., 1976; Sammut and Nickless, 1978). Concentrations of PHCs in surface sediments in areas outside the immediate influence of coastal point sources are generally in the range of 1 to 20 ~g/g (Mille et al., 1982; Saliot et al., 1985; Burns and Villeneuve, 1986). As a conservative estimate of the i nvent ory of PHC in coastal sediments we used the data reported in Burns and Villeneuve (1983) from a core t a k e n o f f t h e coast of Monaco, an area with little direct input from coastal point sources. Total PHCs in this core to a depth of 20 cm were 360 mg/m 2 of sediment surface. This c o n c e n t r a t i o n applied to 26% of the total area yields a minimum stock of 234 × 10ST in coastal sediments bordering industrialized countries. To estimate the i n v e n t o r y in open-sea sediments we used the data from the core tak en under 2900 m of water in the western basin (Burns and Villeneuve, 1986). Integrating to 9 cm depth yields approximately 64 mg/m 2. Applying this

151

to the remaining 74% of the area yields a total stock of 117 x 103T contained in the open-sea sediments. The hydrocarbon compositions of deep sediments are highly modified compared to the PHC patterns in surface flocculent layers (Burns, 1986) and represent a long time interval of continuous inputs. Biomass

To estimate the amount of PHC contained in biota, we tabulated estimated potential stocks from an FAO (1981) atlas. The total estimated stock of 1.6 x 106T fresh weight per year of commerical species of crustaceans, cephalopods, pelagic fish and demersal fish is twice the reported catch of 0.8 x 106T for 1978 (Gulland, 1983). Assuming dry weight is 20% of fresh weight, then a total biomass for commercial species is about 0.3 x 106 T. Viale (1983) reported that a significant population of cetaceans inhabit the Mediterranean. Assuming these mammals would have a biomass of between 1 and 10% of potential food stocks means they are a small portion of total biomass. We assumed the same for seabirds. By far the largest standing stock of animals is the zooplankton. Using the average of 1 mg/m 2 yields a total estimated zooplankton biomass of 2.5 x 106 T. Burns et al. (1985) and Burns and Villeneuve (1986) report estimates for PHCs in zooplankton. The average of 17 reported analyses was 423 + 447 ~g/g. Applying this average concentration to the total estimated biomass yields an inventory of approximately 1 x 103T. Higher levels of PHC have been reported in coastal fish and shellfish (e.g. Risebrough et al., 1983). Thus the content of PHC pollutants has economic impact due to tainting of edible species and possible toxicity; but in terms of the total yearly ecosystem inventory the residues in biota represent only about 0.1%. We also cite the voluminous literature on the potential of marine species to metabolize certain PHCs as reviewed by Malins (1977), Saliot (1981) and NRC (1985). Near surface seawater

Many authors reported the levels of PHCs in Mediterranean seawater using a variety of analytical techniques, but there is no convergence in results and it is impossible to estimate a reasonable average level from the published data. Most monitoring programs have reported concentrations using UVF techniques on small-volume samples as summarized by Levy et al. (1981). The necessity of large-volume samples of seawater analyzed by selective and quantitative methods was discussed by de Lappe et al. (1980). To arrive at a reasonable average value, we considered only data generated from relatively largevolume samples (60 to 1501), analyzed by GC and for which PHCs could be calculated separately from biogenic hydrocarbons (Burns and Villeneuve, 1982; H6 et al., 1982; Albaiges et al., 1984; Burns, 1986; Burns et al., 1985; Saliot et al., 1985; Sicre et al., 1985). We believe a reasonable average value for PHC would be 1 ~g/1 in the open sea in areas devoid of local inputs. Values higher than this are frequently measured and must reflect local inputs of degradable

152 hydrocarbons and the long-term accumulation of residual hydrocarbons in the water column. Burns et al. (1985) argue for a residence time of PHCs in surface waters on the order of 1 year based on sedimentation rates. Thus, to calculate the in v en to r y we applied a c o n c e n t r a t i o n of 1/~g PHC/1 to the total area integrated over 50 m depth (125 × 103 T/y).

Subsurface seawater Uncertainties in the concentrations of PHCs in deep waters are even greater t h a n in surface waters due to the increased difficulties of obtaining largevolume samples u n c o n t a m i n a t e d by the sampling gear and platforms. We can force the mass balance and assume t hat all inputs not yet counted in other ecosystem compartments (147 × 103 T/y) are contained in subsurface seawater. Divided by the total volume of subsurface water (3.6 × 1015m 3) this could indicate a minimum c o n c e n t r a t i o n of 0.04/~g/1 in deep waters. A few measurements using relatively high-volume sampling techniques have been reported with values for PHCs ranging from 0.7 to 16.6 #g/1 and the majority of residues in deep water present in the dissolved phase (Burns, 1986; Saliot et al., 1985; Burns and Villeneuve, 1986). This data base is probably the most critical in terms of total inventory, yet it is the least known. If the reported measurements are accurate, then the deep waters, like the sediments, must be an important repository for residual organic contaminants. As a conservative estimate for PHC residues contained in deep waters we used a c o n c e n t r a t i o n of 0.7 #g/1 and multiplied it by the volume to obtain a minimum inventory of 2.5 × 106 T. DATA REEVALUATION Having calculated the mass balance we can now reevaluate the data base by looking for agreement between the various calculations. For example, the analysis of dated sediment cores indicated t h a t the input of PHCs into marine ecosystems correlates with global patterns of industrialization. From the concentrations of hydrocarbons in a dated sediment core published by Burns and Villeneuve (1983) we can estimate a relative rate of input over time. The PAHs are relatively stable and provide excellent markers of industrial activity. We TABLE IV Adjusted input rates a

Sequential

20

10

10

10

30

30

100

96

59

23

15

5

y e a r s past

Percentage of modern

a Input r a t e s of P H C s as a p e r c e n t a g e of m o d e r n . E s t i m a t e s are based on P A H profiles f r o m a dated

sediment core taken in the Mediterranean as explained in the text.

153

calculated the change in PAH input over past time by subtracting background levels in the deep layers deposited before the Industrial Revolution and dividing by the average concentrations of PAHs in surface layers. We assumed total PHC input would also be related to the general level of industrialization even though the sources of these hydrocarbons found in marine ecosystems are not always the same. The adjusted rates of input of PHC as a percentage of modern are shown in Table IV. With this refinement for past input rates we can now return to the inventory estimates and evaluate them in terms of expected cumulative totals. We estimated t h a t modern sedimentation processes deliver 31 x 103 T/y for burial in sediments after 86% of the total delivered is oxidized at the sediment surface. According to Table IV we would expect a total sediment inventory of 1326 x 103 T. The calculated inventory is 26% of that expected. This difference could be explained by diagenesis processes acting on the more labile PHC fractions within the sediment column after burial. By forcing the mass balance we estimated that 42 x l0 s T/y of residuals are transported to and accumulated in deep waters. Again according to Table IV we would expect a total inventory of 1.8 x 106 T. Our estimate, based on an average water concentration of 0.7 pg/1, is close to the expected value. From these evaluations we believe the mass balance is reasonably accurate. CONCLUSIONS

Several implications arise from the budgets as calculated with respect to sampling strategies and in predicting the effects of organic contaminants in ocean ecosystems. In spite of the large petroleum input to the Mediterranean, output processes continuously remove most petroleum-derived hydrocarbons from the surface ecosystem compartments on a seasonal basis. Thus with adequate analytical precision, it should be feasible to measure reduced inputs as reductions in standing inventories over a time scale of a few years. This is not true for deep sediments and waters. A certain percentage of inputs is transported to the ocean depths and desorbed from both slowly and rapidly settling particles; residual components accumulate in deep sediments and in the water column at rates proportional to their inputs. The distribution processes result in extremely high levels of organic contaminants at the sea/ sediment and air/sea interfaces. Concentrations at these interfaces are higher than in adjacent compartments even though at these sites degradation processes are intense. For example, the PHC concentrations measured in the flocculent particles of sediment cores from the western Mediterranean were as high as those measured in other coastal sediments subjected to an oil spill and shown to be toxic to benthic animals (Krebs and Burns, 1977). If only the levels of contaminants in compacted sediments were measured, as is usually done in pollution-monitoring programs, then this observation would have been missed. Boehm et al. (1982) pointed out that flocculent layers are important in the toxicity of hydrocarbon contaminants and their incorporation into the tissues

154 of benthic animals. Of course the interfaces are extremely difficult to sample quantitatively, and environmental assessment must depend on extensive integration of data from independent measurements including vertical fluxes, sediment accumulation rates and in situ degradation rates. This is clearly seen when considering the implications of the model in establishing accurate concentrations of contaminants in deep waters. The model predicts that current inputs inject 42 × 103 T/y of residual PHC into the deep water column. If the total inventory is on the order of 2.5 × 106 T, then the yearly addition is less than 2% of the total. The ability to detect a change in this important reservoir of organic contaminants would require extreme precision in seawater sampling and analysis. It is necessary to confirm the composition and estimated levels in deep-ocean waters by developing accurate large-volume sampling techniques; but concentration trends will be better assessed by improving the estimates of current fluxes through ocean systems in terms of air/sea exchange and sedimentation processes. Similar conclusions were reached by Burns and Villeneuve (1986) who developed a partial budget for PCBs in the Mediterranean. In terms of long-term accumulation of organic contaminants, the important classes of hydrocarbons to monitor are the PAH combustion products, other highly residual compounds including many of the halogenated hydrocarbons, and the oxidation products of hydrocarbons for which analyses are not routinely done in monitoring programs. This requires continued advancement in both sampling and analytical methodology and an emphasis on the analysis of individual marker compounds. The agreements between calculated fluxes, inventories and input time scales demonstrate the potential usefulness of organic contaminants as biogeochemical markers in the development of global flux models. ACKNOWLEDGMENTS The impetus to write this manuscript was provided by our association with the ad hoc Groups on Mass Balances and Individual Organic Contaminants of the Intergovernmental Oceanographic Commission's Group of Experts on Methods, Standards and Intercalibration (GEMSI). We t h a n k GEMSI for providing the forum for many useful discussions and the opI)ortunity for us to collate the results of many years of organic contaminant measurements made in the Mediterranean by our separate research groups. REFERENCES Albalges, J., Grimalt, J., Bayona, J.M., Risebrough, R., de Lappe, B. and Walker II, W., 1984. Dissolved, particulate and sedimentaryhydrocarbonsin a deltaic environment.Org. Geochem., 6:23%248 Aloisl, J.-C., Cauwet,G., Gadel, E., Got, H., Monaco,A., Vile, F., Causse, C. and Pagnon, M., 1976 Contribution ~ l'~tude de la s~dimentationrdcente et de la pollution sur le plateau continental du golfe du Lion entre Fos-sur-meret S~te. Bull. B.R.G.M.(2), IV: 69-83. Atlas, R.M. and Bartha, R., 1972. Blodegradationof petroleum m sea water at low temperatures. Can. J Microbiol., 18. 1851-1855

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