Francisco Delgadillo-Hinojosa

Alberto Zirino

Cristina Nasci

Copper Complexation Capacity in Surface Waters of the Venice Lagoon

Keywords: Copper, Complexation capacity, Copper ion, Venice Lagoon, Adriatic Sea

Total copper (Cu T ), copper ion activity (pCu) and the copper complexation capacity (CuCC) were determined in samples of seawater collected in July 2003 from the Venice Lagoon. Cu T and CuCC showed considerable spatial variability: Cu T ranged from 1.8 to 69.6 nM, whereas the CuCC varied from 195 to 573 nM. pCu values varied from 11.6 to 12.6 and are consistent with those previously reported in estuarine and coastal areas (10.9 -14.1). The range of Cu T values compares well with those reported in the past in the lagoon and in the adjacent Adriatic Sea. The highest concentrations of Cu T were found in samples collected near the industrial area of Porto Marghera, whereas the lowest were measured near the Chioggia and Malamocco inlets, where an intense tidally-driven renewal of seawater takes place. Although CuCC showed a high degree of spatial variability, the values recorded in the Venice Lagoon are comparable to those reported in other estuarine systems. In addition, CuCC was positively correlated with dissolved organic carbon (DOC), suggesting that organic ligands responsible for Cu complexation are part of the bulk organic matter pool in the lagoon. The CuCC:Cu T molar ratio was, on average 55:1, suggesting that a large excess of complexation capacity exists in the Venice Lagoon. The high levels of CuCC and the narrow range of pCu indicates the importance of the role played by organic ligands in controlling the free ion Cu concentrations in the lagoon, and as a consequence, regulating its availability and/or toxicity.

Introduction

In any attempt to describe and quantify the bio-geochemical cycles of trace metals, the physico-chemical form or speciation, is an extremely important characteristic that has to be taken into account [START_REF] Mueller | Chemical speciation of copper and zinc in surface waters of the western Black Sea[END_REF]. It is widely accepted that trace metals can exist in dissolved and/or particulate forms in natural waters; however, much of the uncertainty about the relationship between total metal concentration and toxicity to aquatic organisms results from lack of definitive knowledge of the physico-chemical properties of these metals in natural waters (Donat et al., l994;[START_REF] Tessier | Trace Metal Speciation and Bioavailability in Aquatic Systems[END_REF].

As the toxicity of metal ions is reported to be far greater than those of its complexes with naturally occurring ligands [START_REF] Campbell | Interactions between trace metals and aquatic organisms: A critique of the free-ion activity model[END_REF], complexation of trace metals by organic ligands in situ, has been considered the main control on bioavailability and toxicity of trace metals to phytoplankton [START_REF] Sunda | Trace Metal Phytoplankton interaction in the Sea[END_REF][START_REF] Campbell | Interactions between trace metals and aquatic organisms: A critique of the free-ion activity model[END_REF]. It is hypothesized that toxicity to microorganisms decreases because complexation of a metal by organic ligands diminishes its mobility through the cell wall. In addition, the agglomeration and ultimate sedimentation of metal-organo complexes further decreases toxicity by diminishing the concentration of metal ion in the bulk solution.

Previous studies of metal the complexation capacity in seawater have focused on the measurement of the complexation capacity of copper (CuCC) because Cu is toxic to microorganisms even at low levels and is able to form stable complexes with organic ligands [START_REF] Van Veen | Interlaboratory comparability of copper complexation capacity determination in natural waters[END_REF]. Most researchers have reported that more than 80% (generally 99%) of dissolved copper in surface waters is organically complexed [START_REF] Coale | Copper complexation in the Northeastern Pacific[END_REF][START_REF] Donat | Speciation of dissolved copper and nickel in South San Francisco Bay: A multi-method approach[END_REF][START_REF] Baeyens | Effect of organic complexation on the behavior of dissolved Cd, Cu and Zn in the Scheldt estuary[END_REF], Zirino et al., 1998a). [START_REF] Moffett | Cu speciation and cyanobacterial distribution in harbors subject to anthropogenic Cu inputs[END_REF] also demonstrated that Cu-binding ligands can be released by cyanobacteria into seawater and, when the sequestering capacity of these ligands is exceeded, Cu can exert acute toxic effects on cyanobacteria above a free copper ion level of 10 -11 M [START_REF] Mann | Copper toxicity and cyanobacteria ecology in the Sargasso Sea[END_REF]. In coastal areas, high concentrations of CuCC generally reduce metal toxicity; therefore, our ability to monitor and regulate copper as a toxin depends on our understanding of the spatial and temporal distributions as well as the sources and sinks of the Cu-binding ligands (e.g. [START_REF] Kogut | Strong copper-binding behavior of terrestrial humic substances in seawater[END_REF]. Thus, the aim of this study was to determine the extent and spatial variability of CuCC in the Venice Lagoon, an area strongly influenced by human activities.

2. Material and Methods.

Study Area.

The Venice Lagoon is a large, semi enclosed water basin (~550 km 2 ) bordering the Northern Adriatic Sea (Figure 1). It is approximately 1 m in depth and it includes canals, marshes, and islands. This transition ecosystem is connected to the Adriatic Sea by three inlets, namely Lido, Malamocco and Chioggia. Water exchange with the Adriatic Sea is governed by a semidiurnal tidal regime [START_REF] Gacic | Temporal variations of water flow between the Venetian lagoon and the open sea[END_REF] ranging from 35 cm during neap tides to 100 cm during spring tides, which combined with the shallowness of the lagoon results in large areas of the lagoon are alternately submerged and exposed daily. An underlying network of navigation channels connects the inlets with the main industrial zone of Porto Marghera and the city of Venice, which is situated in the north-central part of the lagoon. Hydrodynamic residence times vary from 1-3 days near the inlets to 20-30 days in the most interior northerly and southerly portions of the lagoon (Cucco and Umgiesser, 2006). Because in the last 500 years, the major rivers that emptied into the lagoon have been diverted, fresh water inflow is small (average = 3 x 10 6 m 3 d -1 ; [START_REF] Zuliani | Freshwater discharge from the drainage basin to the Venice Lagoon (Italy)[END_REF], less than 1 percent of the flow coming from the Adriatic Sea through the inlets (average = 6.2 -9.0 x 10 8 m 3 d -1 ; [START_REF] Gacic | Temporal variations of water flow between the Venetian lagoon and the open sea[END_REF]Cucco and Umgiesser, 2006). Hence, the Venice Lagoon differs from other typical estuaries in that it receives very limited freshwater input. In last 15 years, the lagoon has been recovering from heavy anthropogenic activities and uncontrolled discharges of pollutants from both diffuse and point sources, which increased greatly during the post WW II period following urban, industrial, and agricultural development. For example, the direct release of organic micropollutants [START_REF] Belluci | Polychlorinated dibenzo-p-dioxins and dibenzofurans in surficial sediments of the Venice Lagoon[END_REF], heavy metals [START_REF] Bloom | Seasonal cycling of mercury and monomethyl mercury in the Venice Lagoon (Italy)[END_REF][START_REF] Carrer | Characterizing the pollution produced by an industrial area: Chemometric methods applied to the Lagoon of Venice[END_REF] and nutrients [START_REF] Socal | Effects of thermal pollution and nutrient discharges on a spring phytoplankton bloom in the industrial area of the lagoon of Venice[END_REF] from the industrial area of Porto Marghera is well documented. Recent work done in the lagoon has pointed out that the Venice Lagoon is a mesotrophic environment with an estimated 85 percent of its nutrient nitrogen coming from the agricultural basin surrounding it (Collavini et al., 2005;[START_REF] Zirino | The monitoring programme in the Venice Lagoon: striving towards a comprehensive knowledge of the lagoon ecosystem[END_REF]. Its waters have a relatively large amount of nitrogen and ammonia all year-round [START_REF] Pastres | Long-term changes of inorganic nutrients in the Lagoon of Venice (Italy)[END_REF][START_REF] Acri | Plankton communities and nutrients in the Venice Lagoon. Comparison between current and old data[END_REF] and therefore, a relatively high rate of primary production [START_REF] Martin | Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)[END_REF].

Experimental

Sample Collection

The "Magistrato alle acque di Venezia" and "Consorzio Venezia Nuova" set up a water quality monitoring in order to gain a better understanding of the Venice Lagoon [START_REF] Bendoricchio | A water-quality model for the Lagoon of Venice, Italy[END_REF]. Briefly, the MELa1 (Monitoraggio dell'Ecosystema Lagunare) program was carried out for more than 3 years (September 2000-October 2003) and it included the monitoring of temperature, salinity, redox, plus approximately 20 nutrient-related macrovariables, and 8 trace variables (seven metals + As) once a month in a network of 30 stations covering the whole lagoon (Figure 1). Stations labeled B (for bassifondi; 20) are located in shallow areas; those labeled C (8) are representative of channels, whereas M1 and M2 are located just outside of the inlets of Lido and Chioggia, respectively. A detailed description of major objectives and the experimental design used to carry out MELa1 program can be seen in Solidoro et al. (2004a) and [START_REF] Zirino | The monitoring programme in the Venice Lagoon: striving towards a comprehensive knowledge of the lagoon ecosystem[END_REF]. Thirty samples of surface seawater from the Venice Lagoon were collected in July 2003 at the regularly monitored, 30 MELa1 stations. Briefly, at each station, samples of seawater were collected by using a 5L Niskin sampling bottle; subsamples to be measured for Cu T and CuCC were placed directly in previously acid-washed polyethylene bottles, quick-frozen and shipped under dry ice to the Scripps Institution of Oceanography, where they were maintained frozen until analysis.

Reagents and cleaning of materials.

The water used for reagent preparation and rinsing was deionised water (resistivity 18.2 MΩ cm -1 ) obtained from a MilliQ™ system. Reagents were Ultrex™-grade unless indicated otherwise. Cu standard solution used in the voltammetric and potentiometric determinations was prepared by dissolving analytical grade CuSO 4 in Milli-Q™ water, diluted to give the desired Cu concentration and acidified to pH ~2.0 using 6M HCl. All materials were acid-cleaned in both solutions of HCl and HNO 3 (10% v/v). All sample manipulations were carried out under a class-100 laminar airflow hood to minimize risks of contamination.

Copper Activity.

The Orion 94-29 copper ion selective electrode (Cu-ISE) was used to determine the Cu ++ activity directly from the samples. Previously, the Cu-ISE electrode was calibrated against two Cu-activity buffers prepared in filtered seawater (0.45 µm) following the procedure described by [START_REF] Belli | Behaviour and calibration of the copper (II) ion-selective electrode in high chloride media and marine waters[END_REF]. These Cu activity buffers were prepared 2 x 10 -4 M in Cu and either 1 x 10 -3 M glycine or ethylenediamine in filtered seawater (Zirino et al., 1998b). A specific ion interaction model was used to calculate pCu (i.e., -log[Cu ++ ] aq ) at the pH and concentrations of major ions in each seawater buffer and, therefore, the calibrated response of the Cu-ISE is reported as the pCu of the solution (Rivera-Duarte and [START_REF] Rivera-Duarte | Response of the Cu(II) ion selective electrode to the Cu titration in artificial and natural shore seawater and in the measurement of the Cu complexation capacity[END_REF]. The range of pCu's covered by this calibration was from 9.4 to 13; however, the response of the electrode to these buffers has been demonstrated to be linear over a wider range [START_REF] Belli | Behaviour and calibration of the copper (II) ion-selective electrode in high chloride media and marine waters[END_REF].

A double H-cell was used for all calibrations and sample measurements. The first H-cell contained a standard calomel electrode (SCE) in one arm of the H and a saturated KCl solution in the second arm. The two were connected via a sintered glass frit. A second and narrower, Hcell made from flat-bottomed test tubes, contained "clean" seawater in one arm and the Cu-ISE plus the sample, in the other arm. The two arms were separated by another glass frit. The seawater level in the sample arm was always lower than in the reference arm in order not to contaminate the reference. The Cu-ISE nearly filled all space in the arm and reached the bottom of the arm with its sensing crystal placed just above a very small (2mm x 5 mm) stirring bar.

This permitted the use of small samples volumes (c.a. 2 ml). A magnetic stirrer was placed below the arm and rotated at high speeds. The two H-cells were connected to each other via a salt bridge made out of a Tygon™ tube filled with KCl-saturated agar. The system was allowed to come to equilibrium over months. When measuring, the seawater sample was allowed to equilibrate for two minutes with the ISE and the electrode potential was then recorded. At the end of each suite of measurements, the slope of the Cu-ISE electrode was determined using the Cu-seawater buffers. The average slope for the 28 calibrations carried out was 29.3 ± 1.5 mV decade -1 , which agrees well with the expected Nernstian slope of 29.6 mV decade -1 .

Copper Titrations.

All copper titrations were made on unfiltered samples because we were interested in assessing the total binding capacity of the samples. Copper titrations were conducted in the batch mode as follows: Briefly, sub samples of 10 mL were pipetted into a series of twelve to sixteen 60 ml FEP Teflon bottles. Incremental amounts of a copper standard solution were added to all but two bottles, giving an approximate concentration range of 0 -800 nΜ. The addition of spikes changed the pH of the samples by less than 0.1 units. Samples were allowed to equilibrate overnight. At the beginning of next day, the Cu-ISE was conditioned using clean coastal seawater for half an hour before starting the protocol described above. Then, each sub-sample, in incremental Cu concentrations, was introduced into the measurement arm and the potential of the Cu ISE was recorded. (Samples previously measured were removed from the arm by vacuum aspiration through a glass capillary tube). Every determination of the Cu-CC was generated by measuring at least 12 different subsamples. Potentials were then converted to [Cu ++ ] and the ligand saturation was indicated by a change in slope on a plot of [Cu ++ ] against Cu T . The least square method was used to calculate the straight lines that best indicated where the CC has been exceeded [START_REF] Van Veen | Temporal variations of copper and zinc complexation capacity in the Humber estuary[END_REF]. Two straight lines were fitted to the titration data and the intersection point between them was the determined CuCC (See figure 3). In addition, we did not find that freezing the samples had an effect on the CuCC measurement. Briefly, one liter of natural seawater collected at La Jolla, CA, was divided in four aliquots, two were analyzed for CuCC immediately after collection (average = 225 ± 6 nM) and two were processed one week later after being frozen for 8 days (Average = 232 ± 3 nM). There was no statistical difference between these two means.

Determinations of Total Cu

The initial total Cu concentrations (Cu T ) in unfiltered samples were determined by differential pulse anodic stripping voltammetry (DPASV). The measurements were carried out using a CV50W polarographic system (BAS, Inc), connected to a C3 stand. The working electrode was a thin mercury film electrode (TMFE), the reference electrode was a Ag/AgCl electrode and a Pt-wire was used as the auxiliary electrode. Briefly, 10 mL samples were aliquoted in a Teflon™ cell and acidified to pH <2 by addition of 200 µL of Ultrex Grade HCl [START_REF] Scarano | Determination of copper in seawater by anodic stripping voltammetry using ethylenediamine[END_REF]. After first purging the sample for 6-8 min with ultrapure nitrogen, the Teflon™ stirring bar was switched on, and the Cu was concentrated on the working electrode for off and 25 s later, the voltage scan was started in the positive direction in the differential pulse mode. The scan parameters included a scan rate of 10 mV s -1 , a pulse amplitude of 25 mV and final potential of -0.01 V. The copper oxidation wave was recorded between -0.25 and -0.05 V; the signal height peak of each voltammogram was processed manually. In order to calculate the total concentration of Cu in each sample, the standard addition method was applied with at least, two additional spikes of Cu added (Figure 2). Usually, the coefficients of correlation (r 2 ) were better than 99%. Replicated analysis of a single sample were different by less than 7% and the accuracy of the method was assessed by analyzing a certified reference material (CASS3) from the National Research Council of Canada and values obtained were within certified 95% confidence intervals.

Results and discussion

The validity of pCu and CuCC determinations in seawater using the Orion 94-29 Cu-ISE has been established previously (Zirino et al., 1998b;[START_REF] Eriksen | Continuous flow methods for evaluating the response of a copper ion selective electrode to total and free copper in seawater[END_REF][START_REF] Rivera-Duarte | Response of the Cu(II) ion selective electrode to the Cu titration in artificial and natural shore seawater and in the measurement of the Cu complexation capacity[END_REF]. [START_REF] Eriksen | Copper speciation and toxicity in Macquarie Harbor, Tasmania: an investigation using a copper ion selective electrode[END_REF] have also shown that the Cu ISE provide a reproducible measure of the toxicity of Cu in cultures of marine phytoplankton (Nitzchia closterium and Dunaliella tertiolecta), whereas Rivera-Duarte et al. (2005) showed that the equivalence point determined with the Cu-ISE coincided with the results obtained using U.S. EPA-approved bioassay methods. In this study, the determined pCu values varied from 11.6 to 12.6 (Table I), and are consistent with those previously reported in estuarine and coastal areas (10.9 -14.1; Rivera-Duarte et al., 2005 and references therein). Thus, the free ion concentrations of Cu recorded in the Venice Lagoon were lower than 10 -10 M, which are below the concentrations of toxic responses reported from laboratory experiments with Monochrysis lutheri [START_REF] Sunda | Effect of complexation by natural organic ligands on the toxicity of copper to a unicellular alga[END_REF], Gonyaulax tamarensis [START_REF] Anderson | Copper Sensitivity of Gonyaulax tamarensis[END_REF], several marine diatoms as Asterionella glacialis and Bacteriastrum delicatulum [START_REF] Brand | Reduction of marine phytoplankton production rates by copper and cadmium[END_REF] and, the cyanobacteria Synechoccocus spp [START_REF] Moffett | Production of strong, extracellular Cu chelators by marine cyanobacteria in response to Cu stress[END_REF][START_REF] Moffett | Cu speciation and cyanobacterial distribution in harbors subject to anthropogenic Cu inputs[END_REF][START_REF] Croot | Uptake and efflux of 64Cu by the marine cyanobacterium Synechococcus (WH7803)[END_REF] and Prochlorococcus spp [START_REF] Mann | Copper toxicity and cyanobacteria ecology in the Sargasso Sea[END_REF].

The ligand concentrations in natural waters are often quantified by various titration methods [START_REF] Buffle | Complexation Reactions in Aquatic Systems: An Analytical Approach[END_REF]. The Cu-ISE Cu titrations conducted on water samples from the Venice Lagoon behaved as typical titration curves with a higher slope with increasing Cu concentration (Figure 3). All samples showed weak curvature at the lower added Cu concentrations, thus indicating the presence of natural Cu binding ligands. Although the presence and the exact value of the constant of stability of the different type of ligands present in the samples cannot be accurately described with our methodology, these metal titrations provide relevant information about the total metal buffering capacity within the range of the copper additions used. Thus, only ligand concentrations data will be further discussed in the present work.

Spatial distribution of Cu T and CuCC in the Venice Lagoon.

The average Cu T in this study was 15.4 ± 3.0 nM, ranging between 1.8 nM measured at the southern area of the lagoon to 69.6 nM recorded off the industrial area of Porto Marghera (Table I). These values compare well with those reported previously by Martin et al. (1994;[START_REF] Eyre | Regional evaluation of nutrient transformation and phytoplankton growth in nine river-dominated sub-tropical east Australian estuaries[END_REF], who collected several samples in the north part of the lagoon in two surveys carried out in 1992 under spring (average Cu = 14.4 ± 4.4 nM) and winter (average Cu = 8.1 ± 2.3 nM) conditions. Our data set also compares favorably with those reported for surface waters of the northern Adriatic Sea [START_REF] Tankere | Distribution of dissolved Cd, Cu, Ni and Zn in the Adriatic Sea[END_REF][START_REF] Zago | Heavy metal distribution and speciation in the northern Adriatic Sea[END_REF], which were measured under winter and summer conditions (average Cu = 7.7 ± 3 nM).

The lower Cu T concentrations were found close to the Malamocco (average = 6.4 ± 3.1 nM; n = 2) and Chioggia (6.1 ± 2.6 nM; n = 2) inlets, which are subject to an intense tidal water renewal [START_REF] Gacic | Temporal variations of water flow between the Venetian lagoon and the open sea[END_REF]Solidoro et al., 2004b;Cucco and Umgiesser, 2006). In contrast, at the Lido inlet, the average Cu T was 22.1 nM (n = 2; Table I). The highest Cu T was 69.6 nM measured at station 8B located off Porto Marghera, which is the industrial area. This high concentration of Cu may be linked to the disposal of industrial wastes and the fact that this area is poorly flushed by tides and freshwater inputs from Mestre-Marghera [START_REF] Sfriso | Heavy metals in sediments, SPM and phytobenthos of the Lagoon of Venice[END_REF][START_REF] Carrer | Characterizing the pollution produced by an industrial area: Chemometric methods applied to the Lagoon of Venice[END_REF]. Our findings are consistent with previous reports, which have documented the direct release of organic micropollutants [START_REF] Belluci | Polychlorinated dibenzo-p-dioxins and dibenzofurans in surficial sediments of the Venice Lagoon[END_REF] and of heavy metals [START_REF] Sfriso | Heavy metals in sediments, SPM and phytobenthos of the Lagoon of Venice[END_REF][START_REF] Bloom | Seasonal cycling of mercury and monomethyl mercury in the Venice Lagoon (Italy)[END_REF] from the industrial zone of Porto Marghera. For instance, [START_REF] Bernardello | The changing state of contamination in the Lagoon of Venice. Part 2: Heavy metals[END_REF] recently measured the Cu concentration in sediments, and they found a clear gradient decreasing from Porto Marghera toward the sea inlets, suggesting the primary role of the industrial zone as a source of heavy metals to the Venice Lagoon.

The CuCC ranged from 195 nM at Malamocco inlet to 573 nM recorded at station 20B in the southern lagoon (Table I). These CuCC values are comparable to those reported in other coastal lagoons and estuarine systems. For example, values of CuCC ranging between 98 and 338 nM have been reported in San Diego Bay [START_REF] Rivera-Duarte | Copper toxicity to larval stages of three marine invertebrates and copper complexation capacity in San Diego Bay, California[END_REF]. van Veen et al. salinity was lower than 32. In particular, CuCC values as high as 520 nM were measured in the brackish area (S <32) close to the mouth of Sile River. These results could suggest that Cubinding ligands may be produced in the estuarine area inside the lagoon. Moreover, these findings also suggest that the Venice Lagoon is functioning as a source of Cu-binding ligands to the Adriatic Sea (see further discussion below).

Relationship between Cu T and CuCC.

In this study, the CuCC was not correlated with Cu T as reported in other coastal systems.

For example, in Newport Bay, RI, the concentrations of Cu complexation ligands increased with increasing total Cu concentrations [START_REF] Kozelka | Chemical speciation of dissolved Cu, Zn, Cd, Pb in Narragansett Bay, Long Island[END_REF]. Likewise, in Galveston Bay, which is a shallow lagoon much more similar to the Venice Lagoon, [START_REF] Tang | Distribution and partitioning of trace metals (Cd, Cu, Ni, Pb, Zn) in Galveston Bay waters[END_REF] found high CuCC associated with Cu T . However, in the Venice Lagoon, the CuCC:Cu T molar ratio was much higher, 55:1 on average, indicating that a large excess complexation capacity exists and because of this excess of CuCC, almost all Cu is likely to be organically complexed. Similar results have been reported for the adjacent Adriatic Sea [START_REF] Zago | Heavy metal distribution and speciation in the northern Adriatic Sea[END_REF] and the eastern coast of USA (Shank et al., 2004a). For example, in Cape Fear River, strong Cu-complexing ligands concentrations ranging from 7 to >200 nM were also found in excess of dissolved Cu concentrations (3 -25 nM; Shank et al., 2004a). These results suggests that the ecological importance of the CuCC may go beyond Cu, since the excess of ligands detected with Cu ++ does not necessarily mean that the organic ligands are specific to Cu but may also bind other trace metals.

Relationship between DOC and salinity.

The average DOC concentration in this study was 308 ± 35 µM, ranging between 99.5 µM, measured at station 1C located in the northern area, and 766 µM recorded at station 20B in the southern part of the lagoon (Table I). In addition, higher values of DOC (339 ± 90 µM) were generally recorded within the estuarine part of the lagoon whereas the lower were found in stations near to the inlets (201 ± 36 µM). These DOC concentrations compares favorably with those reported previously by [START_REF] Martin | Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)[END_REF], who measured DOC concentrations between 116 and 312 µM along a transect from the Silone channel to the Lido inlet. However, it is worth noting that DOC concentrations measured in the Venice Lagoon are higher than other Mediterranean coastal regions [START_REF] Martin | Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)[END_REF] and the adjacent northern Adriatic Sea [START_REF] Pettine | Seasonal variations of dissolved organic matter in the northern Adriatic Sea[END_REF][START_REF] De Vittor | Dissolved organic carbon variability in a shallow coastal marine system (Gulf of Trieste, northern Adriatic Sea)[END_REF]. The difference in DOC concentration between the lagoon and the Adriatic Sea also indicates that the Venice Lagoon may be a locally important source of organic matter to the adjacent Adriatic Sea.

In order to understand better the DOC distribution in the lagoon, we used the MELa1 data set for the period from September 2000 to October 2003 (n = 1170; 30 stations sampled in 39 campaigns). The distribution of DOC versus salinity exhibited an inverse relationship, with lower values in the marine end member and higher in the estuarine part of the lagoon (Figure 4a). River inflow probably does not account for these high DOC concentrations found in the estuarine area of the Venice lagoon, because in the past the main freshwater tributaries were diverted out of the lagoon and it receives very limited freshwater input. Compared with the flow exchanged with the Adriatic Sea through the inlets [START_REF] Gacic | Temporal variations of water flow between the Venetian lagoon and the open sea[END_REF], the freshwater input is less than 1 percent [START_REF] Zuliani | Freshwater discharge from the drainage basin to the Venice Lagoon (Italy)[END_REF]. An estimate of the increase of DOC in surface water of the Venice Lagoon as a result of riverine input, can be made by taking the mean DOC riverine concentration, the mean flux of freshwater (average = 3 x 10 6 m 3 d -1 ; [START_REF] Zuliani | Freshwater discharge from the drainage basin to the Venice Lagoon (Italy)[END_REF], an average hydrodynamic residence time of 15 days (Cucco and Umgiesser, 2006) and, the mean depth of the lagoon (~1.0 m). Although DOC data reported for the riverine inputs arriving to the Venice Lagoon are scarce, an average of 296 ± 40 µM can be calculated from those reported for the Silone (179 ± 57 µM; [START_REF] Martin | Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)[END_REF][START_REF] Dai | Significance of colloids in the biogeochemical cycling of organic carbon[END_REF] and Osellino (416 µM;[START_REF] Critto | Effect of a contaminated site (the San Giuliano Landfill, Venice, Italy) on the interaction between water bodies in a coastal aquifer system[END_REF] channels. Thus, the DOC concentration in the surface water of the Venice Lagoon would have an average increase of approximately 24 µM in 15 days (1.6 µM d -1 ), or an increase of only 11% of the mean DOC concentration (223 ± 45 µM) measured during all MELa1 campaigns, suggesting that the riverine input of DOC is not so important for the surface waters of the lagoon.

The mixing diagram also showed that DOC distributions deviated positively from conservative mixing (Figure 4a), indicating production of these constituents within the lagoon.

Similarly, [START_REF] Martin | Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)[END_REF] reported a non conservative behavior of DOC within the northern part of the Venice Lagoon. Thus, these findings indicate that the origin of organic matter in the Venice Lagoon is mainly autochthonous. Moreover, for the same MELa1 data set for the period from September 2000 to October 2003, we found high values of DOC associated with high chlorophyll concentrations (r = 0.80; p<0.05; n = 30 stations; Figure 4c), substantiating the idea that DOC is partially produced by phytoplankton primary producers within the lagoon (Figure 4b). The high primary production of the lagoon largely determines the distribution of DOC and Chl a, leading to spatial distribution patterns with maxima at intermediate salinities rather than at the river end member as in other estuaries (Figure 4a,b). Hence, primary production in the Venice Lagoon probably plays an important role as a source of organic matter and as a consequence, of CuCC.

Relationship between DOC and CuCC.

Although concentrations of Cu complexing ligands measured in this study were only a small fraction (0.1 -0.5%) of the total water column DOC concentrations (Table I), figure 5 shows that CuCC varied in proportion to changes in DOC concentrations (r = 0.60, n = 25, p<0.05). As DOC in the Venice lagoon is, on average, 82% of the total organic carbon, it is very likely that the ligands responsible for Cu complexation are part of the bulk organic matter pool. Our results are consistent with other studies, which have also shown a positive relationship between Cu complexation and bulk organic matter concentrations [START_REF] Gerringa | Relations between free copper and salinity, disolved and particulate organic carbon in the Oosterschelde and Westerschelde[END_REF]Shank et al., 2004a, b). For instance, in the organic-rich Cape Fear estuary (USA), DOC and ligand concentrations exhibited strong linear correlations (r 2 = 0.93, p<0.01) among transect samples at DOC concentrations <1000 µM and among all size fractions of ultrafiltered estuarine samples (r 2 = 0.94; Shank et al., 2004a). Similarly, [START_REF] Tang | Organic complexation of copper in Galveston Bay waters[END_REF] have demonstrated the importance of colloidal ligands in the organic complexation of Cu in brackish waters of Galveston Bay (USA) and Zirino et al. (1998a) reached the same conclusions in San Diego Bay. In the Venice Lagoon, [START_REF] Dai | Significance of colloids in the biogeochemical cycling of organic carbon[END_REF] reported that a significant part (46%) of the measured dissolved Cu was actually associated with the organic colloidal material. Thus, colloidal organic carbon may be the major vehicle in the formation of organic-metal complexes in the water column of estuarine systems [START_REF] Mackey | Comments on trace element speciation, or do 'onions' grow in the sea?[END_REF].

Although it has been known for some time that organic ligands control Cu speciation in most natural waters, the sources of these ligands are not fully understood. In estuarine environments, ligands may derive from anthropogenic and terrestrial sources (Shank et al., 2004a, b), sediment [START_REF] Skrabal | Fluxes of copper-complexing ligands from estuarine sediments[END_REF], and from water column processes [START_REF] Dryden | Interactive regulation of dissolved copper toxicity by an estuarine microbial community[END_REF]. For example, Shank et al. (2004b) showed that an important source of CuCC complexing ligands to the waters in the South Atlantic Bight is provided by the discharge of dissolved organic matter (DOM) by the rivers along the coastal North Carolina (USA). This is not the case for the Venice Lagoon because the organic matter content (Critto et al., 200;[START_REF] Dai | Significance of colloids in the biogeochemical cycling of organic carbon[END_REF] and the freshwater input [START_REF] Zuliani | Freshwater discharge from the drainage basin to the Venice Lagoon (Italy)[END_REF] are very much reduced.

Sediments are also likely to play an important role as a source of organic matter to the water column in the Venice Lagoon. The prevalence of a semidiurnal tidal regime combined with an average water depth of 1m, generates vigorous currents which are effective in mixing the water column, and at the same time, provokes that large areas in the lagoon are alternately submerged and exposed on a daily basis (Cucco and Umgiesser, 2006). Consequently, Cucomplexing ligands released from sediment pore waters may also influence Cu speciation. Five samples of porewater collected at 5 different locations in the lagoon resulted in range of CuCC values of 315 to 676 (average = 506 ± 141 nM). These CuCC levels in porewater from the Venice Lagoon fall in the range reported for porewater from Chesapeake Bay (135 -12500 nM; [START_REF] Skrabal | Pore water distributions of dissolved copper and copper-complexing ligands in estuarine and coastal marine sediments[END_REF]. Although our data set is very limited for the number of samples analyzed, it shows high variability and suggests that sediments could be a potentially important source of CuCC to the overlying water column in the Venice Lagoon. Other researchers have previously demonstrated that bottom sediments are an important source of CuCC in coastal systems as Chesapeake Bay [START_REF] Skrabal | Fluxes of copper-complexing ligands from estuarine sediments[END_REF][START_REF] Eyre | Regional evaluation of nutrient transformation and phytoplankton growth in nine river-dominated sub-tropical east Australian estuaries[END_REF] and Cape Fear River Estuary (Shank et al., 2004c) in USA. In Chesapeake Bay, for example, the measured benthic fluxes of CuCC ranged between 300 and 1200 nmol m -2 d -1 , suggesting that benthic fluxes may supply from 10 to 50% of the standing stock of CuCC in this bay [START_REF] Skrabal | Fluxes of copper-complexing ligands from estuarine sediments[END_REF]. Finally, it is important to note the important difference in electrochemical methodologies used in these studies. [START_REF] Skrabal | Fluxes of copper-complexing ligands from estuarine sediments[END_REF][START_REF] Eyre | Regional evaluation of nutrient transformation and phytoplankton growth in nine river-dominated sub-tropical east Australian estuaries[END_REF] and Shank et al. (2004c) were examining certain ligand types that may or may not be the same as the CuCC measured in this study carried out in the Venice Lagoon.

The high variability of CuCC in the Venice Lagoon can be explained partially because there is a superimposition of the labile DOM released by biological activity and the pool of more resistant refractory organic material. Both fractions of organic matter may become associated with Cu with different efficiencies of complexation. The more labile organic material is found in productive surface waters released by the recent biological activity of some species of phytoplankton [START_REF] Brand | Reduction of marine phytoplankton production rates by copper and cadmium[END_REF]Moffet et al., 1997), macroalgae [START_REF] Gledhill | The release of copper-complexing ligands by the brown alga Fucus vesiculosus (Phaeophyceae) in response to increasing total copper levels[END_REF], bacterioplankton [START_REF] Dryden | Interactive regulation of dissolved copper toxicity by an estuarine microbial community[END_REF] and heterotrophic bacteria [START_REF] Gordon | Dissolved copper-complexing ligands in culture of marine bacteria and estuarine water[END_REF], all of which have the potential to contribute to the pool of strong Cu complexing ligands in natural waters. In the past, the Venice Lagoon was heavily influenced by the massive proliferation of the macroalgae Ulva rigida. Now, a remarkable disappearance of this species has been reported [START_REF] Sfriso | Decline of Ulva growth in the Lagoon of Venice[END_REF][START_REF] Sfriso | Temporal and spatial changes of macroalgae and phytoplankton in a Mediterranean coastal area: the Venice lagoon as a case study[END_REF]; therefore, other seaweed and phytoplankton communities are now the most important primary producers in the lagoon [START_REF] Facca | Changes in abundance and composition of phytoplankton and microphytobenthos due to increased sediment fluxes in the Venice Lagoon, Italy[END_REF]. Thus, it is expected that the spatial and temporal variation in CuCC is associated with the occurrence or variability of the different producers of organic matter in the lagoon.

The exact nature and sources of the ligands in the water column of the Venice Lagoon remain largely unknown. The high levels of CuCC and the narrow range of pCu recorded in this study suggest the importance of the organic ligands controlling the free ion Cu concentrations in the Venice Lagoon, and as a consequence, in regulating its availability and/or toxicity. Thus, the observations and conclusions drawn here may apply to other lagoons characterized by the presence of high organic carbon concentrations. For example, the contribution of additional organic ligands may reduce copper availability and toxicity to primary producers in lagoons subjected to very high organic matter release.

Conclusions.

This work focuses on studying the spatial variability of Cu T , pCu and CuCC from the Venice Lagoon in July 2003. Cu T and CuCC showed considerable spatial variability and the range of Cu T values compares well with those reported in the past in the lagoon and in the adjacent Adriatic Sea. pCu values varied from 11.6 to 12.6 and are consistent with those previously reported in estuarine and coastal areas. The highest concentrations of Cu T were found in samples collected near the industrial area of Porto Marghera, whereas the lowest were measured near the Chioggia and Malamocco inlets, where an intense tidally-driven renewal of seawater takes place.

Although CuCC showed a high degree of spatial variability, the values recorded in the Venice Lagoon are comparable to those reported in other estuarine systems. In addition, CuCC measured in this study was positively correlated with DOC, suggesting that organic ligands responsible for Cu complexation are part of the bulk organic matter pool in the lagoon. The CuCC:Cu T molar ratio was, on average 55:1, suggesting that a large excess of complexation capacity exists in the Venice Lagoon. However, the exact nature and sources of the ligands in the water column of the Venice Lagoon remain largely unknown. Finally, the high levels of CuCC and the narrow range of pCu suggest the importance of role played by organic ligands controlling the free ion Cu concentrations in the lagoon, and as a consequence, regulating its availability and/or toxicity. [START_REF] Eyre | Regional evaluation of nutrient transformation and phytoplankton growth in nine river-dominated sub-tropical east Australian estuaries[END_REF], based on DOC average values previously reported for the Northern Adriatic Sea (151 ± 40 µM; [START_REF] Pettine | Seasonal variations of dissolved organic matter in the northern Adriatic Sea[END_REF] and for the freshwater input arriving to the Venice Lagoon (296 ± 40 µM; [START_REF] Martin | Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)[END_REF]Critto et al., 2005). Table I.

(

  2001) found maximum values between 200 and 420 nM in the lower salinity samples collected at the Humber estuary. Similarly, in the Venetian lagoon, CuCC values between 358 and 556 nM were found in waters with salinities lower than 32 (Table I). Based on the similarity of the temporal evolution of nutrients and chlorophyll-a (Chl-a) collected during 2001 in the same MELa1 sampling grid, Solidoro et al. (2004a) grouped the stations in the lagoon into several clusters. Among them were "marine" stations, physically located closest to the Adriatic Sea, "estuarine" stations, situated close to the tributary discharges, and stations placed in the southern part of the lagoon. Accordingly to the Solidoro et al. (2004a) classification, the "marine" samples had values of CuCC ranging between 195 and 312 nM (average = 267 ± 40 nM; n = 6) and were statistically lower (t student ; p<0.05) than those found south of Malamocco inlet in the southern area of the lagoon (average = 500 ± 40 nM; n = 6) and lower than those from the estuarine region (average = 439 ± 62 nM; n = 4) of the lagoon, where

Figure 1 .

 1 Figure 1.-Study area and location of MELa1 sampling stations in the Venice Lagoon. Symbols indicate hydrographic sampling stations classified as function of salinity according to Solidoro et al. (2004a). Marine stations are those with salinity >33.0 ( ), estuarine stations with salinity <31.0 ( ) and intermediate stations with salinities between 31.0 and 33.0 ( ). B stands for stations located in shallow areas, whereas those marked as C were positioned at the channels.

Figure 2 .

 2 Figure 2.-Representative ASV-standard addition curves for the determination of total Cu concentration in acidified (pH <2) samples collected at stations 19B and 8C from the Venice Lagoon in July 2003.

Figure 3 .

 3 Figure 3.-Representative titration curves of Cu complexation capacity in samples collected at stations 19B and 8C from the Venice Lagoon in July 2003.

Figure 4 .

 4 Figure 4.-Relationship between (a) the monthly dissolved organic carbon (DOC) and (b) chlorophyll a concentrations and salinity in the Venice Lagoon from September 2000 to October 2003. (c) Relationship between the monthly DOC and chlorophyll a concentrations during the same period. Each point represents the average of thirty-nine samplings (2000-2003) of chlorophyll a and DOC concentrations at each of the thirty lagoon stations. The brackets indicate the standard error. The station symbols indicate the relative salinities; marine stations are those with salinity >33.0 ( ), estuarine stations with salinity <31.0 ( ), and intermediate stations with salinities between 31.0 and 33.0 ( ). The conservative line in (a) was constructed using a simple two end-member mixing model (e.g. Eyre, 2000), based on DOC average values previously reported for

Figure 5 .

 5 Figure 5.-Relationship between CuCC and DOC in the Venice Lagoon. Marine stations are those with salinity >33.0 ( ), estuarine stations with salinity <31.0 ( ) and intermediate stations with salinities between 31.0 and 33.0 ( ) according to Solidoro et al. (2004a).

  Figure 1.

Table I .

 I -Salinity, Cu complexation capacity (CuCC), total Cu, pCu, chlorophyll a and dissolved organic carbon (DOC) for samples collected in the Venice Lagoon during July 2003. pCu = -log[Cu ++ ] aq .

	Sample #	Station	Salinity	CuCC (nM)	total Cu (nM)	pCu	Chl a (µg/L)	DOC (µM)
		13B	36.75	195	9.6	12.04	1.4	---
	2	4B	33.02	266	20.6	11.56	16.0	174.7
	3	2C	28.11	282	17.0	12.08	21.0	283.1
	4	3C	35.09	266	12.9	12.20	14.0	141.4
	5	1M	36.48	312	22.1	12.23	2.2	158.0
	6	2M	36.84	257	4.3	11.64	2.2	174.7
	7	8B	31.18	400	69.6	11.98	43.0	307.9
	8	20B	34.47	573	60.9	12.13	3.6	766.1
	9	5C	35.20	446	23.5	11.81	7.7	182.9
	10	4C	36.28	279	22.1	11.53	1.0	224.7
	11	12B	36.20	274	4.2	11.69	1.4	124.7
	12	8C	36.37	288	27.1	12.05	1.3	249.7
	13	14B	36.96	473	3.4	11.38	1.9	199.5
	14	10B	31.03	556	25.9	11.79	15.0	474.4
	15	3B	37.15	368	4.0	11.81	4.3	166.3
	16	2B	32.71	358	---	12.07	12.0	291.3
	17	19B	36.83	484	8.0	11.76	1.5	341.2
	18	7C	37.03	272	6.2	11.99	1.7	---
	19	5B	33.06	523	3.9	11.96	9.2	224.5
	20	17B	36.28	---	3.7	12.20	2.3	---
	21	6C	35.34	517	5.5	12.17	5.8	216.1
	22	1C	36.61	451	7.4	11.89	2.1	99.5
	23	6B	33.43	---	7.0	12.09	8.1	---
	24	15B	36.32	464	1.8	12.52	3.3	441.2
	25	11B	36.06	497	12.8	12.62	2.7	657.8
	26	16B	34.64	514	6.1	12.19	3.3	324.5
	27	18B	36.58	495	7.1	12.36	1.3	616.2
	28	1B	27.68	516	5.5	12.23	32.0	---
	29	7B	33.71	407	22.3	12.48	11.0	474.6
	30	9B	34.01	503	21.8	12.19	11.0	382.8


S C Johannessen

R W Macdonald

B Burd

A Van Roodselaar

Biogeochemical cycling in the Strait of Georgia

Fraser River discharge dominates the particulate and dissolved organic carbon that enter the Strait of Georgia system, but organic carbon is also produced in situ by primary production [START_REF] Johannessen | A sediment and organic carbon budget for the greater Strait of Georgia[END_REF]. Quarterly measurements throughout the Strait of While benthic macrofauna appear to thrive under conditions of high river discharge, the abundance of reef-forming, siliceous hexactinellid sponges is low under the Fraser River plume, where the sedimentation rate is high (Cook et al., this issue). The sponges do survive there, apparently physically adapted to the turbidity, if not in great numbers. The biggest threat to these sponge reefs and their associated fauna appears to be mechanical trawling, which breaks the fragile sponges, destroying both the sponges and the habitat for rockfish and other animals that was formerly provided by the sponges.

The signature of the municipal outfall can be seen in a sediment core collected within 0.5 km of the Iona outfall in enhanced organic carbon oxidation and fluxes of elements like

ACCEPTED MANUSCRIPT

Cd, Ag and U, which are sensitive to redox conditions in sediments. Other redox elements like Mn, Re, Mo, total sulphur or acid-volatile sulphides, provide little indication of significant oxygen depletion due to the addition of labile organic carbon from wastewater (Macdonald et al., this issue). Within the Greater Strait of Georgia there is a wide range of natural variability in redox marker distributions and fluxes due to large natural variations in organic flux, making it difficult to find any significant impact from organic loadings from outfalls.

Although there is no evidence of significant oxygen depletion in the sediments near the Iona Island wastewater outfall, over a finer spatial scale, a somewhat lower oxygen concentration around the outfall appears to recorded in iron oxide or hydroxyl-oxide staining on the shells of the infaunal bivalve Axinopsida serricata (Burd et al., 2008 c).

This staining, which depends on the reduction of iron to soluble forms that diffuse and reprecipitate, may provide a simple means to conduct preliminary surveys of organic loadings around point sources like marine outfalls.

Polybrominated diphenyl ethers (PBDEs), principally used as flame retardants, are a concern, as their use and emissions to the environment have been growing rapidly [START_REF] De Wit | An overview of brominated flame retardants in the environment[END_REF]. These contaminants, which are commonly found in households and businesses, and readily "go down the drain" in cleaning water, mean that municipal wastewater provides a major pathway for PBDEs into the environment generally [START_REF] Chapman | Emerging substances -emerging problems?[END_REF] and into the Strait of Georgia in particular (Johannessen et al.,

  Biogeochemical cycling in the Strait of Georgia -Special Issue S. C. Johannessen, R. W. Macdonald, B. Burd and A. van Roodselaar The encroachment of human activities on coastal waters is a worldwide problem (GESAMP, 1990) that requires site-specific consideration of how multiple discharges from of land-based sources affect marine systems. Appropriate policy to protect marine resources requires, among other things, scientific data to understand the functioning of coastal waters (e.g., baselines, residence times, sinks, pathways) and the means to detect and quantify the impacts of using the coastal ocean as a direct or indirect means to dispose of waste. The papers in this special issue present interpretations of scientific observations carried out during a five-year project undertaken in the Strait of Georgia, British Columbia, Canada (Figure 1) as a collaboration among Fisheries and Oceans Canada, Natural Resources Canada and Metro Vancouver (formerly the Greater Vancouver Regional District). The objectives of the project were to provide context for local monitoring of discharges into the Strait of Georgia from Metro Vancouver's wastewater outfalls, and also to enhance confidence in the scientific findings through the process of peer-review conducted during publication of results in international journals. Furthermore, the project provided the means to advance our understanding of underlying sedimentological, geochemical and benthic processes taking place in the Strait of Georgia. Four of the papers presented here address directly the effects of the wastewater on the local environment, while other papers provide overviews of the ambient sedimentological and geochemical conditions and the benthic biota and sponge reefs of the greater Strait of Georgia. Natural sediment transport and deposition in the Strait of Georgia, which are complex, form a crucial background process onto which municipal outfall particulates load. Riverborne sand and coarse silt are deposited on the Fraser River delta, from where they are moved northward along the bottom by the asymmetrical tidal currents. The finer material remains longer in the surface plume before it is deposited and, in some parts of the Strait, becomes extensively reworked. Hill et al. (this issue) combine data from sedimentological, multibeam mapping and seismic profiling studies to give a comprehensive description of local sediment dispersal.

  Georgia and Haro and Juan de Fuca Straits show that, compared with the Fraser River and marine primary production, very little organic carbon enters the Strait of Georgia with the inflowing seawater(Johannessen et al, this issue b). The Strait is dynamic with respect to organic carbon, with an annual flux through the system of approximately twice the standing inventory. The organic carbon supports an active benthic community. Burd et al. (this issue a) provide an extensive review of the benthic biota and their habitat in the Strait of Georgia, based on data collected over more than 20 years. The review links the organisms with the sediment conditions that surround them, illustrating the importance of sediment geochemistry and sedimentation to an understanding of benthic biomass and community structure. Benthic biota in the immediate area of the Iona Island municipal wastewater outfall show effects of the effluent. The total benthic biomass is depleted, and the population is dominated more by polychaetes and less by bivalves and ophiuroids, relative to other parts of the Strait (Burd et al., this issue b). Biomass at sites close to anthropogenic discharges, such as municipal outfalls, pulp mills and the abandoned Britannia copper mine, is not predicted by the organic flux -biomass relationship that describes more remote sites. In addition to local organic flux, biomass and other biological characteristics are affected by the quality and source of organic matter (lability) and by the contribution of terrigenous material.

  this issue a). Because PBDEs have been introduced into wide-scale use relatively recently, ACCEPTED MANUSCRIPT they are now found in highest concentration around current points of entry, including the Iona Island outfall. Polychlorinated biphenyls (PCBs), conversely, no longer have a local source because they have been banned in North America for about thirty years, and their concentration in sediments is primarily controlled by the continuing flux of PCB into coastal waters from revolatilization of contaminated soil reservoirs, and by sedimentation and mixing rates in surface sediments. Sediment core profiles from the Strait of Georgia show that while PCB concentrations continue to decrease slowly, PBDEs are presently increasing rapidly. Collectively, these papers demonstrate that the fate of particles, organic carbon and contaminants discharged into the coastal ocean, and the response of benthic biota, depend not only on the contaminants themselves but also on the nature of the receiving environment. A low sedimentation rate in the northern Strait of Georgia, combined with benthic mixing, for example, produces high concentrations of PCBs in surface sediments that have little to do with local pollution. The sedimentation rates themselves are controlled by a complex combination of fluvial discharge, tidal currents, and underwater slope stability. And the biota associated with reef-forming sponges are limited by turbidity near the Fraser River plume, while benthic infauna thrive near the river. This complexity requires that management decisions concerning the Strait of Georgia be informed not only by the nature of a particular stressor but also by an understanding of the dynamics and cycling of particles, organic matter and benthic communities in the Strait.