significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the...

Significance of Colloids in the Biogeochemical Cycling of Organic Carbon and Trace Metals inthe Venice Lagoon (Italy)Author(s): Jean-Marie Martin, Min-Han Dai and Gustave CauwetSource: Limnology and Oceanography, Vol. 40, No. 1 (Jan., 1995), pp. 119-131Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2838254 .

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Limnol. Oceanogr., 40(1), 1995, 119-131 C) 1995, by the American Society of Limnology and Oceanography, Inc.

Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)

Jean-Marie Martin and Min-Han Dai Institut de Biogeochimie Marine, Unite Associee au CNRS No. 386 and Unite de Recherche Marine IFREMER No. 6, Ecole Normale Superieure, 1, Rue Maurice Amoux, 92120 Montrouge, France

Gustave Cauwet Groupement de Recherches: "Interactions Continent-Ocean," GDR CNRS No. 909, Laboratoire Arago, Universite Pierre et Marie Curie, 66650 Banyuls-sur-mer, France

Abstract Colloidal organic C and trace metals from the waters of a highly productive coastal environment (the

Venice Lagoon, Italy) have been separated by a cross-flow ultrafiltration device. On average, 18% of organic C, 34% of Cd, 46% of Cu, 87% of Fe, 18% of Ni, 58% of Pb, and 54% of Mn which previously would have been considered in the dissolved phase are actually associated with colloidal material. Thus, past studies overestimate the dissolved trace-metal concentration in the nearshore environment. Compared to total concentration, the proportion of the colloidal fraction represents on average 15% of organic C, 18% of Cd, 28% of Cu, 1 1% of Fe, 1 1% of Ni, 29% of Pb, and 12% of Mn. This fraction acts differently from the truly dissolved and macroparticulate phases. The behavior of organic C and trace elements during mixing between freshwater and seawater is more complicated than expected when a colloidal fraction is involved. The flocculation of colloids, encountered normally during estuarine mixing, is not very significant on the time scale of mixing in the lagoon. Conversely, the interaction between colloidal and truly dissolved phases seems important. The partitioning of trace metals between different fractions of organic C appears variable, Fe and Mn are preferentially tied to macroparticulate organic matter, and Cu and Cd are preferentially tied to colloidal organic matter in seawater. Truly dissolved organic C appears to be important for Ni. Pb is mainly associated with macroparticulate organic matter at most stations except in the highly productive region where Pb prefers colloidal organic C.

Colloids are operationally defined as particles between 1 nm and 1 ,um (Vold and Vold 1983). This family of particles is thus not fully retained by filters with pore sizes between 0.22 and 0.7 ,um, which leads to their inclusion in the so-called dissolved fraction in most aquatic chemistry studies. To clarify notation, we define the filter-pass- ing fraction (<0.4 ,um) as the total dissolved fraction and the permeate from the ultrafiltration (<104 Daltons) as the truly dissolved fraction. Consequently, speciation, bioavailability, and partitioning between dissolved and particulate phases for a given chemical element or com- pound may not be assessed correctly. Although the role that colloidal material plays in ocean biogeochemistry remains controversial, several papers have emphasized its significance in understanding the biogeochemical cycling of organic C, trace elements, and radionuclides (Lee and Wakeham 1992; Buffle and Van Leeuwen 1992; Ben- ner et al. 1992; Moran and Buesseler 1992, 1993).

Acknowledgments This work was carried out in the framework of the UNESCO

Venice Lagoon Ecosystem project, a part of the Venice Lagoon System project coordinated and financed by the Italian Ministry for the Universities and for Scientific and Technological Re- search. We thank W. W. Huang for assistance during the field sampling and analytical work. Ray C. Griffiths is especially appreciated for valuable comments. Finally, we also acknowl- edge B. P. Boudreau, R. Cole, and the four anonymous reviewers for critical and constructive comments on the manuscript.

Indeed, colloids have been reported to be ubiquitous components of seawater (Koike et al. 1990; Wells and Goldberg 1991, 1992), freshwater (e.g. Orlandini et al.- 1990), and groundwater (e.g. McCarthy and Zachara 1989), as well as pore waters (e.g. Chin and Gschwend 1991). Several studies have shown that macromolecular organic matter accounts for 5-45%, but more typically 15-20%, of the DOC (dissolved organic C) in marine environments (Carlson et al. 1985). Sigleo et al. (1983) concluded that 50% of the DOC in the waters of Ches- apeake Bay is colloidal, which could be isolated by ultrafiltration. In the Lena River, 57% of the DOC was colloidal material (Cauwet unpubl. data). These organic- rich colloids are expected to be partly responsible for the transport and ultimate fate of trace metals in aquatic systems. Another striking aspect of these macromolecular-colloidal materials is that they are extremely reactive and have short residence times; hence they might play a fundamental role in the biogeochemical cycling of organic C (Moran and Buesseler 1992, 1993; Benner et al. 1992).

Colloidal trace metals are also commonly found in estuarine and riverine waters (Sigleo and Helz 1981; White- house et al. 1990; Benoit et al. 1994). For various trace metals, the colloidal fraction is predominant in the so- called dissolved fraction. Hart et al. (1992) reported that 62% of Cu, 99.6% of Fe, and 99.9% of Zn in the total dissolved phase (<0.2,um) were actually associated with colloidal material in polluted sections of the Tambo Riv- er.

119



120 Martin et al.

Although information on colloids in the aquatic environment is growing rapidly, we still have a poor understanding of the role of colloids in various biogeochemical processes. This is particularly true in estuarine and coastal systems, where colloids may govern the fluxes of organic matter and trace metals to the ocean. This study illustrates the distribution and significance of colloidal organic C and associated trace metals compared to their truly dissolved counterparts. The role of colloids in scavenging trace metals and partitioning metals between dissolved and solid phases is addressed. To facilitate this study, we selected a highly productive and organic-rich environment (Venice Lagoon, Italy), where colloids were expected to be abundant.

Materials and methods

Environmental background-The Venice Lagoon is a complex enclosed embayment connected to the north- western Adriatic Sea through three narrow entrances: the Lido, Malamocco, and Chioggia inlets. The whole area ( 550 kM2) is subject to tidal intrusion and characterized by an average water residence time of 1 d. The lagoon is surrounded by a heavily populated area associated with an active harbor and many industrial and agricultural activities. As a consequence, the quality of the lagoon has been seriously modified by intense eutrophication as well as by the development of a widespread and temporary anoxia (Degobbis et al. 1986; Sfriso et al. 1988, 1989). Heavy metals have been discharged into the lagoon during past decades, mainly from the Marghera industrial area (Donazzolo et al. 1984). However, some recent studies of trace-metal concentration in water of the central part of the lagoon did not show any significant enrichment compared to other coastal environments. It has been as- sumed that these low values were the consequence of the short flushing time of the lagoon by Adriatic seawater and of the strong binding of trace metals with the sediments (Martin et al. 1994).

Sampling and prefiltration - Sampling was done in July 1992. Seven stations were selected along the salinity gradient from Porto di Lido, which can be regarded as the Adriatic seawater end-member, to the Silone Channel, which represents one of the sources of freshwater to the northeastern part of the lagoon. Brackish samples were collected in the mixing zone near Crevan Island in the Burano Channel and at Palude della Rosa (Fig. 1).

The study area might differ from other typical estuaries in that it receives very limited freshwater inflow; hence, the telluric sources of either trace metal or organic C are limited. Additionally, the study area contains a highly productive mixing zone, Palude della Rosa, which is one of the most eutrophic parts of the lagoon. Consequently, organic material, including colloids, should be mostly autochthonous. These features could significantly influence the cycling of trace metals and organic C in this area.

Surface samples were collected by hand from a small rubber boat with acid-cleaned 10-liter polypropylene bottles. One subsurface sample (station 1, referred to as 51-

2) was taken at a depth of 2 m with an air-driven Teflon pump fitted with PTFE tubing and connectors. Within 2 h of sampling, 15 liters of water were prefiltered in the laboratory with acid-cleaned Nuclepore filters (0.4 ,m, 4 142 mm) and a Teflon filter holder in a closed pressurized system (pure N2 gas). Both the filter holders and connectors were made of PTFE and were precleaned by soak- ing in acid. To avoid any adsorption or release onto or from the Nuclepore filter, we discarded the first 1-2 liters of filtrate. Then we collected 50 ml of filtrate for DOC analysis, with HgCl2 added as a preservative. We acidified 500 ml of prefiltrate to pH 2 with concentrated HNO3 (Suprapur) for total dissolved trace-metal analysis. An- other portion of the samples (- 1 liter) was passed through precombusted Whatman glass-fiber filters (GF/F, 0.7 ,um) on a glass filter holder (Millipore); these filters were stored for later POC (particulate organic C) measurement. The remaining prefiltrate (- 10-12 liters) was processed by cross-flow ultrafiltration to isolate the colloids from the truly dissolved phase.

Cross-flow ultrafiltration (CFF)-Our CFF system, modified from the Millipore Pellicon cassette system (Millipore Corp.), is similar to those used by other in- vestigators (e.g. Whitehouse et al. 1990; Moran 1991). The Millipore system is connected to an air-driven Teflon pump and to a polypropylene reservoir by Teflon valves and tubing. The permeation pressure is displayed by a pressure gauge installed on the upstream end of the membrane. A Teflon gauge-protector is fitted between the pressure gauge and the Millipore cassette to prevent contact between the metal gauge and the sample. This cassette system contains a PTGC-00005 filter cassette in an acrylic housing that contains a polysulfone filter with a nominal molecular weight cutoff (MWCO) of 104 (corresponding to a pore size of -3 nm) and a filter area of 0.462 M2.

Before sampling, the whole CFF system was cleaned with several acid washes. Between samples, the system was cleaned by flushing with Milli-Q water, cycling with 0.1 N HNO3, then with Milli-Q water, and finally with

1,000 ml of sample to condition the system. The CFF system was completely closed to avoid any risk of contamination. Two three-way Teflon valves were installed on both the permeate and retentate lines to allow easy change between flushing, recycling, and concentrating modes. Another three-way valve on the permeate line provided access to 500- or 1,000-ml collection bottles (Nalgene) while maintaining sealed conditions. Ultrafil- tration was then carried out continuously under the concentration mode until the sample volume was reduced 1 liter. During this operation, 500 ml of the permeate

for trace-metal analysis and 50 ml for organic-C measurement were collected. The prefiltered (JV,) colloid con- centrate volumes (Vr) were recorded to determine the concentration factor (F = V,/ Vr).

All sample handling was carried out in laminar air-flow clean benches in the laboratory to minimize contamination hazards.

Chemical analysis-Organic C was determined in the so-called total dissolved fraction <0.4 ,um (DOC), in the



Colloidal organic C and trace metals 121

.. ... ....... . ... .. . .... .... .......... ..................... ..........- .......... -.. .............. ............... - ... ... ... ... ... ... .. .. .. ...... ...... ... ..... ... ...... .... ... km ...... .................. .......... ............. --- .................- ............ ............... ............. ..................... ........................................... .................. ........................................ ........... -,-- ................. ............. ...... ............... ................. . ...... .... . . ... ... ... 11 .... . ... .. .. ... ..... .... .......... . ............. .................... .................. ... ....... . ....... ........... ........... . ... ...... .................... ............. . . .......... .............. . ... ............ ............................................. .. . ....... ....... .................. ..........

... .................. . .... ............ ............ . . ... ......... .............. ---- % .:, . ............ . .......... - ........ ..... ....... ...... ........ . . . . . . . . . ... . . . . . . ............. ......... ..................... ..................

............................ ........... ............ --- .......... V E N IC E ................. -- ....... ............... ....... Palude ......................... LAGOON di Cona ............... ........... ............... .......... ........ ... ............. X ......... . ............ ... A . .... ........ ... . .. ......

de ............. Palu della Rosa

.............. 20 130 140 1 5 ............. rx ..... ....... Ore .................. . ............. ........... .... ......

............... ...... ............... ...... ............... .................. .... ..... .. . .. .... .. .. .. ... .. ... ... ..... . ..... . . .. .. ....... ....................... .. .. .. ...........

.......... ...... 0 - ..........

.......... . . . . . . . . . . . . . . . . . . ............ Porto ........ ......... . .. . . . . . . . . . . . .. . . . . ......... ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . ... - - - - - -- di Lido S t. 1 ........ ........... ........... ...... ....... N o rth . ...... ... .. .... .. . .. .. .. .. . ... .. .. . . . . . . .. . .. . .. . . . .. . . . . . . . . . . . . . . . . . . .. . . . ........... ...... .... .... ....-.. . ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... - 1- - ............ A d ria tic S e a ..................... ............-...... ............. ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... ........ ........ . .... . ... .. ..... .. ... . ..... .. .. .. . .............. .............. ....... ADRIATIC SEA .... ....... . . . . . . . . .. .. .. . . . . .. . ....... ....... - 1- ........... .. ...... . . . . . . . . . . . . . . .. . . . . . . . . . . .. ............. .........I............. . . ............. 11- 1.1 .............. ......... .. ...................... ............. .......... . . . . . . . . . .. . . . .. . . . .. . . . . . .. . . . . .. .. .. . ................. ..........

Fig. 1. Sampling stations in the Venice Lagoon.

ultrafiltrate (UOC < 3 nm), and in the colloidal (COC, between 3 nm and 0.4 ,xm) and particulate fractions (POC > 0.7 ,Am). DOC, UOC, and COC were measured by a high-temperature catalytic oxidation method (HTCO) with a TOC-5000 Instrument (Shimadzu). The oxidation of organic compounds was performed by direct injection of the sample into a furnace at 680?C on a catalyst made of 1.2% Pt coated on SiO2. The CO2 thus formed was analyzed with a nondispersive infrared (NDIR) detector. More detailed information on this technique is given by Cauwet (1994). POC was measured according to the method of Cauwet (1983).

Mn, Fe, Cu, Ni, Cd, and Pb in the prefiltrate and ultrafiltrate were measured by graphite furnace atomic-ab- sorption spectrophotometry (GFAAS) after extraction in a Class 100 clean room using a method modified from Danielsson et al. (1982). Extractions were done at pH 4.5 ?0.5 for Fe, Cu, Ni, Cd, and Pb and at a pH range between 9.0 and 9.5 for Mn. Recoveries of 85-100% were obtained when compared to the NASS-4 standard seawater reference material. The colloidal concentration was calculated as the difference between the total dissolved and truly dissolved concentration. Total trace-metal concentrations in particulate matter (>0.4 ,.m) were also determined by GFAAS after digesting the filter with a mixture of HNO3, HC104, and HF.

Blank check-Contamination risks in the CFF system were tested in two ways. First, a test was designed to determine the maximum blank of the whole system. We recycled 10 liters of Milli-Q water continuously for 2 h

to reach an equilibrium between the system and the water. Concentrations of organic C and trace metals were then determined (for trace-metal determination, preconcen- tration is necessary). Organic C and trace-metal concentrations in the Milli-Q water before cycling were 18 ,uM for organic C, 0.9 pM for Cd, 22 pM for Cu, 100 pM for Fe, 14 pM for Mn, 21 pM for Ni, and 7 pM for Pb. As a comparison, blanks after the cycling were 23 zM for organic C, 1.19 pM for Cd, 40 pM for Cu, 189 pM for Fe, 25 pM for Mn, 34 pM for Ni, and 14 pM for Pb. These concentrations after the recycling step included blanks resulting from water and reagents and those due to the CFF system itself. These blank levels are reasonably low for most trace-element determinations in coastal waters, indicating the utility of the CFF system for trace analyses.

The second test involves calculation of a recovery factor (R%) for samples during ultrafiltration:

R(%) = 100 x (F- 1) x C + Cr F x Cp

F is the concentration factor, Cu is the concentration of the truly dissolved fraction (<3 nm), Cr is the concentration of retained material (concentrated solution or colloids), and Cp is the concentration of the solution after prefiltration (<0.4 ,tm). After HgCl2 is added as a preservative, the retentate COC sample can be stored for several months without precipitation. However, precipitation occurs in the concentrated samples for trace-metal analysis even after acidification. Consequently, the re-



122 Martin et al.

14 12 -u-C

-*- Cd

V 8

0 x 3 g -S- g~~~~x C

L. ~~~~~~~~~~~~0Fe

0 30 59 94 136

Time (min)

Fig. 2. Trace-metal concentration variations in the ultrafiltrate during cross-flow ultrafiltration cycling as a function of time. Test samples taken from the River Seine.

covery check was only available for organic C. A loss of material occurs when R% < 100; conversely, the system has been contaminated when R% > 100.

For most samples, the recovery factors are 85-98%, with an average recovery of 91% (?5%), indicating that there is no significant organic-C contamination or loss during ultrafiltration and that organic C can be quanti- tatively recovered by the ultrafiltration experiment. An exception is for the freshwater sample (station 7), where recovery was relatively low indicating a slight loss (- 18%) of organic C during the CFF treatment. We have obtained similar low recovery of organic C in a River Rhone water sample (Dai et al. 1995).

We used freshwater from the River Seine (France) to test for possible adsorption of metals onto the CFF membrane. Concentration variations during CFF recycling were determined in the ultrafiltrate as a function of time (up to 1 3 6 min). Concentration variations were not significant (Fig. 2); therefore, the adsorption of metals onto the membrane is probably not important on a time scale of min- utes to 2 h. In fact, another study has already calculated the mass-balance from CFF processing (a membrane similar to ours), and the recoveries have been shown to be acceptable (72-124%, Whitehouse et al. 1990).

Results

Organic carbon-DOC, COC, and UOC concentrations and their variations are shown in Table 1. Com- pared to other Mediterranean coastal regions, the Venice Lagoon is characterized by a high level of organic material, most likely related to its relatively high primary production (10 times higher than in the Adriatic Sea). For example, the average total [DOC] in the area studied (- 200 AM) is almost twice that in the Rhone delta (- 1 19 AM; Dai et al. 1995). k

COC, with an average concentration of 33.4 AM, represents 10.4-26.0% of the DOC, with a maximal percentage in riverine water (station 7). This concentration indicates that most of the total DOC has a low nominal molecular weight (<104) and corresponds to truly dissolved organic C. Sempere (1991) used hollow-fiber filtration through a metallic membrane (MWCO = 104 Dal- tons) to obtain a similar percentage of COC (26%) in the River Rhone, while Dai et al. (1995) found that some 30% of total DOC was in colloidal form.

Because of the different techniques and membranes used in the ultrafiltration experiments, any comparison between results from different studies is difficult. For in- stance, some workers use 1,000 Daltons as the boundary between colloids and truly dissolved solutes (Benner et al. 1992; Guo et al. 1994). Nevertheless, it is now evident that COC contributes significantly to the total dissolved fraction. Benner and Hedges (1993) reported that 76% of total DOC in the Amazon River system was in a colloidal form, with particles from 0.2 Am to 1,000 Daltons. These results are in agreement with the DOC fractionation studies of Carlson et al. (1985). On the basis of our study, COC and POC account for similar proportions of the TOC. This equivalence provides indirect evidence for the importance of COC because POC is considered to play a central role in the cycling of organic C and in the scavenging of trace elements.

For most of ours stations, DOC, UOC, and COC are distributed relatively uniformly except at station 3, where concentrations were high (Fig. 3). It is logical to relate this high organic-C content to biological production. Sta-

Table 1. Organic C fractions in the Venice Lagoon in July 1992: TSM-total suspended matter; TOC-total organic C; UOC- truly dissolved organic C (< 104 Daltons, corresponding to -3 nm); DOC-total dissolved organic C (<0.4 ,um); COC-colloidal organic C (3 nm-0.4 ,m); POC-particulate organic C (>0.7 ,um).

UOC DOC COC

TSM TOC UOC UOC/TOC DOC DOC/TOC COC COC/DOC Station (mg liter-1) (AM) (AM) (%) (AM) (%) (AM) (%)

Lido-SI 5.04 211.75 148.42 70.09 176.42 83.31 28.00 15.87 Lido-Sl-1 3.96 187.75 140.50 74.83 170.42 90.77 29.92 17.56 Lido-S1-2 4.86 188.50 140.08 74.31 175.17 92.93 35.08 20.03 Crevan-S2 10.11 226.75 152.33 67.18 183.33 80.85 31.00 16.91 Palude-S3 23.89 425.42 267.17 62.80 312.25 73.40 45.08 14.44 Silone-S4 11.06 277.67 210.00 75.63 234.33 84.39 24.33 10.38 Silone-S5 5.32 303.83 198.50 65.33 228.17 75.10 29.67 13.00 Silone-S6 6.26 253.50 144.17 56.87 191.92 75.71 47.75 24.88 Silone-S7 3.47 149.75 86.33 57.65 116.67 77.91 30.34 26.00




tion 3 is at the entrance of Palude della Rosa, and a very high primary production associated with a diatom bloom was observed at this location (an average of 101.4 mg C m-3-h-1 in July 1992, Bianchi unpubl. data). Socal et al. (1987) also found the maximum abundance of phytoplankton in the northern basin of the lagoon at this station. Conversely, the organic-C content in freshwater (Si- lone, station 7) was lower than that measured at station 1, which is greatly influenced by the Adriatic Sea. Thus, organic matter (both dissolved and colloidal fractions) appears to originate from in situ biological activity, produced either by the decay of dead organisms or by ex- cretion from plankton. As mentioned by S0ndergaard and Jensen (1986), dissolved organic matter (DOM) in the colloidal range can contribute about a quarter of the DOC excreted by phytoplankton. Another study of colloid composition by Sigleo et al. (1982) demonstrated that plankton can be a main source for colloidal material, while terrestrial organic matter need not be important. Isotopic measurements (Sigleo 1985) further indicated that colloids from surface waters were of planktonic origin, while those in deeper waters corresponded to the less labile components of the organic matter. Indeed, Wells and Goldberg (1991) provided direct evidence that marine colloids are mostly organic. Organic colloids in the ocean are likely to be of biological origin and related to primary productivity. Our study area, where terrestrial inputs are limited, is similar.

Trace metals -Trace-metal colloidal fractions are much more significant than organic C (Table 2). On average, 34% of Cd, 46% of Cu, 87% of Fe, 18% of Ni, 58% of Pb, and 54% of Mn of the so-called dissolved phase are associated with colloidal materials. The proportion of colloidal to total metal concentrations (including particulate) is on average 18% for Cd, 28% for Cu, 1 1% for Fe, 11% for Ni, 29% for Pb, and 12% for Mn. By comparison, contributions of macroparticulate metals to total metal concentrations are 42% for Cd, 36% for Cu, 89% for Fe, 14% for Ni, 52% for Pb, and 77% for Mn.

Colloidal Cu contributes 20.5-59.2% of the total dissolved Cu. The average colloidal Cu concentration (4.98

Table 1. Extended.

COC POC COC/TOC POC POC/TOC POC/TSM

M% (A4M) (%/) (%/) 13.22 35.33 16.69 8.40 15.93 17.33 9.23 5.30 18.61 13.33 7.07 3.40 13.67 43.42 19.15 5.20 10.60 113.17 26.60 5.70 8.76 43.33 15.61 4.70 9.76 75.67 24.90 17.10

18.84 61.58 24.29 11.80 20.26 33.08 22.09 11.40

800

600 E POG

400 E O Elc [D TSM [1nDOC

200

0 5 10 15 20 25 30 35

Salinity Fig. 3. Distributions of particulate organic C (POC, ,uM),

colloidal organic C (COC, AIM), truly dissolved organic C (UOC, AM), total suspended matter (TSM, mg liter-1), and total dissolved organic C (DOC, AM) along with the salinity.

nM) is equivalent to the particulate Cu (4.90 nM). In Silone Channel, 44% of the total dissolved Cu is in the colloidal phase. This proportion is in agreement with values in the Medway River (53%; Whitehouse et al. 1990), the Westfield River (52%; Whitehouse et al. 1990), and the River Rhone (40%; Dai et al. 1995). In seawater, colloidal Cu is even more enriched (59% at station 1). These values confirm that colloidal material plays an important role in the transport and fate of Cu.

Colloidal Cd varies from 3 to 43 pM and typically represents 30-40% of the total dissolved Cd. Like Cu, colloidal Cd exhibits a concentration comparable to the particulate fraction (15.4 pM). Colloidal Cd in seawater is poorly documented, although Cd is believed to be a particle-reactive element (Elbaz-Poulichet et al. 1987). Dai et al. (1995) found a lower and more variable colloidal Cd fraction in surface waters of the Rhone delta.

Total dissolved Ni (6.7-20.9 nM) is found mainly in true solution, with some 17% in a form with a nominal molecular weight _ 104.

The major fraction of total dissolved Fe (from 67.8 up to 99.7%) appears as colloidal iron. It has long been rec- ognized that colloidal Fe predominates in the operationally defined dissolved phase in many areas, such as the Patuxent estuary (Sigleo et al. 1982), some French rivers (Figueres 1978), and the Ob and Yenisey Rivers (Dai and Martin unpubl. data). Fox (1988) concluded that many previous studies overestimated the dissolved Fe concentration in rivers because dissolved Fe is not effectively separated from colloidal dispersed Fe by conventional filtration techniques. This study provides direct evidence for this point of view. At stations 3, 4 and 5, almost all the total dissolved Fe is in colloidal form.

From 24.1 to 76.8% of total dissolved Mn is present in the colloidal state. These percentages are much higher than in Scotian shelf (11%) and Halifax Harbor (2 1 %) waters (Whitehouse et al. 1990). Colloidal Pb also accounts for most (up to 94%) of the total dissolved Pb, but it shows greater variations than Fe.



124 Martin et al.

Table 2. Trace metal concentrations in the truly dissolved (Ca, nM), total dissolved (Cd, nM), colloidal (Ce, nM), and macroparticulate fractions (PP, nM, per water volume, and PP, ,ug g-1, per macroparticulate mass) in the Venice Lagoon (the unit of macroparticulate Fe concentration per particulate mass is %).

CC/Cd Station C. Cd C, (%) PP (nM) PP (gg g-1)

Cd Lido-SI 0.075 0.110 0.035 31.82 0.014 0.85 Lido-S 1 - 1 0.079 0.080 0.001 1.25 Lido-S1-2 0.083 0.126 0.043 34.13 Crevan-S2 0.061 0.077 0.016 20.78 0.030 0.90 Palude-S3 0.007 0.010 0.003 30.00 0.022 0.36 Silone-S4 0.025 0.039 0.014 35.90 0.029 0.39 Silone-S5 0.007 0.019 0.012 63.16 Silone-S6 0.014 0.024 0.010 41.67 Silone-S7 0.016 0.028 0.012 42.86 0.044 1.24

Cu Lido-SI 4.50 11.05 6.55 59.24 2.41 83.12 Lido-S 1 - 1 3.76 5.64 1.88 33.37 Lido-Sl-2 4.52 10.52 6.00 57.05 Crevan-S2 4.56 5.74 1.18 20.52 2.39 40.99 Palude-S3 3.15 5.69 2.54 44.64 2.00 18.74 Silone-S4 4.22 8.54 4.32 50.59 31.80 239.98 Silone-S5 6.07 13.08 7.00 53.55 Silone-S6 7.07 15.41 8.34 54.11 Silone-S7 8.85 15.82 6.96 44.04 5.48 87.02

Fe Lido-SI 1.46 11.94 10.48 87.74 255.33 0.78 Lido-S l- 1 2.48 7.69 5.21 67.75 Lido-Sl-2 2.50 22.95 20.45 89.12 Crevan-S2 2.63 9.63 7.00 72.72 908.50 1.37 Palude-S3 4.04 299.27 295.23 98.65 2,037.36 1.68 Silone-S4 3.25 1,124.92 1,121.67 99.71 2,361.09 1.57 Silone-S5 5.60 544.24 538.64 98.97 Silone-S6 5.76 58.81 53.06 90.21 Silone-S7 11.37 62.29 50.92 81.74 1,149.27 1.61

Ni Lido-SI 7.10 8.62 1.51 17.55 1.35 43.08 Lido-Sl-l 6.50 6.72 0.22 3.23 Lido-Sl-2 8.13 11.89 3.76 31.63 Crevan-S2 6.59 6.66 0.08 1.13 0.96 15.22 Palude-S3 12.62 14.73 2.11 14.33 2.19 18.94 Silone-S4 11.02 14.57 3.55 24.36 3.38 23.60 Silone-S5 11.46 17.25 5.79 33.55 Silone-S6 12.62 17.41 4.79 27.52 Silone-S7 19.17 20.89 1.72 8.24 2.57 37.73

Pb Lido-SI 0.033 0.050 0.017 34.00 0.08 8.76 Lido-Sl-l 0.049 0.060 0.011 18.33 Lido-Sl-2 0.045 0.115 0.070 60.87 Crevan-S2 0.062 0.064 0.002 3.13 0.14 7.90 Palude-S3 0.021 0.368 0.347 94.29 0.40 12.08 Silone-S4 0.100 1.078 0.978 90.72 0.63 15.50 Silone-S5 0.024 0.441 0.417 94.52 Silone-S6 0.026 0.120 0.094 78.33 Silone-S7 0.156 0.306 0.150 49.02 0.22 11.61

Mn Lido-Si 5.93 21.58 15.65 72.51 31.10 928.62 Lido-Sl-l 4.95 13.59 8.63 63.55 Lido-Sl-2 6.61 11.12 4.50 40.52 Crevan-S2 9.02 13.69 4.68 34.14 58.65 870.81




Table 2. Continued.

Cc/Cd Station C. Cd Cc (%) PP (nM) PP (pg g-1)

Palude-S3 6.39 22.55 16.16 71.67 320.79 2,599.31 Silone-S4 11.68 33.96 22.28 65.60 176.98 1,154.71 Silone-S5 8.00 34.42 26.42 76.75 Silone-S6 27.79 44.26 16.47 37.21 Silone-S7 24.26 31.95 7.69 24.06 63.44 871.33

Discussion

Significance of the colloidal material-UOC was far more abundant than COC or POC and represented the most important pool of either DOC or TOC. The cor- relations between POC and the total dissolved fraction (POC = 0.50 x DOC - 44.83, R2 = 0.68) and between DOC and TSM (DOC = 138.27 + 7.36 x TSM) were positive and significant (R2 = 0.75), showing a link between suspended matter and total DOC (including colloidal C). This relationship was even more obvious when

all organic fractions and TSM were plotted against salinity (Fig. 3). All exhibited a maximum around station 3 linked to higher productivity. Such a distribution of organic C is unlike many other estuaries, where terrestrial sources of organic C dominate. The distribution of COC was somewhat more complicated because it is also controlled by various physicochemical processes during the mixing of freshwater and salt water.

As already mentioned, COC and POC represented an equivalent percentage of TOC. Because colloids offer a much greater surface area than large particles per unit

0.1 y=0.02+ 234x R2=0.59 1 y=26761.49x R2=0.90

0.128

0.10

0 0.06/

0~~~~~~~~~~~~~~ 0.0 0o^U4

0.020 . 0

0.00 0.01 0.02 0.03 0.04 0.0 0 2 4 6 8 10

Colloidal Cd (n.NM) Colloidal Cu (nM)

1200 y2440+1. =1.00 50 y=11.55+1.01x R2-0.46 0

1000 40 -

; 800 / e 30

-600-

X 400 -

200 /

0 0 10 20 30 0 200 400 600 800 1000 1200 Colloidal Mo (nM)

Colloidal Fc (iiMI)

30y= 8.93 + 1.63x K2= 0.41 1.2 y = 0.05+ 1.03x R2= 0.98

1.0

00

0

0 0~~~~~~~~~~~~0 00

0 000

0 1 2 3 4 0.0 0.2 OA 0.6 0.0 f O Colloidal Ni (n.NM) Colloidal Pb (nM)

Fig. 4. Relationship of trace metals between dissolved and colloidal fractions.



126 Martin et al.

20

i 0 0 0 O

10- rA ~~~~~~~~~~0

0co

y =15.68 -0 24x R2 = 71

0O- 0 10 20 30 40

Salinity

30-

20

0~~~~~~~ 0 .3

10~~~~~~~~~~ _10-

y=20.00-031x R2=083

0- 0 10 20 30 40

Salinity

Fig. 5. Dissolved Ni concentration as a function of salinity.

mass, it is likely that COC plays a role as important as POC in the transport and biological cycling of chemical elements and compounds.

Despite its relatively low percentage abundance, COC (<26% of DOC) appeared to play a major role in the transfer and transport of some trace metals (especially Fe, Pb, Mn, and Cu). For example, the concentration of almost all total dissolved trace metals was dominated by their colloidal fraction (Fig. 4).

The maximum colloidal concentrations of Fe, Mn, Pb, and Cd were found at station 4, at the entrance of the Silone Channel and upstream of the Palude della Rosa. Station 4 corresponds to the interface between freshwater and seawater. Conversely, the highest COC content was encountered at station 3 rather than at station 4. If we assume that the organic colloidal material is of local biological origin, the concentration of these four colloidal trace metals should be controlled by both biological activity and physicochemical conditions. Maximum colloidal Cu and Ni concentrations were encountered in lower salinity water. Nevertheless, all the colloidal trace-metal maxima were at neither the highest salinity nor in the freshwater but at the boundary between river water and seawater.

It is somewhat surprising that colloidal metals occur in seawater at levels similar to those measured at other stations, e.g. station 1 (S- 35, Porto di Lido). Although we are not sure of the origin of these highly stable colloids

9-

8

a 7

, 6\

0 3 O 10 20 30 40

Salinity

16 - - - --

14

a 12

0~~~~~~~~~~~ 10-~~~~

6- 8 Cb

4 0 10 20 30 40

Salinity

Fig. 6. Dissolved Cu concentration as a function of salinity.

in seawater, some points must be considered. In a highly productive area such as the Venice Lagoon, it might be expected that most colloids are of in situ biological origin as noted above. They could be stable in the lagoon. In addition, tidal currents (up to 2.69 m s-'; Tosi 1970) may prevent colloids from settling.

Role of colloids in scavenging trace metals during mixing offreshwater and seawater-Ni displays conservative behavior for both the truly and the total dissolved fraction during mixing between seawater and freshwater (Fig. 5). Conversely, Cu exhibits nonconservative behavior especially for the truly dissolved fraction (Fig. 6), which is scavenged from the dissolved phase. This is quite different from the Rhone delta (Dai et al. 1995) and the Ob- Yenisey estuaries (Dai and Martin unpubl. data), where both dissolved and colloidal Cu behave conservatively.

As already mentioned, Cd exhibits a higher concentration in the Adriatic than in the Silone Channel, which may indicate an input of dissolved Cd from the Adriatic. Furthermore, total dissolved Cd is in excess when compared to the mixing line between freshwater and seawater (Fig. 7). Such addition is generally attributed to the desorption of Cd from macroparticles (e.g. Elbaz-Poulichet




0.10 '

0.08

0

U 0.06e -SSS /

* 0.04 0~~~~~~

E 0.02 -

0 0

0.00 0 1o 20 30 40

0.14. -Salinity

0 0.12

: 0.10

0.08 , -

? 0.06., Ar ,-

I

c 0.04. ,

0.02 0 0

0.00., 0 10 20 30 40

Salinity

Fig. 7. Dissolved Cd concentration as a function of salinity.

et al. 1987). Such desorption may also occur in the Venice Lagoon because the distribution coefficients (see later) are relatively low at higher salinities (stations 1 and 2). In addition, the desorption of Cd from macroparticles might be related to the affinity of Cd for both total DOM and POM (i.e. there might be competition between DOM and POM for complexation with Cd). Such a mechanism is suggested by the trends in Fig. 8.

Total dissolved and colloidal Fe and Pb behave simi- larly, but their truly dissolved fractions showed no significant relationship (Fig. 9). The nonconservative behavior of Fe and Pb (especially Fe) in estuaries is a well documented and almost universal feature during estuarine mixing (Boyle et al. 1977). This removal was also observed in the Venice Lagoon, but surprisingly, only the truly dissolved fraction was removed from solution (Fig. 10). As has been noted previously, total dissolved Fe and Pb concentrations were dominated by their colloidal fractions. The distribution patterns oftrace metals were mainly determined by colloids (Fig. 4), with a maximum value in the Palude della Rosa, where biologically produced colloids ultimately controlled the total dissolved concentration.

This difference between the truly dissolved and total dissolved fractions was particularly critical for Mn. An apparent conservative distribution was observed for total dissolved Mn, whereas the truly dissolved Mn and col-

400

=L 0 300\

C , 0

_ 100

y = 342.05 - 183.41x R = 0.87

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Particulate Cd (ppm)

300

v 200

, 100

= 305.27 - 176.71x R 0.92

0 0.0 0.2 (04 0.6 0.8 1.0 1.2 1.4


12-

10~~~~~~~~~~~

8

L 6 ?

FE. 4

2-

y = 2.54 + 6.06x R2 0.65 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4


Fig. 8. Relationship between particulate Cd and various fractions of organic C.

loidal Mn were actually nonconservative. Truly dissolved Mn showed a concave upward regression on salinity, and colloidal Mn was found in excess relative to the theoret- ical dilution line (Fig. 11). This discrepancy probably resulted from a dynamic equilibrium between the colloidal and truly dissolved phases (i.e. a transformation from the truly dissolved phase into the colloidal phase). This transformation results from both adsorption (or complexation) with colloids and uptake by colloid-size



128 Martin et al.

12 y =0.09 + 8.47e-4x R2 =0.95

1.0

.o 0.8

c 0.6/

0~~~~~~ 0.4 0

0

0.2e

0.0........... 0 200 400 600 800 1000 1200

Total dissolved Fe (nM)

1.0y = 3.86e-2 + 8.27e-4x R2 (.98S

0.8

0.6

.0~~~~~~ r0.4 0

0

02 O

0 200 400 (600 800 1000 1200 Colloidal Fe (nM)

Fig. 9. Relationship between Fe and Pb in various fractions.

phytoplankton species that are included in the colloidal fraction. The formation of fresh colloids via precipitation of Mn oxides is also possible. The in situ biological production of colloids is likely more important than input from freshwater. Thus, flocculation of riverine colloids is less important than in other estuaries. As a result, total dissolved Mn behaved conservatively.

Figure 11 illustrates a critical point about the kinetics of metal scavenging in terms of "colloidal pumping": the transfer of dissolved species to large aggregates via colloidal intermediates as proposed recently by Honeyman and Santschi (1989). Colloidal pumping involves two steps: the rapid formation of metal-colloid surface site complexes (i.e. adsorption) and the slow agglomeration and coagulation of these colloids into macroparticles. The second step limits the scavenging rate. In the Venice La- goon, the coagulation of colloids appeared to be unim- portant, so most of the trace metals remained in colloidal form on the time scale of estuarine mixing. That is to say, the second step of this pumping process is not always observable because of the rather slow rate of coagulation, while truly dissolved Mn is converted rapidly to colloidal form in the Venice Lagoon. Nelson et al. (1985) also reported that COC in natural aquatic systems could in- hibit adsorption of reduced plutonium onto suspended

12

_ 10

GJ 8

06- ? 0

,4 \-

2-

0 0 10 20 30 40

Salinity

0.2

I-

CE0.1

r \

0 L a

0.0 o 10 20 30 40

Salinity

Fig. 10. Concentration of Fe and Pb in truly dissolved fractions as a function of salinity.

sediment particles. However, Moran and Buesseler (1993) reported that the residence time of colloidal 234Th in con- tinental shelf waters off New England was O. 5 d. They suggested that colloid aggregation might not be rate lim- iting in controlling the scavenging of 234Th and, by anal- ogy, other particle-reactive trace metals. These counter-

50,

^ Total dissolved \In

40- Colloidal Mn

30 Ideal mixin' line f'or truly dissolved M4n

o .S

0

1~~~~~ ~~0 20-

0 20 30 Salinity 40

Fig. r 1d. Concentration of total dissolved (), truly dissolved (@), and colloidal (0) Mn as a function of salinity.




Table 3. Distribution coefficients (normalized to organic C) beween macroparticulate and truly dissolved trace metals [Ko,, (Mp/ POC)/(Md/UOC)] and between colloidal and truly dissolved trace metals [Koc, (MC/COC)/(Md/UOC)] where M, is the macroparticulate metal concentration, Mc is the colloidal trace metal concentration, and Md is the truly dissolved metal concentration.

Cd Cu Fe Ni Pb Mn

Station Ko, Kop Koc Kop Koc Kop Koc Kop Koc Kop Koc Kop Lido-SI 2.47 0.78 7.70 2.24 37.93 732.60 1.13 0.80 2.73 9.80 13.98 6.62 Crevan-S2 1.29 1.70 1.27 1.84 13.10 1,213.37 0.06 0.51 0.16 7.98 2.55 9.11 Palude-S3 2.54 7.32 4.78 1.50 433.50 1,191.75 0.99 0.41 97.92 44.41 14.99 126.03 Silone-S4 4.83 5.69 8.83 36.50 2,982.23 3,525.04 2.78 1.49 84.40 30.53 16.46 26.98 Silone-S7 2.13 7.22 2.24 1.62 12.742 263.77 0.26 0.35 2.74 3.75 0.90 2.85

examples imply that colloidal pumping (transformation from dissolved to particulate phase through a colloidal intermediate) is largely related to the physicochemical features of the waters in which the colloids are dispersed.

It might also be concluded that the flocculation of colloids in our study area was not important as is implied by the distributions of Fe, Pb, and Mn. But the transformation between colloids (including colloid-size phytoplankton) and the truly dissolved phase is important. This could be explained by the original characteristics of the colloids as proposed earlier and by the low input of colloids (particles) by freshwater. At the same time, it is worth noting that the behavior (conservative or nonconservative) of a specific element (e.g. Cd) during estuarine mixing is related to its reactivity with respect to different fractions of organic materials.

Partition of trace metals between different organic C fractions-To assess the distribution of metals between various fractions of organic C (to compare the capacity of different fractions of organic C to complex with trace metals), we define two distribution coefficients normalized to organic C: Koc and Ko, The first represents the distribution coefficient between macroparticulate and truly dissolved and the second that between colloidal and truly dissolved trace metals (Table 3). These two coefficients depict the complexing capacity of DOC and COC as compared to UOC. Except for Ni, all the Koc and Ko, values are > 1, suggesting that the particulate phases (either colloidal or macroparticulate) have a stronger affinity for metals than the truly dissolved organic matter does. The trace-metal affinity for colloidal organic matter and for macroparticulate organic matter is highly variable between stations and between elements. The data clearly show that Fe and Mn are preferentially associated with macroparticulate organic matter. Cd also seems to prefer macroparticles, with the exception of the seawater station (station 1). This exception may be related to the nonconservative behavior of Cd during estuarine mixing, as noted earlier. Conversely, Cu and Ni prefer COC, especially in seawater (e.g. station 1). In freshwater, Cu and Ni seem to prefer macroparticulate organic matter. The partitioning picture of Pb between the three organic fractions is less systematic. Generally, Pb tends to associate with macroparticulate organic matter. However, at highly pro-

ductive stations, Koc for Pb is two times larger than K0,, which may be related to biological activity.

Conclusions

Our data indicate that the Venice Lagoon ecosystem (at least in our study area) does not seem to be significantly polluted with respect to the trace metals we studied. This situation has already been described by Martin et al. (1994). It is especially noteworthy that this remains valid when the truly dissolved concentration is considered. Conversely, the total DOC concentrations in the lagoon, in Palude della Rosa in particular, are markedly higher than other Mediterranean coastal regions due to high primary productivity. In turn, this high primary productivity largely determines the distribution of organic C, leading to spatial distribution patterns with maxima at intermediate salinities (Palude della Rosa) rather than at the river end-member as in other estuaries. This characteristic indicates that organic matter is mainly autochthonous in the study area.

The colloidal fraction behaves differently from the truly dissolved fraction, although this behavior differs from one element to another. Information on the involvement of colloids in biogeochemical processes greatly enhances the understanding of their distribution and behavior (e.g. conservative or nonconservative) in the aquatic environment. COC has been shown to act as a major ligand for the formation of organic-metal complexes in spite of its relatively low mass (<26% of the DOC).

A critical aspect of the Venice Lagoon is the importance of the interaction between solutes and colloids. There is no evidence for any significant flocculation of colloids on the time scale of estuarine mixing, suggesting high stability of colloidal material in the study area. Such stability may come about because organic matter is produced in situ rather than being derived from terrestrial sources.

The competition between different fractions of organic C for complexation with trace elements seems rather variable. Fe and Mn are preferentially tied to macroparticulate organic matter, whereas Cu and Cd are preferentially tied to colloidal organic matter in seawater. UOC appears to be important for Ni. Pb is mainly associated with macroparticulate organic matter at most stations,



130 Martin et al.

with the exception of the highly productive region, where Pb prefers COC.

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Submitted: 6 August 1993 Accepted: 16 June 1994 Amended: 19 July 1994

Erratum In the note by Hansell and Newton (September 1994: Vol. 39, No. 6), figure 2 should be as shown here.

H-- 7.0 cm --

A Swimmer

r t 27 scavenging 2.5 ~~~~~surfaces

Particle Passageway

T I | Exit hole to 2.2 cm <>$] 3.7 swimmer collection 22cm F<hnh

Pat of em chamber _Path of .................... (2.0 cm diameter) swimmer .H-1.6 -i

I t cm \ Wall of inner

Particles chamber (0.33 cm thick)



significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the...

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