Significance of Colloids in the Biogeochemical Cycling of Organic Carbon and Trace Metals in the Venice Lagoon (Italy)

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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 .Accessed: 18/06/2014 03:13Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp .JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org. .American Society of Limnology and Oceanography is collaborating with JSTOR to digitize, preserve andextend access to Limnology and Oceanography.http://www.jstor.org This content downloaded from 195.78.109.49 on Wed, 18 Jun 2014 03:13:29 AMAll use subject to JSTOR Terms and Conditionshttp://www.jstor.org/action/showPublisher?publisherCode=limnochttp://www.jstor.org/stable/2838254?origin=JSTOR-pdfhttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jspLimnol. 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 chem- istry studies. To clarify notation, we define the filter-pass- ing fraction (120 Martin et al. Although information on colloids in the aquatic en- vironment is growing rapidly, we still have a poor un- derstanding of the role of colloids in various biogeochem- ical 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 scav- enging trace metals and partitioning metals between dis- solved 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 dur- ing past decades, mainly from the Marghera industrial area (Donazzolo et al. 1984). However, some recent stud- ies 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 gra- dient 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 influ- ence 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 bot- tles. 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 con- nectors 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 mem- brane. A Teflon gauge-protector is fitted between the pres- sure 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 con- tamination. 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 con- centration 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 mea- surement 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 contami- nation hazards. Chemical analysis-Organic C was determined in the so-called total dissolved fraction 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 ul- trafiltrate 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 sea- water reference material. The colloidal concentration was calculated as the difference between the total dissolved and truly dissolved concentration. Total trace-metal con- centrations 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 concen- trations 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 wa- ters, indicating the utility of the CFF system for trace analyses. The second test involves calculation of a recovery fac- tor (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 (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 ultrafil- trate 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 mem- brane. 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 mem- brane 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 sim- ilar to ours), and the recoveries have been shown to be acceptable (72-124%, Whitehouse et al. 1990). Results Organic carbon-DOC, COC, and UOC concentra- tions 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 ma- terial, 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, rep- resents 10.4-26.0% of the DOC, with a maximal per- centage in riverine water (station 7). This concentration indicates that most of the total DOC has a low nominal molecular weight (Colloidal organic C and trace metals 123 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 phyto- plankton in the northern basin of the lagoon at this sta- tion. 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, pro- duced 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 or- igin, while those in deeper waters corresponded to the less labile components of the organic matter. Indeed, Wells and Goldberg (1991) provided direct evidence that ma- rine 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 partic- ulate) 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 dis- solved 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 dis- solved 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 val- ues 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 im- portant 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 operation- ally 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 concen- tration 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 ac- counts for most (up to 94%) of the total dissolved Pb, but it shows greater variations than Fe. This content downloaded from 195.78.109.49 on Wed, 18 Jun 2014 03:13:29 AMAll use subject to JSTOR Terms and Conditionshttp://www.jstor.org/page/info/about/policies/terms.jsp124 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 This content downloaded from 195.78.109.49 on Wed, 18 Jun 2014 03:13:29 AMAll use subject to JSTOR Terms and Conditionshttp://www.jstor.org/page/info/about/policies/terms.jspColloidal organic C and trace metals 125 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 be- tween suspended matter and total DOC (including col- loidal 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 con- trolled 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. This content downloaded from 195.78.109.49 on Wed, 18 Jun 2014 03:13:29 AMAll use subject to JSTOR Terms and Conditionshttp://www.jstor.org/page/info/about/policies/terms.jsp126 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 (Colloidal organic C and trace metals 127 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 sig- nificant relationship (Fig. 9). The nonconservative be- havior of Fe and Pb (especially Fe) in estuaries is a well documented and almost universal feature during estua- rine 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 frac- tions. 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 concen- tration. 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 Particulate Cd (ppm) 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 Particulate Cd (ppm) 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 col- loidal 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 This content downloaded from 195.78.109.49 on Wed, 18 Jun 2014 03:13:29 AMAll use subject to JSTOR Terms and Conditionshttp://www.jstor.org/page/info/about/policies/terms.jsp128 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 pro- duction 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 col- loidal 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 frac- tions 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. This content downloaded from 195.78.109.49 on Wed, 18 Jun 2014 03:13:29 AMAll use subject to JSTOR Terms and Conditionshttp://www.jstor.org/page/info/about/policies/terms.jspColloidal organic C and trace metals 129 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 macropar- ticulate 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 col- loids in our study area was not important as is implied by the distributions of Fe, Pb, and Mn. But the transfor- mation between colloids (including colloid-size phyto- plankton) 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 col- loids (particles) by freshwater. At the same time, it is worth noting that the behavior (conservative or noncon- servative) 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 normal- ized to organic C: Koc and Ko, The first represents the distribution coefficient between macroparticulate and tru- ly dissolved and the second that between colloidal and truly dissolved trace metals (Table 3). These two coeffi- cients 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 (ei- ther 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 be- tween 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 noncon- servative behavior of Cd during estuarine mixing, as not- ed 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 partition- ing picture of Pb between the three organic fractions is less systematic. Generally, Pb tends to associate with ma- croparticulate 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 pri- mary productivity. In turn, this high primary productivity largely determines the distribution of organic C, leading to spatial distribution patterns with maxima at inter- mediate salinities (Palude della Rosa) rather than at the river end-member as in other estuaries. 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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

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