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Metals content in surface waters of an upwelling system of thenorthern Humboldt Current (Mejillones Bay, Chile)

Jorge Valds a,, Domingo Romn b, Gabriel Alvarez c, Luc Ortlieb d, Marcos Guiez a,e

aLaboratorio de Sedimentologa y Paleoambientes, Instituto de Investigaciones Oceanolgicas, Facultad de Recursos del Mar,

Universidad de Antofagasta, Casilla 170, Antofagasta, Chileb Departamento de Qumica, Facultad de Ciencias Bsicas, Universidad de Antofagasta, Antofagasta, Chile

c Departamento de Geomensura, Facultad de Ingeniera, Universidad de Antofagasta, Antofagasta, Chiled

UR 055, PALEOTROPIQUE, Institut de Recherche pour le Developpement, Francee Programa de Doctorado en Ciencias Aplicadas, Facultad de Recursos del Mar, Universidad de Antofagasta, Chile

Received 1 August 2005; received in revised form 26 April 2007; accepted 11 May 2007

2 Abstract

3 Physicalchemical parameters (temperature, salinity, dissolved oxygen, nutrients, and chlorophyll concentration) of surface4 waters were used to evaluate the influence of biological and physical processes over the metal concentrations (Cd, Ni, V, Mo, Mn,5 and Fe) in different periods of a normal annual cycle (June 2002 and April 2003), in Mejillones Bay (23 S), one of northern6 Chile's strongest upwelling cells. Two points were sampled every 2 months, but statistical analysis of these parameters did not

7 show any spatial differences in surface water composition (annual average) in this bay. The order of total and dissolved metals by8 abundance (annual mean) in the Mejillones Bay surface waters during the sampling period was CdbNibMnbFebVbMo.9 The surface concentration of metals does not appear to be explained by anthropogenic inputs (at least not during the year of this0 work), and variability observed in this study appears to be natural. The lack of correlation between physicalchemical parameters

and metals could indicate a more complex combination of factors acting on surface concentrations. Moreover, the mixture of water2 masses and the Oxygen Minimum Zone which characterizes the Mejillones bay should have an important influence on surface3 distribution of trace metals and can explain the high temporal variability observed in most of the metals analyzed in this work. A4 two-box conceptual model is proposed to suggest possible influences on metals in surface waters of this coastal ecosystem.

2007 Elsevier B.V. All rights reserved.6

7 Keywords: Trace metals; Upwelling; Humboldt system; Marine surface water

9 1. Introduction

0 The continental margin is a boundary where severalsources may affect the chemical characteristics of the

32water and bring contaminating materials. Of these, the33most important are continental sources (atmospheric34transport, river runoff), marine (upwelling), and diage-35netic exchanges at the watersediment interface. These36sources provide organic and inorganic materials that37 play an important role in biogeochemical cycles. For38example, nutrients and trace metals are essential to the39ecosystem, but should be carefully monitored because40they are potential contaminants in the coastal environ-41ment (Cotte-Krief et al., 2000). In both cases, and in the

Journal of Marine Systems xx (2007) xxxxxx

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Corresponding author. Tel.: +56 55 637865; fax: +56 55 637804.E-mail addresses: [email protected] (J. Valds),

[email protected] (D. Romn), [email protected] (G. Alvarez),[email protected](L. Ortlieb), [email protected](M. Guiez).

0924-7963/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jmarsys.2007.05.006

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Please cite this article as: Valds, J. et al. Metals content in surface waters of an upwelling system of the northern Humboldt Current (MejillonesBay, Chile). J. Mar. Syst. (2007), doi:10.1016/j.jmarsys.2007.05.006
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.jmarsys.2007.05.006http://dx.doi.org/10.1016/j.jmarsys.2007.05.006http://dx.doi.org/10.1016/j.jmarsys.2007.05.006http://dx.doi.org/10.1016/j.jmarsys.2007.05.006mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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42 absence of contamination processes, upwelling is the43 principal source of surface nutrients and trace metals44 from the deep ocean.45 In general, all metals present in marine waters are in46 dissolved and particulate forms. Many of these trace47 metals are classified as micronutrients because they are48 essential for phytoplankton growth. Most are compo-49 nents of the enzyme system, which catalyzes important50 biochemical reactions such as glycolysis, photosynthe-51 sis, and protein metabolism. Partitioning between dis-52 solved and particulate forms of trace metals depends on53 the physical and chemical conditions of the marine54 environment and the nature of each metal (Libes, 1992).55 Moreover, any metal can assume various chemical56 forms, being able to form a wide variety of ions, com-57 pounds, or complexes, or being associated with different58

mineralogical or organic phases (Grotti et al., 2001).59 The concentration of these metals can be variable as a60 consequence of changing inputs and/or seasonal effects61 involving biological, geochemical, and physical inter-62 actions (Hatje et al., 2001).63 Knowledge of the biogeochemical cycle of trace64 metals in coastal environments is needed in order to65 identify pollution sources and to explore biological and66 dynamical processes.67 This paper aims to study the influence of biological68 and physical processes over the some metals content in69 different periods of a normal annual cycle, in surface70 waters of one the most productive coastal systems of71 northern Chile.

721.1. Regional setting

73The Humboldt Current is one of the most productive74systems in the world. In the northern section of this75system, many permanent upwelling cells support im-76 portant pelagic fisheries (Strub et al., 1998). Punta77Angamos and Mejillones Bay (Fig. 1) form the most78productive upwelling system of northern Chile. Many79studies concerning biological and physical characteris-80tics of this system have been developed in recent years,81improving our understanding about their response to82ocean-climatic variability, principally, El Nio events83(Escribano et al., 1998; Gonzalez et al., 1998, 2000;84Iriarte et al., 2000; Sobarzo and Figueroa, 2001; Ulloa85et al., 2001; Pizarro et al., 2002; Gonzalez et al., 2004;86Iriarte and Gonzalez, 2004). This area borders one of the87

world's most arid regions, and continental input to the 88ocean is restricted to minor atmospheric transport of89lithogenic particles (Vargas et al., 2004). For this reason,90upwelling seems to be one of the most important pro-91cesses influencing the chemical composition of the sur-92face waters. Mejillones Bay is a hotspot of biological93 productivity. This bay is considered an upwelling94shadow system (UPS), which responds with more95intense biological productivity due to its internal thermal96stability (Marn and Olivares, 1999; Marn et al., 2003).97Studies of the Mejillones Bay have been, principally,98focused on the structure and composition of marine99sediments and their potential for paleo-ocean-climatic100reconstructions of the last millennia (Ortlieb et al., 2000;

Fig. 1. Oceanographic setting of Mejillones Bay, showing schematic location of upwelling cell and location of sampling stations: A is coastal and C

pelagic. Bathymetry's lines each 25 m are showed. The dashed line represents the position of the thermal front, which generates an upwelling shadow(UPS) in Mejillones Bay (Marn et al. 2003).

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01 Valds et al., 2000; Valds and Ortlieb, 2001; Valds02 et al., 2003, 2004; Vargas et al., 2004). As part of the03 latest studies, redox-sensitive metals preserved in04 sediments have been analyzed in order to calibrate05 proxies of paleoxygenation (Valds, 2004) and to asses06 potential problems of marine contamination derived07 from intense industrial development in recent years08 (Valds et al., 2005).09 Industrial activity in the area is represented by aqua-0 culture and the fishmeal industry, power thermoelectric1 generation, and mineral exports. The real impact of this2 recent development on the marine ecosystem is still3 unknown.

4 2. Materials and methods

5

2.1. Sample collection

6 Surface water samples were collected from two7 sites in Mejillones Bay (Fig. 1), using clean, 5 l inert8 sampling bottles (Niskin, General Oceanic) hung on9 stainless steel hydrowire. Between June 2002 and April20 2003, samples for nutrients, chlorophyll-a, temperature,21 dissolved oxygen and salinity were collected monthly,22 while samples for trace metals were collected every23 2 months. Samples were stored at low temperatures,24 in high density polyethylene bottles treated with nitric25 acid. Temperature, salinity, and dissolved oxygen data26 were collected with a CTD (Sea Bird, 19 plus) at the27 same sampling locations.

28 2.2. Analytical procedures

29 2.2.1. Trace metals

30 For dissolved concentrations, the samples were31 filtered with 0.45 m Millipore membranes in Nalgene32 Polisulfone (PSF) systems and acidified to pH 2 with33 nitric acid. This was done the same day as the collection34 in a clean laboratory environment, inside a laminar

35 flow hood (Labconco, Purifier Class II). The samples36 were maintained at 4 C inside the hood. Immediately37 before the ICPOES measurements, the samples were38 acidified to 1 M with HNO3.39 For total concentrations, whole water samples were40 acidified to pH 2 with nitric acid, and then a 250 ml sub41 sample was digested with 20 ml of nitric acid at 90 C in42 a BOD bottle for 2 h in a stainless steel air forced oven43 (WTC Binder); the final acidity of this medium was 1 M44 HNO3.45 Fe and Mo were determined according to the method46 described by Romn et al. (2003). Fe was measured with47 Hydraulic High Pressure Nebulization, Flame Furnace,

148Atomic Absorption Spectrometry (HHPNFFAAS)149 before off-line separation and preconcentration with150ammonium pyrrolidine dithiocarbamate (APDC) and151extraction with methyl isobutyl ketone (MIBK). Mo was152measured with Hydraulic High Pressure Nebulization,153Atomic Absorption Spectrometry (HHPNAAS) before154off-line separation and preconcentration with MIBK;155afterwards, the sample was acidified to 7 M with HCl.156V and Mn were determined by Inductively Coupled157Plasma, Optical Emission Spectrometry (ICPOES) ac-158cording to the matrix matching technique, using purified159seawater (PSW) (1) for blank measurements and the160 preparation of the standards for optimization and161calibration.162Ni was determined by adsorptive accumulation of the163respective complex with dimethylglyoxime at the drop164

mercury electrode (DME) by Adsorptive Differential 165Pulse Polarography (AdDPP) (2), and Cd was measured166 by Potentiometric Stripping Analysis (PSA) in the167derivate signal mode (3, 4).168CASS 4 (coastal seawater) and NASS 4169(oceanic seawater) certified reference seawater from the170Canadian National Research Council (NRCCNRC)171were used to validate the techniques and for quality172control of the analytical determinations of Mn, Fe, Ni,173Cd, and Mo. Vanadium's quality control was done with174the spiking technique. Purified coastal seawater (PSW)175was prepared with an additional solid phase extraction176step, using C18 3 M Empore Bakerbond (J. T. Baker)177discs (1) to improve the uptake of residual trace metals,178and was used for blanks, the ICPOES calibration ma-179trix, with spikes as a secondary reference material for180quality control.181Before each series of ten measurements routine182controls of precision and accuracy were done. If, in four183consecutive tests, the precision varied from the expected184value by 20%, all analytical procedures were reviewed,185and any having suspicious measurements were repeated.186When the committed relative error was 15% with

187respect to the certified value, all the analytical pro-188cedures were reviewed, and any suspicions ones were189repeated.190All metal concentrations are expressed in g l1.

1912.2.2. Nutrients and chlorophyll

192Water samples for nutrients were filtered through a1930.45 m membrane filter and concentration was194determined using a Spectronic 20D Spectrophotometer,195according to methodology proposed by Parsons et al.196(1984). Chlorophyll-a was measured in a Shimadzu RF-1975301 Spectrofluorometer, after filtering samples in the198dark through a 0.45 m membrane filter, following

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199 Holm-Hansen et al. (1965). The results of nutrients are200 reported as moll1; chlorophyll-a is reported asg l1.

201 2.3. Statistical analysis

202 Temperature, dissolved oxygen, salinity, metals,203 nutrients, and chlorophyll were each analyzed by one-204 way analyses of variance (ANOVA), to test the signif-205 icance of spatial and temporal variability. Pearson corre-206 lations and cluster analyses were performed to evaluate207 the relationships between all parameters measured at208 each station.

2093. Results and discussion

2103.1. Statistical validation

211Descriptive statistics of all parameters measured in212both stations in Mejillones Bay are shown in Table 1.213The comparison of these results with previous work in214this area (Rodriguez et al., 1991; Marn and Olivares,2151999; Marn et al., 2003) indicates that the sampling216period corresponds to a normal year, without influence217of El Nio event, which drastically changes water218column structure in this area.

Table 11:1Descriptive statistics of all parameters measured in this work1:2

1:3 Parameter Mean SD Range

1:4 ST. A ST. C ST. A ST. C ST. A ST. C

1:5 Temperature (C) 17.02 17.04 1.79 1.56 14.4719.92 15.1820.061:

6 Salinity (psu) 34.79 34.77 0.11 0.10 34.6434.98 34.6534.981:7 Dis. Oxygen (ml l1) 6.40 7.24 2.31 2.02 2.2210.47 3.2010.681:8 Nitrate 2.13 1.04 2.95 1.15 0.068.70 0.023.661:9 Nitrite 0.11 0.08 0.14 0.07 0.000.46 0.000.191:10 Phosphate 1.40 1.13 0.74 0.49 0.202.70 0.301.181:11 Silicate 16.55 17.39 21.86 18.61 0.9272.60 0.0050.501:12 Chlorophyll-a 3.2 2.73 2.08 1.68 0.676.36 0.245.311:13 V Total 7.33 7.36 1.91 3.21 4.409.23 3.4812.801:14 Dissolved 4.95 3.78 1.47 1.53 3.596.61 2.116.391:15 Mn Total 1.49 1.99 0.73 1.18 0.672.54 0.874.091:16 Dissolved 0.94 1.03 0.31 0.33 0.561.32 0.561.381:17 Fe Total 2.77 2.69 0.41 0.67 2.413.51 1.963.581:18 Dissolved 1.88 1.75 0.58 0.20 1.172.41 1.572.111:19 Ni Total 0.88 0.83 0.05 0.23 0.840.98 0.360.981:20 Dissolved 0.57 0.57 0.23 0.25 0.200.88 0.210.821:21 Mo Total 13.85 15.28 1.79 5.02 11.1915.36 9.4321.741:22 Dissolved 7.40 10.57 1.88 3.78 4.188.97 5.7315.441:23 Cd Total 0.27 0.21 0.30 0.12 0.040.80 0.100.391:24 Dissolved 0.04 0.04 0.04 0.03 0.010.11 0.010.07

SD = standard deviation; nutrients are expressed in mol l1; Chlorophyll is expressed in g l1; metals are expressed in g l1.1:25

Table 22:1

Spatial variability: one-way ANOVA of all parameters measured in Mejillones Bay2:

2

2:3 Temperature Oxygen Salinity Chlorophyll-a Nitrate Nitrite Phosphate

2:4 F 0.000 0.75 0.14 0.31 1.18 0.392:5 P 0.977 0.398 0.715 0.588 0.291 0.539

2:6 Silicate VTotal MnTotal FeTotal NiTotal MoTotal CdTotal Vdissolved

2:7 F 0.01 0.00 0.79 0.06 0.32 0.38 >0.26 1.832:8 P 0.927 0.986 0.396 0.812 0.587 0.553 0.621 0.206

2:9 Mndissolved Fedissolved Nidissolved Modissolved Cddissolved

2:10 F 0.27 0.26 0.00 3.39 0.012:11 P 0.617 0.619 0.963 0.095 0.930

Pb0.05.2:12


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9 The results of the ANOVA showed no significant20 differences for oceanographic parameters between the21 two sampling stations (Table 2). In other words, the22 surface waters of Mejillones Bay, at averaged over a23 year, present homogeneous spatial conditions for all24 parameters measured. However, the results suggest25 some differences related to warm and cold periods26 which are characteristics of this zone.27 Table 3 presents the results of Pearson's Coefficient28 of Correlation for all parameters measured in this study.29 In spite of number of parameters measured in this study,30 significant correlations were found in few cases. Only31 temperature and chlorophyll correlate with more than32 two other parameters.

2333.2. Oceanographic variability

234The annual mean surface temperature, salinity, and235dissolved oxygen levels found are characteristic of a236normal (non-El Nio) year, whereas the ranges of values237reflect seasonality and upwelling events influences.238Considering the results of the ANOVA test, cluster239analysis was done with all physicalchemical parameters240combining both sampling stations. The result (Fig. 2)241showed a clear group formed by temperature,oxygen, and242chlorophyll, which present positive and significant243correlations, except in case of oxygen and chlorophyll244(Table 3). The others parameters didn't showed a clear245relationship, but nitratenitrite (significant correlation,

Fig. 2. Single-linkage dendrogram of similitude showing the results of clustering analyses of grouped physical

chemical parameters (not metals). Thebest line of similitude is shown.

Table 31Pearson's Coefficient Correlation of all parameters measured in Mejillones Bay2

3 Temperature Oxygen Chlorophyll Nitrate Nitrite Mntot Motot Cdtot

4 Oxygen 0.535 Chlorophyll 0.804

6 Nitrate 0.601 0.6517 Nitrite 0.868 Phosphate 0.79 Silicate 0.710 Mntot 0.63911 Fetot 0.82212 Nitot 0.65213 Cdtot 0.65514 Vdis 0.67 0.71515 Mndis 0.825 0.83216 Modis 0.606 0.59817 Cddis 0.656 0.699

Only significant values (Pb0.05) are showed.18


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246 Table 3), and phosphatesilicate (no significant correla-247 tion) suggest some relationship (Fig. 2). Supported in248 these results we interpret that Mejillones Bay system249 presents two oceanographic scenarios: one characterized250 by a low influence of physical processes and rich nutrient251 surface waters (cold season) and another characterized by252 strong influences of physical processes coupled to253 biological factors (warm season), the last one represented254 in Fig. 2 by group formed by temperature, oxygen, and255 chlorophyll.256 Temperatures ranged between 14.5 C (winter) and257 20.1 C (summer) and dissolved oxygen between 2.2 ml258 l1 and 10.7 ml l1 (both in summer; Table 1, Fig. 3). A259 considerable decrease of these parameters was regis-260 tered in February. This situation suggests upwelled261 waters present inside the bay. At this month, it also262

registered an increase of phosphate (Fig. 3). The neg-263 ative correlation between dissolved oxygen and phos-264 phate has been used as an indicator of upwelling in this265 bay (Rodriguez et al., 1991).266 During cold season (AprilSeptember) nutrients267 concentration is high because productivity is inhibited268 due to a lack of radiation. The low intensity of pro-269 ductivity generates low chlorophyll concentrations270 (Fig. 3). During warm season (OctoberMarch), when271 radiation is optimum, the productivity increases and,272 consequently, the nutrient concentrations decrease and273 chlorophyll increases (Fig. 3). A similar pattern, es-274 pecially between nitrate and chlorophyll, was observed275 by Rodriguez et al. (1991), during 198788.

276 3.3. Metal content in surface waters

277 The concentrations of total and dissolved metals at278 each station are showed in Table 1. As was the case for279 oceanographic parameters, ANOVA did not show280 significant differences in metal concentrations between281 both sampling stations (Table 2). In order of abundance,282 the total and dissolved trace metals (annual mean) in the

283 surface waters of Mejillones Bay (20022003) were284 CdbNibMnbFebVbMo.285 In general, metals in seawaters can be classified286 according to their interactive properties with biological287 and physicalchemical processes. Two groups can be288 identified: conservative and non-conservative. The first289 group, including Mo, presents few interactions with the290 biological cycle and its horizontal and vertical distribu-291 tion is governed by physical processes such as advection292 and turbulent mixing (Libes, 1992). The second group,293 including V, Ni, Cd, Fe, and Mn, is affected by bio-294 logical processes, whether via scavenging and/or295 biological uptake (Brown et al., 1994). In the latter

296case, phytoplankton affects the distribution of dissolved297metals, using most of them as micronutrients (Gonzalez-298Davila, 1995). These intrinsic characteristics of metals299must be considered when interpreting this study's300results.

3013.3.1. Vanadium

302The predominant chemical form of V in oxic waters303is vanadate, H2VO4

(Morford and Emerson, 1999),304whose behavior is mostly conservative with some305 possible nutrient cycling (Bruland, 1983). Total and306dissolved V variability presents a general trend char-307acterized by high concentrations in cold season and low308concentrations in warm season, similar to nutrient309behavior. The partitioning between the dissolved and310 particulate phases appears to be relatively constant311

throughout the year with exception of February in 312 pelagic station (Fig. 3). This situation points to an313increase of the particulate form, which could be asso-314ciated to the influence of upwelled water transported315into the bay by onshore advection.

3163.3.2. Manganese

317Mn is present in oxic waters as MnO2 (Russell and318Morford, 2001). According to Kremling (1985) and319Libes (1992), this metal is enriched in coastal surface320waters due to a large metal supply from external sources.321Variability was more marked for total Mn than for its322dissolved form, which could be explained by the323 presence of particulate Mn in surface waters during324periods of high total Mn concentrations (i.e. October).325According to Schenau et al. (2002), the most important326primary source of Mn to the ocean is the product of327continental weathering. Valds et al. (2005) found a328significant correlation between this metal and Al in the329Mejillones Bay marine sediments, which is a signal of330lithogenic input. In Mejillones the continental transport331to the ocean is controlled almost exclusively by winds332(Vargas et al., 2004). The strongest winds from the south

333and south-west develops during spring and summer334conditions (Escribano et al., 2004) mostly in Septem-335berOctober, which could to explain the increase of336particulate Mn registered in October.

3373.3.3. Iron

338The annual variation of Fe was different at each339sampling station. At the coastal station, both total and340dissolved iron concentrations were low in winter and341increased during springsummer. On the other hand, the342pelagic station showed an opposite trend: high total343concentrations during winter and low total concentra-344tions during springsummer, with constant dissolved


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Fig. 3. Surface temporal variability of parameters measured at both Mejillones Bay stations during 20022003. Temperature in degrees Celsius;dissolved oxygen in ml l1; chlorophyll-a in g l1; nutrients in mol l1; metals in g l1. Black circle is total metal and white circle is dissolvedmetal. Arrows indicates satellite image dates (See Fig. 4).


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345 concentrations year-round (Fig. 3). Martn and Fitzwater346 (1988) have shown that phytoplankton growth is limited347 by iron availability, especially in the California and348 Humboldt Current Systems (Hutchins and Bruland,349 1998). However, the lack of correlation between Fe and350 nutrients and chlorophyll would to indicate that this351 metal has no relationship with biological factors, at least352 in this bay. By other hand, Des Combes et al. (1999)353 indicate that the supply of this metal is governed,354 principally, by continental input probably in the same355 way as Mn in Mejillones Bay. However, no significant356 correlation was found between Fe and Mn (Table 2). In357 the California Current Johnson et al. (1999) found358 that Fe was introduced by resuspended bottom sediment359 during coastal upwelling. A similar mechanism should360 be acting in Mejillones bay, because the microxic con-361

dition of bottom environments (Valds, 2004) and the362 persistence of the upwelling should to introduce Fe into363 the bay from offshore zone.

364 3.3.4. Nickel

365 Ni occurs in oxic waters as Ni2+ and NiCl+ (Calvert366 and Pedersen, 1993). At the Mejillones Bay coastal367 station, total Ni was constant during the entire study368 period, whereas its dissolved form showed high369 concentrations during winter and low concentrations370 during springsummer. The pelagic station registered a371 highly variable Ni concentrations, with decay during372 October and high concentrations before and after this373 month (Fig. 3). The distribution of Ni is strongly374 mediated biologically (Kremling, 1985), and its behav-375 ior as a micronutrient indicates that it is removed from376 surface waters by plankton growth (Saager et al., 1992;377 Morley et al., 1993). It seems like lowest dissolved Ni378 concentration at station A should be accompanied by a379 high chlorophyll concentration (Fig. 3), which sug-380 gest some influence of biological productivity on Ni381 behavior.

382 3.3.5. Molybdenum383 Molybdenum (VI) appears in the stable oxidation384 state in oxic waters and is found as MoO4

2 (Morford385 and Emerson, 1999; Russell and Morford, 2001;386 Nameroff et al., 2002). Mo is a conservative metal387 only affected by physicalchemical factors (Libes,388 1992; Brown et al., 1994; Adelson et al., 2001).389 Because of its conservative nature, it is possible to390 assume that coastal surface circulation controls the Mo391 distribution in Mejillones Bay surface waters. Concen-392 trations of dissolved and total Mo were less variable at393 the coastal than at the pelagic station (Fig. 3), which is394 closer than upwelling cell (Fig. 1). During warm season

395the upwelling events are more frequent and intense396(Marn et al., 2003), which influence the onshore397advection and probably the Mo content in surface398waters principally in the pelagic station.

3993.3.6. Cadmium400Of all metals measured in this study, Cd had the401lowest concentration. The total concentration presented402a more marked variability than its dissolved form at both403stations (Fig. 3), as a consequence of the variation of404 particulate Cd in Mejillones Bay surface waters. The405typical oceanic Cd distribution presents low or depleted406values near the surface (Delgadillo-Hinojosa et al.,4072001) and is regulated by marine biogeochemical pro-408cesses, namely uptake by phytoplankton in surface409waters (Abe, 2001). Cd is a nutrient-like metal that410

presents only one oxidation state in seawater (Cd II) and 411is found as CdCl+ (Morford and Emerson, 1999; Russell412and Morford, 2001; Nameroff et al., 2002). This413 behavior indicates that Cd has a short residence in414surface waters because it is removed rapidly by plankton415growth (Calvert and Pedersen, 1993). Temporal Cd416variations were associated with chlorophyll-a concen-417tration and temperature fluctuation, suggesting that418 phytoplanktonic biomass production is the principal419factor controlling Cadmium concentration in surface420waters of Mejillones bay. However, the two highest total421Cd values measured in December and April in station A422seem too high to be attributed to phytoplankton, because423according to Cullen and Sherrell (1999) in a work424developed in California Current, the Cd content of425marine particles is less than 10 ng l1. It is possible that426upwelled waters transported by onshore advection have427some degree of influence in Cd content in Mejillones428bay, as account in others coastal environments (Boyle,4291988; Van Geen et al., 1992; Van Geen and Husby,4301996; Takesue et al., 2004).

4313.4. Factors controlling seasonal variability of trace

432metals in Mejillones bay

433One possible source of trace metals in Mejillones434Bay is the input of lithogenic material. However, Valds435et al. (2000) and Valds (2004) compared V, Ni, Mo, and436Cd with aluminum (a marker of lithogenic sources437according to Dean et al. (1997) in marine sediments,438without finding any significant correlations). The439authors concluded that continental input in the sedi-440ments of Mejillones Bay is not the principal source of441these trace metals. Moreover, Vargas et al. (2004) in-442dicated that lithogenic debris in the bottom marine443sediments of Mejillones Bay represents less than 5% of


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44 the total bulk sediments, which suggests that metals in45

lithogenic minerals transported from the continent to the46 ocean constitute a minor source in this coastal area. By47 other hand, Valds et al. (2005) demonstrated that48 contamination doesn't explain the concentration of these49 trace metals in this bay. However, some influence of50 waste water treatment plant effluent should be consid-51 ered specially in station A. Even if influenced to some52 extent by atmospheric inputs and/or contamination53 processes in the surface waters, the distribution of V,54 Ni, Cd, and Mo in Mejillones Bay at the moment55 of sampling, can be broadly and principally accounted56 by a combination of natural oceanic processes and57 mechanisms.58 Seasonal variability was described by grouping pa-59 rameters into two general annual periods: the cold60 (autumnwinter) and warm (springsummer) seasons.61 Fig. 4 shows typical satellite images (from EOS Aqua62 Spacecraft; second and third order polynomic correction63 process) of chlorophyll-a distribution during the cold64 period (August 28, 2002) and the warm period65 (February 28, 2003), coincident with the sampling of66 metals in surface waters. During the cold season low67 levels of chlorophyll were found in the surface waters of

68 the Punta Angamos upwelling cell and Mejillones Bay69 (in accordance with chlorophyll measured in situ),70 which indicated that productivity was low during this71 period. On the other hand, during the warm season, well72 productive waters are common, with a filament of high73 levels of chlorophyll displaced to the north-east, gen-74 erating an upwelling shadow system (UPS) in the bay, as75 proposed by Marn et al. (2003). Low levels of dissolved76 oxygen and high levels of phosphate registered during77 warm season, particularly in February, are characteristic78 of upwelling events (Fahrbach et al., 1980).79 The lack of a similar pattern of temporal variability of80 metals in both sampling stations seems to be the most

481important characteristic. This situation could indicate482

that there is a combination of factors with different 483temporal variation patterns which influence the presence484of metals in surface waters. Based on the area's oceanic485characteristics and the results obtained in this study, a486conceptual two-box model is proposed to explain the487variation of metals in surface waters of this coastal488ecosystem (Fig. 5). The two-box model considered in489this work was based on the area's UPS interpretation490 proposed by Marn et al. (2003), which separates the491Mejillones Bay from the Punta Angamos upwelling492system. The upwelling event of Punta Angamos causes493a biological productivity increment into Mejillones bay,494and the development of an efficient retention zone of495phytoplanktonic organisms (Marin and Olivares, 1999;496Escribano et al., 2000). These characteristics are497modulated by the upwelling shadow condition present498in this bay, which is defined as a confined zone of an

Fig. 5. Conceptual scheme of mechanisms which influence temporalvariability of trace metal concentrations in surface waters of MejillonesBay. TTM= Total TraceMetal; PTM= Particulate TraceMetal; DTM=Dissolved Trace Metal; 1 = offshore deep water upwelling; 2 = onshoreadvection; 3 = desorption of particulate metals; 4 = phytoplankton

uptake; 5 = scavenging; 6 = exchange at sediment

water interface 7 =wind transport; 8 = expected anthropogenic input.

Fig. 4. Satellite image of chlorophyll-a in Punta Angamos upwelling system during August 2002 (left) and March 2003 (right).


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499 active coastal upwelling system, whiting which upwell-500 ing is less intense (Marin et al., 2001). According to this501 situation the upwelled waters are transported inside the502 bay by onshore advection. The upwelling condition503 founded inside the bay in February, according to PO4504 and dissolved oxygen (Fig. 3) represents the spin-up505 phase of the develop of the upwelling shadow (Marn506 et al., 2001).507 Several studies have shown that coastal surface508 waters contain higher concentrations of dissolved trace509 metals (DTM) than open ocean waters (Kremling and510 Hydes, 1988; Kremling and Pohl, 1989; Muller et al.,511 1994; Cott, 1997; Le Gall et al., 1999). The offshore512 box presents two ways of metal transport: surface513 circulation entering and vertical circulation via the514 upwelling of deep water, which results in different515

intensities depending on the wind direction and strength516 (Strub et al., 1998). Both mechanisms transport water517 masses with a much defined chemical composition518 (metals included). For example, in the case of DTM, at a519 global scale, Martn and Thomas (1994) estimated that520 the oceanic input would dominate sources for the521 oceanic margin. Unfortunately, information about the522 metal composition of marine waters in the Humboldt523 system is lacking and it was not possible to compare our524 results with other works. In the inside the bay box,525 surface metal concentrations result from very low526 lithogenic input, uncertain industrial contamination pro-527 cesses (a continuing possibility), ingestion by phyto-528 plankton, desorption from particles, and scavenging due529 to high organic matter production. In this last case,530 Bacon and Anderson (1982) propose the reversible531 scavenging model, in which the settling and regener-532 ation of biogenic particles are important factors in533 removing trace metals from surface waters in the ocean.534 These factors are combined with surface water masses535 present inside the bay. During period of study the water536 column of Mejillones bay showed a mixture of Sub-537 tropical Surface Water (STSW), Subantarctic Surface

538 Water (SASW), and Equatorial Subsurface Water539 (ESSW), which are commonly observed offshore of540 Mejillones Bay (Sobarzo and Figueroa, 2001). Consid-541 ering that the bay have less than 100 m deep, it is542 convenient to talk about the types of water associated to543 these water masses. Their original properties are modi-544 fied by a mixture of processes, and/or due to the atmo-545 spheric conditions. The composition of these surface546 water masses has a temporal and spatial variability that547 is subject to seasonal atmospheric variations such as548 solar radiation, wind intensity and a mixture of effects549 caused by the bay's dynamics and coastal upwelling550 processes. The features of the water-mass core defined

551in the oceanographic literature do not correspond to the552bay's interior. However, it is possible to recognize these553traits in spite of the water mixture. In general terms, it554can be said that there is a predominance of the STSW at555the surface level. However, this level is also character-556ized by minimum salinities related to the minimum557salinity water levels (MSW) mentioned atSobarzo and558Figueroa's(2001) work corresponding to some type of559SASW. This situation should have an important560influence on surface distribution of trace metals and561can explain the high spatial and temporal variability562observed in most of the metals analyzed in this work.563Due to the fact that vertical mixing of water column564is an important characteristic in Mejillones bay, another565factor that should be considered is the influence of566microxic condition of subsurface waters (Valds, 2004),567

because all the metals analyzed in this work are redox- 568sensitive (Morford and Emerson, 1999; Bostick et al.,5692000; Brown et al., 2000, Crusius and Thomson, 2000;570Hammond et al., 2000; Zheng et al., 2000, Morford571et al., 2001; Russell and Morford, 2001). In the shallow572zone (between 0 m and 45 m water depth) sediments are573deposited under oxic condition. In this zone Ni, V, Cd574and Mo are resolubilized during the diagenetic process575remaining in solution, while Mn and Fe precipitate to576the sediments. Hatje et al. (2001) suggest that Fe is577oxidized faster than Mn in natural systems, which578indicates that Mn could be transported further away579from coast before precipiting in the bottom sediments.580In the deeper zone of Mejillones bay prevails a581microxic/anoxic sedimentwater interface condition582which promotes precipitation of Ni, Cd, Mo and V. Fe583and Mn remain in solution and are resolubilized during584the diagenetic process.585The combination of these mechanisms and temporal586variation could explain the complex behavior of these587metals in this upwelling system of northern Chile.

5884. Conclusions

589The temporal distribution of some metals in the590coastal waters of the northern Humboldt Current591(Mejillones Bay, Chile) has been described in relation592to physicalchemical and biological processes.593In Mejillones Bay, nutrients are present year-round in594surface waters and are not a limiting factor for high595 primary productivity. Radiation could be a limiting596factor explaining the opposing trend of nutrients and597chlorophyll; low radiation in autumn and winter cor-598responds to high nutrients and low chlorophyll, whereas599high radiation in spring and summer corresponds to low600nutrients and high chlorophyll.


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01 The surface concentration of metals does not appear02 to be explained by anthropogenic inputs (at least not03 during the year of this work), and variability observed04 in this study appears to be natural. The lack of05 correlation between physicalchemical parameters and06 metals could indicate a more complex combination of07 factors acting on surface concentrations. It is possible,08 also, that the temporal variability of metals corresponds09 to a time scale other than those considered in this study.0 However, although not significant, a more intense1 variability in metal concentrations was observed at the2 pelagic station, which is under the direct influence of3 the Punta Angamos upwelling cell located off Mejil-4 lones Bay.5 A two-box conceptual model is proposed to explain6 the variation of metals in surface waters of this coastal7

ecosystem. In the offshore box, surface circulation8 and deep water upwelling transport water masses with9 different metal contents and different temporal fre-20 quencies into the bay. In the inside the bay box,21 surface metal concentrations result from very low22 lithogenic input, ingestion by phytoplankton, desorp-23 tion, and scavenging due to high organic matter24 production. The development of upwelling events in25 this area during the sampling period had different26 effects on each metals and sampling point. Moreover,27 the mixture of water masses and the Oxygen Minimum28 Zone which characterize the Mejillones bay should29 have an important influence on surface distribution of30 trace metals and can explain the high temporal31 variability observed in most of the metals analyzed in32 this work.

33 5. Uncited References

34 Abe and Matsumaga, 198835 Martn and Knauer, 198436 Segovia-Zavala et al., 199837 Takesue and Van Geen, 2002

38 Acknowledgements

39 This study was supported by grant PEI 1340-02,40 from the Universidad de Antofagasta, Chile, and UR41 055 PALEOTROPIQUE, IRD, France. Support was also42 provided by IRD through the JEAI (Jeune Equipe43 Associe l'IRD) program. We thank Freddy Rabast44 and Keyla Majluf for their help in sampling and45 analytical support in nutrient and chlorophyll determi-46 nations. Special thanks to anonymous reviewers for47 their comments and significantly improving the48 manuscript.

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