Barnacles as biomonitors of trace metal availabilities in Hong Kong coastal waters: changes in space and time

The use of selected organisms as biomonitors of trace metal bioavailabilities allows comparisons to be made over space and time. The concentrations of 11 trace metals (arsenic, cadmium, chromium, cobalt, copper, iron, lead, manganese, nickel, silver, zinc) were measured in the bodies of two barnacle species, Balanus amphitrite and Tetraclita squamosa, from up to 18 littoral sites from Hong Kong coastal waters in April 1998. These data provide evidence on the geographical variation in metal bioavailabilities at this time, and are compared selectively against historical data sets for 1986 and 1989. Geographical variation in bioavailabilities is clear for several metals, with hotspots for arsenic, copper, nickel and silver at Chai Wan Kok, and for lead in Junk Bay. Victoria Harbour sites head the rankings for silver and arsenic, and Tolo Harbour sites exhibit relatively elevated cobalt, manganese and zinc. Many bioavailabilities of trace metals to barnacles are lower in Hong Kong coastal waters in 1998 than in 1986. The two barnacle species are widespread and the extensive data set presented is a benchmark which can be compared to the results of similar biomonitoring programmes elsewhere in the Indo-Pacific and beyond.     

Spatial and seasonal variations in depth profile of trace metals in saltmarsh sediments from Sapelo Island, Georgia, USA

This study was undertaken to elucidate the impact of early diagenetic processes on the accumulation of trace metals in Sapelo Island saltmarsh sediments as a function of time, space and sediment properties. Samples were collected from three sites in summer (May 1997) and winter (January 1998) along a transect from an unvegetated Creek Bank through a vegetated Tidal Levee to the vegetated midmarsh with evident lateral heterogeneity caused by hydrologic regime, macrophytes and microbial and macrofaunal activities. A suite of trace metals (As, Ba, Cr, Co, Cu, Cd, Mo, Ni, Pb, Th, Ti, U, V, Zn and Zr) was analyzed to obtain their depth-distribution at the three sites. Spatially marked differences were observed, that were primarily related to hydraulic flushing of trace metals away from the sites in high-energy regimes, rapid downward mixing and reworking of sediment via bioturbation, and below-ground degradation and production of Spartina biomass. Although sulfate reduction and the formation of acid volatile sulfide and pyrite were dominant processes throughout the marsh, the trace metal scavenging role of sulfides was not apparent. However, possible sulfurization of organic matter, leading to enhanced trapping of trace metals with organic carbon, may have played an important role in sequestration of trace metals.No similarity was observed visually between the depth trends of trace metals and sediment properties (grain size, iron-oxyhydroxide content, acid volatile sulfides and pyrite content) that are known to play a major role in trace metal partitioning. Only organic carbon content closely followed the trace metal profiles at all the three sites. Minor variation in depth-integrated sediment trace metal content was observed seasonally at each of the three sites. Furthermore, the depth trend of profiles of individual trace metals also did not vary significantly over the seasons either.     

Uptake and distribution of N, P and heavy metals in three dominant salt marsh macrophytes from Yangtze River estuary, China

We examined the variation in aboveground biomass accumulation and tissue concentrations of nitrogen (N), phosphorus (P), copper (Cu), zinc (Zn) and lead (Pb) in Phragmites australis (common reed), Spartina alterniflora (salt cordgrass), and Scirpus mariqueter throughout the growing season (April-October 2005), in order to determine the differences in net element accumulation and distribution between the three salt marsh macrophytes in the Yangtze River estuary, China. The aboveground biomass was significantly greater in the plots of S. alterniflora than in the plots of P. australis and S. mariqueter throughout the growing season (P<0.05). In August, the peak aboveground biomass was 1246+/-89 gDW/m(2), 2759+/-250 gDW/m(2) and 548+/-54 gDW/m(2) for P. australis, S. alterniflora and S. mariqueter, respectively. The concentrations of nutrients and heavy metals in plant tissues showed similar seasonal patterns. There was a steady decline in element concentrations of the aboveground tissues from April to October. Relative element concentrations in aboveground tissues were at a peak during the spring sampling intervals with minimum levels during the fall. But the concentrations of total nitrogen and total phosphorus in the belowground tissues were relatively constant throughout growing season. Generally, trace metal concentrations in the aboveground tissues of S. mariqueter was the highest throughout the growing season, and the metal concentrations of S. alterniflora tissues (aboveground and belowground) were greater than those of P. australis. Furthermore, the aboveground pools of nutrients and metals were consistently greater for S. alterniflora than for P. australis and S. mariqueter, which suggested that the rapid replacement of native P. australis and S. mariqueter with invasive S. alterniflora would significantly improve the magnitude of nutrient cycling and bioavailability of trace metals in the salt marsh and maybe transport more toxic metals into the water column and the detrital food web in the estuary.     

Impact of trace metals on denitrification in estuarine sediments of the Douro River estuary,Portugal

Denitrification influences the nitrogen budget in estuaries by removing fixed nitrogen from the inorganic pool; rates are dependent on both geological and geographic conditions as well as increasing anthropogenic impacts. In this study the effects of copper (Cu), chromium (Cr), zinc (Zn), cadmium (Cd) and lead (Pb), on the denitrification pathway were evaluated in subtidal and intertidal sediments of the Douro River estuary. Dinitrogen, N₂O and NO₂⁻ production rates were measured in triplicate slurries of field samples under different treatments of metal concentrations. Results demonstrated that similar metal amendments led to different site responses for denitrification, suggesting that variations in sediment properties (metal concentrations, grain size, organic matter content, etc.) and/or differences in denitrifying community tolerance modulate the level of metal toxicity. Denitrifying communities in subtidal muddy sediments were not affected by increasing concentrations of metals. In contrast, intertidal sandy sites revealed high sensitivity to almost all trace metals tested; almost complete inhibition by Cr (95%) and Cu (85%) was observed for 98 and 79 μg per gram of wet sediment respectively, and by Zn (92%) at the highest concentration added (490 μg per gram of wet sediment). Moreover, the addition of trace metals stimulated N₂O and NO₂⁻ accumulation in intertidal sandy (Zn, Cu, Cr and Cd) and muddy sediments (Cu and Zn), demonstrating a pronounced inhibitory effect on specific steps within the denitrification enzymatic system. In summary, the results obtained suggest that, according to the type of estuarine sediment, trace metals cannot only reduce total N removal from an estuary via denitrification but also can enhance the release of N₂O, a powerful greenhouse gas.     

Bioaccumulation and depuration of Zn and Cd in mangrove oysters (Crassostrea rhizophorae, Guilding, 1828) transplanted to and from a contaminated tropical coastal lagoon

In order to study Zn and Cd accumulation and depuration, a set of oysters, Crassostrea rhizophorae, were transplanted to a metal contaminated coastal lagoon and another one was harvested there and transplanted to a non-polluted site. C. rhizophorae oysters and Perna perna mussels native from both sites were collected in order to monitor variability of metal concentration in resident populations. After three months exposure, oysters transplanted to the polluted site accumulated fourfold Zn (307-1319 microgg(-1)) without reaching the concentration level of resident oysters (9770 microgg(-1)). Cadmium concentrations had a slight but significant decrease during the same period (1.25-0.54 microgg(-1)). Oysters transplanted to the non-polluted site, showed threefold Zn depuration (6727-2404 microgg(-1)), while Cd had no significant variation (0.90-1.45 microgg(-1)). Results showed that transplanted oysters do not reach heavy metal concentrations in indigenous populations suggesting transplanted organisms would be better used to evaluate bioavailability instead of environmental concentrations.     

Etude du transfert de quelques oligo-éléments métalliques entre le milieu sédimentaire estuarien et les poissons plats ‘mangeurs de sédiments’

In order to study the transfer of trace metals (Cd, Pb, Cu, Zn) between sediments and living organisms, flatfishes (flounder, Platichthys flesus and sole, Solea solea) were collected monthly in the Loire estuary (France) for more than a year. Trace metal concentrations were determined in flatfish digestive contents and tissues. The influence of some ecological factors upon bioaccumulation of trace metals was considered.We consider the benthic way of life not to be responsible for a preferential bioaccumulation of trace metals in flatfishes. Although zinc accumulation in external organs is important, the hypothesis of an uptake via food and sediment cannot be rejected.     

Nereis diversicolor: an Unreliable Biomonitor of Metal Contamination in the 'Ría de Bilbao' (Spain)

Abstract. Samples of the polychaete Nereis diversicolor O.F. M üller and surficial sediments from a clean and a polluted estuary were taken during a year at monthly intervals to determine, by comparison, the range of temporal fluctuations in trace metal concentrations. Whereas metal variations in surficial sediments were more pronounced in the polluted estuary, concentrations in N. diversicolor in both estuaries showed a wide range of temporal variation that surpassed that of surficial sediments. A clear decline in all metals studied (except Mn) from polluted sediments was noted after episodes of strong rainfall. In the reference estuary the fluctuations were less pronounced. Metal body burdens in N. diversicolor from the two estuaries were usually quite similar even though total levels in sediments differed considerably, indicating a lack of worm-sediment relationship. The low level of Cu, Cr and Pb in the tissues of N. diversicolor suggest a mechanism which prevents the polychaete from being exposed to these metals in highly polluted sediments. Therefore, it is concluded that N. diversicolor cannot be considered to be an ideal biomonitor of metal contamination in the polluted Bilbao Estuary.     

Sources and timing of trace metal contamination to sediments in remote sealochs,N.W. Scotland

Sediment cores from a transect of sealochs, Etive, Sunart, Nevis, Duich, Broom and Torridon in N.W. Scotland were analysed for the trace metals lead, copper, and zinc. In addition, sediment cores from the Clyde Sea Basin (Loch Fyne, the Gareloch, Clyde dumpsite) and from the Irish and Celtic Seas were analysed as possible source areas for contaminated particles. The sediment cores were dated using excess ²¹⁰Pb and ¹³⁷Cs, and Al normalisation was used to correct for sediment compositional effects. All cores showed increased concentrations of all trace metals (and M/Al) towards the surface, with a maximum of 280–500% above background being observed in Loch Etive. Only cores from Duich and Torridon (Zn/Al and Pb/Al) and Etive (Zn/Al) did not continue to show an increase in trace metals to the uppermost samples. The burden of atmospherically derived trace metals accumulated over the past 100 years was compared with similar published burdens from a series of freshwater lochs adjacent to the sealochs. Plots of excess ²¹⁰Pb (used to correct for sediment focusing) against atmospherically derived Pb showed a series of linear relationships of systematically decreasing gradient towards the north-west, away from potential urban sources of lead. It was calculated that 49% (15–77%) of the Pb but only 3% (1–4%) of the Zn and 2% (0–3%) of the Cu reached the sealochs from atmospheric sources. The remaining trace metals deposited in the sealochs came from marine particles. Using trace metal ratios as an indicator of pollutant provenance, it was shown that the trace metals came predominantly from the Irish Sea. There was no evidence of trace metals derived from the Firth of Clyde basin. It was concluded that trace metal contamination from the Clyde and the surrounding industrialised urban area was mainly deposited in the estuary and the Clyde Sea basin with its adjoining sealochs, such as the Gareloch and Loch Fyne.     

Paleo-redox depositional conditions inferred from trace metal accumulation in two Cretaceous-Paleocene organic-rich sequences from Central Egypt

Cretaceous-Paleocene organic-rich sediments in Egypt occur as an east-west trending belt extending from the Quseir-Safaga district (Red Sea) to the Kharga-Dakhla (Western Desert) region. They are associated with the Duwi Formation (phosphate-bearing) and the overlying Dakhla Formation (deeper epicontinental shale/marl). This study aims to reconstruct the paleo-redox conditions during deposition of these thermally immature organic-rich sediments using carbon-sulfur-iron systematics and trace metal proxies in two cores, one each from the Quseir and Abu Tartur areas. Paleoproductivity, based on P content, seems to have been higher in the Quseir section than in the Abu Tartur section. The Quseir section also records a relatively greater occurrence of anoxic conditions during the accumulation of these sediments than the Abu Tartur section. This difference is indicated by its markedly higher total organic carbon (TOC) content as well as higher contributions of redox-sensitive and sulfide-forming metals (Mo, U, Ni, V, and Co). A weak correlation exists between S and TOC, and a positive S intercept (>1) was observed in most of the rock units of the study sections. A high consistency between the TOC-S-Fe relations and trace metals findings was found. The uppermost Duwi and the lowermost Dakhla strata, which have the highest TOC and represent a maximum sea transgression during the Late Cretaceous, have the highest contents of redox-sensitive trace metals. The carbonate-dominated transgressive Baris and Beida members of the Dakhla Formation record relatively stronger oxygen-depleted conditions during their accumulation than others, which led to relatively higher TOC contents and redox-sensitive metal accumulations. A scenario for the environmental conditions that existed during the deposition of these organic-rich successions, based on compiled trace metals and TOC-S-Fe implications, is reconstructed here.     

Contrasting behaviour of trace metals in the Scheldt estuary in 1978 compared to recent years

Dissolved and particulate trace metals (Cu, Cd, Pb, Zn, Ni, Fe and Mn) measured at six stations along the Scheldt estuary in October/November 1978 are compared with more recent data. Based on Ca content in the suspended matter, three distinct geochemical regions could be distinguished: the upper estuary (salinity 1–7) dominated by fluvial mud, mid-estuary (salinity 7–17) where the composition of the suspended matter remained relatively constant, and the lower estuary where marine mud prevailed. Re-suspension of sediments is the major factor controlling the composition of the particles in the upstream region. Anoxic conditions prevailed in the upper part of the estuary extending to a salinity of 15 in 1978, while at present the seaward boundary of the anoxic water body is located at less saline waters. Furthermore, the present-day metal load is much lower than in 1978. As a consequence of the changed situation, maxima in dissolved concentrations of redox-sensitive metals in the mid/lower estuary have moved as well, which affects the trace metal re-distribution pattern. In the anoxic zone, exchange processes between dissolved and particulate metal fractions were strongly redox regulated, with Fe and Mn as excellent examples. Iron was removed from the dissolved phase in the early stages of mixing resulting in an increase in the suspended particulate matter of the leachable ‘non-residual' Fe fraction from 2 to 3.5%. Due to its slower kinetics, removal of Mn from solution occurred in mid-estuary where oxygen concentrations increased. Cu, Cd and Zn on the contrary were mobilised from the suspended particles during estuarine mixing. External inputs of Pb, and to a lesser extent of Cu, in the lower estuary resulted in the increase of their particulate and the dissolved concentrations. Calculated K_d (distribution coefficient) values were used to assess the redistribution between the dissolved and particulate phase of the investigated metals. Due to the existence of the anoxic water body in the upper estuary, the importance of redox processes in determining the K_d values could be demonstrated. The sequence of K_d values in the upper estuary (Fe, Cd, Zn, Pb > Cu > Ni, Mn) is significantly different from that in the lower estuary (Fe > Mn > Pb, Ni, Zn, Cu, Cd). Thus, in such a dynamic estuary single metal-specific K_d values cannot be used to describe redistribution processes.     

Trace metals and hydrocarbons in sediments from the Southampton water region,with particular reference to the influence of oil refinery effluent

The concentrations of a number of trace metals (Co, Ni, Cu, Zn, Ag, Cd and Hg) have been determined in sediments from cores collected in the Southampton Water region. Measurements of total hydrocarbons were also made on several of the cores. Markedly elevated concentrations of copper, up to 362 μg/g dry wt, were found in sediments which were collected close to the discharge points of oil refinery outfalls and which also showed concentrations of total hydrocarbons considerably greater than the background levels for the estuary. The results of selective chemical extractions showed that much of the copper was present in the non-lattice fraction of the sediment. Although the concentrations of Co, Ni, Zn and Cd were significantly correlated with those of copper in the core showing the highest concentrations of this element, these metals were not enhanced in comparison with the levels found in several other parts of the estuary. Variations in concentrations of metals in sediments throughout the estuary were not correlated with differences in organic carbon content.     

Geochemical signature as paleoenvironmental markers in Holocene sediments of the Tinto River estuary (Southwestern Spain)

High absolute concentrations of metals associated to sulfide deposits located at the river source (Fe, Cu, Zn, Pb, Cd, and As) have been determined from the study of major and trace element concentrations in sediment samples obtained in two vibracores in the Tinto River estuary. In most cases, the enrichment factor (EF) for these metals is over 2. Cu, Zn and As have much higher EF, over 10 in surficial samples. Fe oxy-hydroxide precipitation in high-chlorinity zones and its scavenging effect on metals is the main control process on the chemical composition of sediment. Vertical evolution of the sedimentary and geochemical characteristics of the cores show that the estuary has undergone a process of progressive shallowness and a relative increase in acidic river water volume with respect to seawater. This effect has originated a downstream displacement of acid mixing processes affecting the estuary, which is reflected in higher metal concentrations in sediments of the outer zones of the system.     

夏季珠江口沉积物?水界面重金属分布特征及其影响因子研究

珠江口受复杂径?潮动力耦合作用的影响,河口重金属迁移转化机制复杂多变。本文基于2018年夏季珠江口及其邻近海域海水和沉积物的调查资料,研究了珠江口多动力因子驱动下7种重金属元素汞、砷、锌、镉、铅、铜和铬的沉积、迁移和积累机制。结果表明:相比于溶解态重金属（水体中）的赋存状况,吸附态重金属（沉积物中）更稳定,污染也更严重;采用皮尔逊相关分析和主成分分析计算出重金属元素与环境因子之间的响应关系,溶解态重金属主要以稀释混合过程为主,吸附态重金属受有机碳和氧化还原作用的影响较大;沉积物?水界面重金属的分配系数显示出铅和铬易被吸附在颗粒物上,而镉和汞易溶解在水体中,揭示了河口复杂动力影响下元素在不同介质的形态转换特征;除了镉?铬、镉?铜和铬?砷这3组元素间不显著相关,其他元素间的显著相关性表明了重金属元素具有相似的来源,并采用主成分分析探讨了重金属元素的潜在来源,主要来源为工业废水,农业和大气沉降次之。研究结果可为有效控制重金属的排放和河口污染治理提供重要支撑。     

胶州湾水生系统中Pb、Zn 的分布特征及其在生物体中的浓缩

利用在胶州湾采集的47组同站位多介质样品,进行了痕量金属Pb、Zn及其它环境因子的化学成分测试,讨论了胶州湾水-沉积物-生物系统中Pb、Zn的生物地球化学总体特征及其在各介质平面上的分布,揭示了胶州湾水-沉积物-生物系统对陆源物质输入的响应.垂向上表层沉积物是Pb、Zn组分的富集带,该系统中的生物相对于其所处水环境具有显著的富集作用,生物体中Pb、Zn生物浓缩系数分别为36、935;横向上,Pb、Zn在底层水和沉积物介质中的分布主要受控于河口,即高值区分布于胶州湾的各个主要河口区,特别是在沉积物中金属组分浓度的高值区主要集中分布于胶州湾的东部.而孔隙水中Pb、Zn的高值主要分布于水交替较弱的海域,如红岛前缘.但生物体中的痕量金属组分化学场空间分布规律与上述各介质的化学场均不吻合,亦即是生物体中痕量金属组分的浓度与其所处环境中的同名金属组分浓度无关.作者认为,生物对痕量金属组分的富集并不简单地取决于它所处环境介质中同名金属组分的总量,而存在形态上的选择性.通过回归分析揭示了孔隙水对生物体中Pb、Zn的富集贡献较大.     

Influence of chronic exposure to silver and mercury in the field on the bioaccumulation potential of the bivalve Macoma balthica

In order to assess the adaptation to metals previously observed in the bioindicator organism, Macoma balthica, subjected to chronic contamination by silver and mercury in the French Loire estuary, the bioaccumulation potential of individual organisms originating from the contaminated Loire estuary and a relatively uncontaminated control estuary (Somme) was evaluated using both radiotracers and stable isotopes of Ag (80 μg Ag litre⁻¹) and Hg (100 μg Hg litre⁻¹). Clams from the contaminated estuary were more sensitive to Ag (LT₅₀ = 9d) than those originating from the Somme estuary (LT₅₀ > 15d), even though the former bioaccumulated Ag to a significantly lower degree. This is attributed to a consequence of the chronic stress induced by Ag while clams were living in their natural environment. Therefore, past history of trace metal contamination should be considered when evaluating the susceptibility of M. balthica to heavy metal exposure. Lower uptake rates obtained for Hg (during the initial uptake phase only) and for Ag in clams from the polluted estuary suggest the presence of an adaptive trait for survival in contaminated areas. However, the lower degree of bioconcentration observed for Ag was not sufficiently low to reduce the sensitivity of the organisms to Ag and allow them to resist the toxic stress. Clams that survived Ag or Hg exposure at LT₅₀ did not protect themselves against metal toxicity by accumulating a significantly lesser amount of these metals than clams which did not survive metal stress. The results suggest that the bioaccumulation potential of each individual was not a factor which can explain the survival ability of M. balthica exposed to chronic Ag and Hg contamination in estuaries. In this case, cellular, biochemical and genetic levels of adaptation are presumed to be of greater importance.     

珠江河口微量金属的相态变化与生物效应

通过实验，研究了珠江河口海区悬浮颗粒物质对微量金属Ｃｕ，Ｚｎ和Ｃｄ化学相态和生物毒性的影响，以及腐植酸对微量金属Ｃｕ，Ｚｎ和Ｃｄ络合作用和生物毒性的影响，从而回答一些目前仍有争议的问题。     

Relevance of intracellular partitioning of metals in prey to differential metal bioaccumulation among populations of mummichogs (Fundulus heteroclitus)

Intracellular partitioning of trace metals is critical to metal tolerance in aquatic organisms and may also influence metal trophic transfer in ecosystems. In this study, we tested the relevance of metal (Cd, Cu, Pb, and Zn) intracellular partitioning in prey as an indicator of metal trophic availability to benthic forage fish, mummichogs (Fundulus heteroclitus), in chronically metal-polluted salt marshes in New York, USA. Two common prey of mummichogs in the study area, Palaemonetes pugio and Nereis acuminata, generally stored increasingly higher proportions of non-essential metals (particularly Pb) in insoluble (less trophically available) cellular components, as the whole body burdens increased. In contrast, intracellular partitioning of essential metals (Cu and Zn) in invertebrate prey varied relatively little among sites. Differential Cd and Pb intracellular partitioning patterns within P. pugio among sites were significantly associated with Cd and Pb whole body burdens in mummichogs, respectively (i.e., prey-driven bioreduction of metals), while bioaccumulation of Cu and Zn in mummichogs was similar among populations. The findings in this study suggest that metal intracellular partitioning within prey may be partially responsible for metal trophic availability to a predator in metal-polluted habitats, while there was also evidence that some predator-dependent processes may offset differential trophic availabilities from prey.     

Metallothionein and stress protein levels in shore crabs (Carcinus maenas) along a trace metal gradient in the Fal estuary (UK)

A field study was conducted to assess the potential use of metallothionein (MT) and stress proteins as biomarkers of trace metal exposure. Shore crabs (Carcinus maenas) were collected from five sites: three located in the metal-contaminated Fal estuary (Southwest England) and two control sites located on the south coast of England. Two methods for determining MT levels in crab midgut gland were compared. A direct method in which MT was quantified by spectrophotometric measurement of SH content, and an indirect method in which the metal content (Cu and Zn) of partially purified MT was measured. Stress protein levels (stress-70) were quantified in crab gills using an improved western blotting technique. Elevated Cu and Cd concentrations compared to values in control crabs were found in the midgut glands of crabs from the Fal estuary. No significant variation was seen in Zn concentrations among the five sites. The two techniques used for MT analysis yielded similar results. Metallothionein concentrations were slightly higher in crabs from Restronguet Creek (the most metal-contaminated site) than in crabs from the other four sites, however this difference was not significant (p > 0.05). Stress protein levels did not reflect the metal gradient.     

Mechanisms for trace metal enrichment at the surface microlayer in an estuarine salt marsh

The relative contributions of adsorption to particulate surfaces, complexation with surface-active organic ligands and uptake by micro-organisms were evaluated with respect to their importance in the surface microlayer enrichment (‘partitioning’) of Cd, Pb and Cu. The contributions of each process were inferred from field data in which partitioning of the dissolved and particulate forms of Cd, Pb and Cu, total and dissolved organic carbon, particles and total bacteria were observed. In the South San Francisco Bay estuary, particle enrichment appears to control trace metal partitioning. Trace metal association with the particulate phase and the levels of partitioning observed were in the order Pb > Cu > Cd and reflect the calculated equilibrium chemical speciation of these metals in computer-simulated seawater matrices.     

Influence of halophytes and metal contamination on salt marsh macro-benthic communities

Since an important fraction of the organic matter produced by salt marshes is decomposed in situ, macro-benthic communities are particularly exposed to the trace metals retained by these systems. Yet, few studies have investigated the macro-benthic communities using the between-root sediment habitat of the salt marsh halophytes (salt-tolerant plants), or the effect of trace metal pollution on its population dynamics. In the present study, samples were collected in vegetated and unvegetated sediment, in three salt marshes in the Tagus estuary, for trace metal concentration determination in the sediment and in the halophytes roots, grain size determination and macro-benthic organism identification. Data analysis revealed that the distribution of macro-benthic organisms is mainly determined by metal contamination, metal type and by the presence/absence of halophytes, not by the halophyte species. Five different associations were identified: resistant organisms were associated with the highest concentrations of lead (sediment); tolerant organisms with zinc, copper (sediment and roots) and lead (roots); cadmium in the sediment with a lack of macro-benthic life; sensitive organisms with low levels of metals except for cadmium in the roots; and macro-benthos typical of intertidal mudflats with unvegetated areas with low metal contamination.