Sediment Compartments Research Articles

Nitrogen and plankton dynamics in the lagoon of Venice

The nitrogen cycle in the lagoon of Venice, which is the largest Italian lagoon, was investigated by means of a 3D fully coupled transport – water quality model, which had been validated against a substantial amount of real-world data. Nitrogen fluxes among different ecosystem compartments were computed for each month of a reference year, and for each one of the three sub-basins into which the lagoon is conventionally subdivided. The computation included the loads of nitrogen discharged by the tributaries, the direct inputs from the industrial area and the city of Venice, the atmospheric loads, the fluxes at the three lagoon inlets and the internal fluxes between sediment and water compartments and among the three sub-basins. The results of the analysis show that the lagoon, as a whole, exports nitrogen towards the sea. Approximately 4000 tN/year are recycled by the system, while 4640 tN/year is the net input from the drainage basin and the other sources, thus leading to about 8640 tN/year of dissolved inorganic nitrogen that enter the water compartment. Around half of the this amount is used by primary producers, one fourth is exported towards the sea, and one fourth is transferred into the sediment compartment, or lost to atmosphere. These findings suggest that the exchanges through the inlets play an important role in keeping nitrogen concentration at an acceptable level. A more detailed analysis of the model results shows that the non-homogeneous spatial distribution of tributary discharges and point sources is the main cause of the differences in the ecosystem response and water quality among the three sub-basins. Nutrient poorer sub-basins fix a ration of available inorganic nutrient higher than nutrient rich ones. However, they are more efficient in transferring the biomass to the highest trophic levels. Results also include estimates of fluxes that were not quantified so far (such as grazing and recycling), and a validated model, which could have a practical use, for example for assessing implications of reduction of nutrient loads.

Relative importance of macrofaunal burrows for the microbial mineralization of pyrene in marine sediments: impact of macrofaunal species and organic matter quality

The significance of macrofaunal burrows for microbial organic matter mineralization is well recognized. However, despite the importance of marine sediments as main sinks for organic particle-reactive contaminants such as polyaromatic hydrocarbons (PAHs), the significance of biogenic structures for microbial pollutant mineralization is largely unknown. We measured microbial mineralization of the PAH pyrene in different sediment compartments (surface, burrow and reduced bulk sediments) as a function of contamination history (uncontaminated and pyrene-exposed), macrofaunal species (Amphiura filiformis and Nereis diversicolor) and sediment organic matter quality (labile Tetraselmis sp. and refractory lignin). Sediments were exposed to combinations of the 3 factors for 43 d, after which sediment samples from the 3 compartments were incubated for radio-respirometric measurements in 1 4 C-pyrene-coated flasks to monitor 1 4 CO 2 production (i.e. pyrene mineralization) for 128 d. Pyrene exposure enhanced microbial pyrene mineralization (MPM) rates by an order of magnitude in all compartments, signifying MPM as inducible. MPM rates increased successively from reduced bulk to burrow and surface sediments in uncontaminated treatments, while MPM rates were highest in burrows when exposed to pyrene. This emphasizes the oxygen dependence of MPM and pinpoints burrows as MPM hot spots. Enrichment with labile organic matter doubled MPM rates in pyrene-exposed surface sediment, likely fertilizing MPM where oxygen is readily available. Burrow sediment MPM rates were doubled with A. filiformis, suggesting species-specific stimulation of the microbial community within the burrows. In conclusion, burrow sediment appears to be at least as important as surface sediment for microbial PAH degradation in marine sediments. Furthermore, macrofaunal biodiversity, sedimentation events and the trophic state of the system should be taken into account when the fate and effects of organic contaminants are assessed in marine systems.

Oxygen supply for biostimulation of enzymatic activity in organic-rich marine ecosystems

The depletion of oxygen and the use of high energy cost electron acceptors for mineralisation processes are considered as one of the main reasons limiting degradation rates in aquatic organic-rich ecosystems. In this research the effect of two different biostimulation oxygen-releasing protocols were investigated using extracellular β-glucosidase activity rates. Organic-rich sediment and water was collected from the harbour of Genoa (Italy, North-Western Mediterranean) and oxygenation was generated either by positioning air tubes into the overlying water, or by placing oxygen release compounds (ORC) directly into the sediment, in laboratory microcosms. The increase in enzymatic activity was significant in the water compartment of the oxygenated microcosms (ANOVA, p<0.05) and greater for the ORC treatment. In particular, in the water, air tubes caused an increase in the β-glucosidase activity during the first 24 h (from 18.1 nmol l −1 h −1 ( T=0, before the treatment) to 50.9 nmol l −1 h −1 ( T=24 h)), while ORC induced a β-glucosidase activity increase over the whole period (from 12.4 nmol l −1 h −1 ( T=0) to 63.7 nmol l −1 h −1 ( T=60 d)) together with a reduction in carbohydrate (from 1.91 mg l −1 ( T=0) to 0.76 mg l −1 ( T=60 d)) and a sedimentary pH increase (from 8.04 ( T=0) to 8.22 ( T=60 d)). The response to ORC treatment in the overlying water was also evident in the potential carbohydrate turnover rate, decreasing from 0.86 h ( T=0) to 0.06 h ( T=60 d) and cell specific enzymatic activity, increasing from 3.8 ( T=0) to 22.8 nmol l −1 h −1 cell −1 ( T=60 d). A less marked change was observed in the sediment. This research shows that the supply of oxygen directly into the sediment rather than water aeration is a better way of naturally enhancing enzymatic degradation rates in organic-rich marine ecosystems. The marked effect observed in the water following the sediment treatment suggests the occurring of a close association of decomposition processes within water and sediment compartments.

Distribution of carbon between sediment and water in macrophyte dominated lowland streams

Water and sediment was monitored at monthly intervals from three macrophyte-rich lowland streams with markedly different plant communities and plant biomasses. Despite marked seasonal variations in concentrations of dissolved and sediment C and N, concentration ratios of total dissolved C (TDC) – and the constituent parts dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) – to sediment C (PC) and N (PN) were remarkably constant within each stream, and for all stream data combined. Sediment C and N each varied by more than a factor of 10 but had a constant concentration ratio of 12.9 (r 2=0.94). Overall ratios (g l−1) were 1.3±0.3 (n=102) for TDC:PC, 21±3 (98) for TDC:PN, 0.32±0.05 (103) for DOC:PC, 5.4±0.8 (101) for DOC:PN, 0.98±0.13 (102) for DIC:PC and 16±2 (96) for DIC:PN. The influence of macrophytes on organic-inorganic C transformations and between sediment and water compartments is further shown by the strong linear relationship (r 2=0.66, n=111) between EpCO2 and DOC, and PC and PN.

A dynamic compartment model to predict sedimentation and suspended particulate matter in coastal areas

This paper presents a new dynamic mass-balance model for suspended particulate matter (SPM) and sedimentation in coastal areas handling all important fluxes of SPM to, from and within coastal areas, as such areas can be defined according to the topographical bottleneck method. The model is based on ordinary differential equations and the calculation time (d t) is one month to reflect seasonal variations. An important demand, related to the practical utility of the model, is that it should be driven by variables readily accessed from standard monitoring programs or maps. Added to the dynamic core model are several (static) empirical regressions for standard operational effect variables used in coastal management, such as the Secchi depth, the oxygen saturation in the deep water, and chlorophyll-a concentrations. The obligatory driving variables include four morphometric parameters (coastal area, section area, mean and maximum depth), latitude (to predict surface water and deep water temperatures, stratification and mixing) and Secchi depth or SPM-concentrations in the sea outside the given coastal area. The model is based on four compartments: two water compartments (surface water and deep water; the separation between these two compartments is done not in the traditional manner from temperatures but from sedimentological criteria, as the water depth separating transportation areas from accumulation areas) and two sediment compartments (ET-areas, i.e., erosion and transportation areas where fine sediments are discontinuously being deposited, and A-areas, i.e., accumulation areas where fine sediments are continuously being deposited). The processes accounted for include inflow and outflow via surface and deep water, input from point sources, from primary production, from land uplift, sedimentation, burial (the transport of matter from surficial A-sediments to underlying sediments), resuspension, mixing and mineralization. The model has been validated with good results (the predictions of sedimentation are within the 95% confidence limits of the empirical data used to validate the model) against data collected by sediment traps placed in 17 Baltic coastal areas of different character. The paper also presents sensitivity and uncertainty tests of the model. The weakest part of the model concerns the sub-model to predict the ET-areas. Many of the structures in the model are general and have also been used with similar success for other types of aquatic systems (mainly lakes) and for other substances (mainly phosphorus, radionuclides and metals). We also present approaches to indicate how the model could be modified for coastal areas other than those included in this study, e.g., for open coasts, estuaries or areas influenced by tidal variations.

Uncertainty analysis of parameters for modeling the transfer and fate of benzo( a)pyrene in Tianjin wastewater irrigated areas

A Monte Carlo simulation for uncertainty analysis of three key parameters (local coal consumption rate Q 1 L , dry deposition velocity of aerosol particulate K P and biodegradation rate of benzo( a)pyrene in soil and sediment K R3 ) was conducted in this study. Results of the simulation indicate that the three parameters were influenced by uncertainty and that all equilibrium concentrations in the four bulk compartments and various sub-compartments were log-normally distributed. However, the results also indicated that among the six primary transfer fluxes, erosion associated with solids in soil and deposition associated with solids in water, along with output from sewers were also log-normally distributed, while deposition from air to soil and biodegradation in soil and sediment followed normal distributions. The effect of uncertainty on the model results of the three key parameters was derived using a comparison of upper and lower of confidence interval boundaries at the 95% level of confidence. The results reveal that uncertainty in the key parameters had a more significant influence on equilibrium concentrations of the chemical in the bulk compartments of soil and sediment than on concentrations in the other two bulk compartments, various sub-compartments and the six predominant transfer fluxes.

Microscale distribution of oxygen and nitrate in sediment inhabited by Nereis diversicolor: spatial patterns and estimated reaction rates

Profiles of oxygen and nitrate were measured with microsensors in sediment surrounding burrow structures of the polychaete Nereis diversicolor in a shallow mudflat in Limfjorden, Denmark. Rates and spatial distribution of reactions involving oxygen and nitrate (oxygen consumption, nitrification and nitrate reduction) in the vicinity of burrow structures and surface sediment were calculated and identified from concentration profiles. The burrow walls of the natural N. diversicolor population increased the surface area of the sediment-water interface several-fold and introduced oxic microniches with high metabolic activity into otherwise anoxic sediment. Nitrification and nitrate reduction were spatially separated processes, with nitrification restricted to the 0.9 to 1.5 mm thick oxic zone and nitrate-reduction occurring as a strictly anaerobic process in the nitrate-containing layers next to the oxic zone. At a density of 2560 individuals m-2, burrow walls of N. diversicolor account for 12 to 40% of bulk sediment oxygen uptake and 50 to 77% and 58 to 82% of bulk nitrification and nitrate reduction, respectively. Thus, burrow structures of macro-infauna are potentially a major sediment compartment involved in nitrogen cycling and may promote loss of bioavailable nitrogen from the sediment. [KEYWORDS: NO3-/NO2- biosensor ; Nitrogen cycling ; Sediments ; Bioturbation ; Nereis diversicolor ; Burrow walls]

Sedimentation of Oil-Mineral Aggregates for Remediation of Vegetable Oil Spills

A response alternative for floating vegetable oil spills based on sedimentation of negatively buoyant oil-mineral aggregates followed by anaerobic biodegradation in the sediments is under investigation. Sedimentation of floating canola oil by interaction with montmorillonite was demonstrated, and the oil-binding capacity of wet and dry clay was determined. Only two of the four factors that were investigated produced statistically significant effects on the sedimentation of floating oil: Clay was required for oil sedimentation to occur, and high mixing energy during the initial contact between oil and clay reduced the amount of oil that could be transported to the sediment compartment. The addition of ferric chloride as a coagulant and the mixing energy imparted during the flocculation period did not significantly affect the extent to which oil was transported to the sediment compartment. The maximum oil-binding capacity of dry high-surface-area (220–270 m²/g) clay was about three times higher than for wet clay (1 g oil/g dry clay versus 0.3 g oil/g wet clay). The oil-binding capacity of low-surface-area (20–40 m²/g) clay was less affected by prior interaction with water (0.7 g oil/g dry clay versus 0.6 g oil/g wet clay), but it was much less effective in promoting sedimentation of floating oil. The differences between the oil sedimentation abilities of these two types of clay suggest that the success of this remedial alternative requires a clear understanding of the nature of the oil-mineral interactions.

Development of a numerical model to study the dispersion of wastes coming from a marine fish farm in the Ligurian Sea (Western Mediterranean)

In order to simulate the dispersion of pollutants released from a marine fish farm in the Ligurian Sea (Western Mediterranean), the Lagrangian particle model LAMP3D has been nested into the numerical hydrodynamical model POM. Estimates of the regional dispersion patterns of nitrogen, phosphorus and organic carbon were calculated using a three-dimensional mesoscale grid and compared to in situ experimental data. In the numerical simulations, the dispersion of different aquaculture wastes (dissolved nutrients, faecal matter and feed wastes) was evaluated by changing the rates of particles settling and their condition of release (continuous or periodical). Results of the simulations clearly showed that dissolved particles tend to spread rapidly and undergo rapid dilution depending on dominant wind and surface currents direction. In contrast, settling particles remain mainly confined in the fish farm area and readily sink. Predicted concentrations of wastes were low in both the water column and sediment compartments and never exceed the threshold of environmental crises.

A high-resolution model for estimating the environmental fate of multi-species chemicals: application to malathion and pentachlorophenol

A high-resolution multi-species (HR-MS) model is presented that assesses the fate of up to four inter-converting chemical species. The current model has a more detailed environmental description than previous multi-species models in order to give a more accurate description of environmental fate. Improvements to the model environment include stratified air, soil and sediment compartments, the inclusion of a vegetation compartment, and the separation of the aerosol phase from the gas phase of the atmosphere. Such detailed environmental descriptions are particularly valuable when chemical heterogeneity is expected within environmental media as occurs with more reactive chemicals or local-scale simulations. The HR-MS model is illustratively applied to two situations for which a detailed environmental description is needed to describe the chemical fate accurately. The first example is the estimation of the atmospheric concentrations of malathion and its degradation product malaoxon following a local-scale application. The second example is a regional simulation of pentachlorophenol, which benefits from the more detailed treatment of ionizing chemical in the atmosphere. In both these cases, the HR-MS model is shown to be in good agreement with observed field data and provides a more accurate description of environmental fate than simpler multi-species models.

Metal Sources for Freshwater Invertebrates: Pertinence for Risk Assessment

Ecological risk assessments are likely to be more effective if they are built upon knowledge of from where and in what manner animals take up contaminants. We discuss the relative importance of various metal sources for aquatic invertebrates. First, we address the question do sediment-dwelling animals take up their metals from the overlying water compartment or the sediment compartment or both (both compartments include water and particles). We find that the overlying water column is more important as a metal source for insects, whereas the sediment compartment is more important for oligochaete worms. We explain this tendency by the behaviors of the animals involved. Second, we ask the question do animals take up their metals from food or water within a given compartment. Through case studies on three widespread freshwater invertebrates, we conclude that for some predatory insects food is their major source of several metals, whereas for the crustacean Hyalella both food and water appear to be important depending on the metal involved and the experimental protocol used to study the question. We conclude that ignoring food as a metal source could severely underestimate metal exposures for some animals. We suggest that integrating these complexities into laboratory tests and risk assessment protocols will improve their meaningfulness and thus their ability to protect aquatic ecosystems.

Estimation of PCDD/F distribution and fluxes in the Venice Lagoon, Italy: combining measurement and modelling approaches

The available experimental information on the occurrence of PCDD/Fs in the Venice Lagoon, Italy, was compiled and used to calculate fugacities for the environmental compartments of sediment, suspended particulate matter (SPM), water and air and then used to estimate fugacity ratios and assess the likely net direction of flux between media. The bottom sediment: SPM fugacity ratios for different PCDD/Fs indicate conditions close to equilibrium, suggestive of the close coupling of SPM with re-suspended sediment. Sediment/water and the sediment/air fugacity ratios suggest that net flux directions vary depending on the congener and the location within the lagoon. Net sediment–water–air movement (i.e. re-mobilisation/volatilisation) is suggested for the lighter congeners from the industrial canals, where the highest PCDD/F concentrations in the lagoon occur. The tendency to volatilise increases with decreasing congener molecular weight. In contrast, net deposition (air–water–sediment) appears to be occurring for the heaviest (hepta- and octa-) substituted PCDD/Fs. OCDF represents a marker of the industrial district of the lagoon, decreasing in concentration and as a fraction of total PCDD/Fs with increasing distance. The fugacity-based quantitative water air sediment interaction (QWASI) mass-balance model was applied to the central part of the lagoon. The key parameters for the determination of the model output, identified by a sensitivity analysis, were: the sediment active depth, the sediment re-suspension and deposition rates, and the total input of PCDD/Fs to the system. The QWASI model also indicates the tendency for the lighter PCDD/Fs to be released from surface sediment to the water column.

Determination of Cd, Pb, Zn and Cu in Sediment Compartments by Sequential Extraction and Isotope Dilution Inductively Coupled Plasma Mass Spectrometry (ID-ICP-MS)

Trace concentrations of Cd, Cu, Pb and Zn in four different sediment fractions extracted in sequence were determined by isotope dilution inductively coupled mass spectrometry (IDICPMS). The metals from each fraction were extracted following the sequential extraction procedure recommended by the Bureau Commun de Référence (BCR) of the Commission of the European Communities. As an alternative to external calibration, the elements were quantified by spiking the extracted solutions with 112Cd, 63Cu, 208Pb and 66Zn and application of isotope dilution. The proposed approach was applied to a sample collected from a lake and two standard reference materials, NIST2704 river sediment from the National Institute of Standards & Technology and the BCR-277 estuarine sediment. Detection limits, for each extracted solution, varied from 0.31 to 0.53 mug L¹ for Cd, 0.92 to 2.9 mug L¹ for Cu, 0.22 to 1.1 mug L¹ for Pb and 1.3 to 7.6 mug L¹ for Zn. The sum of the metals concentration in the different fractions was compatible with 95% confidence level found amounts obtained with complete digestion of the samples and with the certified values of the standard reference materials.

Dissolution Kinetics of Heavy Metals in Dutch Carbonate‐ and Sulfide‐Rich Freshwater Sediments

In two sulfide-rich freshwater sediments from the Biesbosch and Kromme Rijn River in the Netherlands differing in carbonate content and acid volatile sulfide (AVS) content, metal and sulfide dissolution kinetics were studied at different acid concentrations by varying both the procedure of acid addition and the extraction time. The establishment of equilibrium was monitored by measuring the pH in time, which reached a near constant value. The equilibrium pH was reached quickly when large amounts of acid were added and slowly when small amounts of acid were added. This observation was confirmed by the yield of extracted metals after either a 45-min or 24-h extraction over a pH range from 0 to 5. The pH factor seemed to be of more influence than time for the dissolution of metals. The amount of extracted metals was highly dependent on the metal itself due to its physico-chemical behavior. Although the sediments studied varied in carbonate content, acid volatile sulfide (AVS), and total metal content, the extracted fraction of metals compared with their total content in the sediment was similar for most metals. Finally, the AVS content as well as the ratio of simultaneously extracted metals (SEM; sum of Cd, Cu, Ni, Pb, and Zn) to AVS decreased with increasing pH. Because the SEM to AVS ratio may be used to set environmental quality criteria for the sediment compartment, this observation is of significance.

Dissolution Kinetics of Heavy Metals in Dutch Carbonate- and Sulfide-Rich Freshwater Sediments

In two sulfide-rich freshwater sediments from the Biesbosch and Kromme Rijn River in the Netherlands differing in carbonate content and acid volatile sulfide (AVS) content, metal and sulfide dissolution kinetics were studied at different acid concentrations by varying both the procedure of acid addition and the extraction time. The establishment of equilibrium was monitored by measuring the pH in time, which reached a near constant value. The equilibrium pH was reached quickly when large amounts of acid were added and slowly when small amounts of acid were added. This observation was confirmed by the yield of extracted metals after either a 45-min or 24-h extraction over a pH range from 0 to 5. The pH factor seemed to be of more influence than time for the dissolution of metals. The amount of extracted metals was highly dependent on the metal itself due to its physico–chemical behavior. Although the sediments studied varied in carbonate content, acid volatile sulfide (AVS), and total metal content, the extracted fraction of metals compared with their total content in the sediment was similar for most metals. Finally, the AVS content as well as the ratio of simultaneously extracted metals (SEM; sum of Cd, Cu, Ni, Pb, and Zn) to AVS decreased with increasing pH. Because the SEM to AVS ratio may be used to set environmental quality criteria for the sediment compartment, this observation is of significance.

The newly proposed EU risk assessment concept for the sediment compartment

Within the frame of the EU Existing Chemicals Regulation (793/93/EEC), a new concept for the assessment of the sediment compartment has been developed. During the assessment process it has become clear that for substances that strongly adsorb or bind to sediment the equilibrium partitioning method (EPM) presently employed is not sufficient. The new approach is therefore based on long-term sediment tests with benthic organisms. A testing strategy has been developed which considers species which represent different habitats and feeding strategies in order to adequately address all possible routes of exposure. The new concept has proven to be valuable in risk assessments, two examples (3,4-DCA, DODMAC) of which are shown in more detail.

Cadmium accumulation by invertebrates living at the sediment–water interface

Benthic animals can take up trace metals both from the sediment compartment in which they burrow and from the water column compartment above their burrows (we define both compartments as containing water and particles). If criteria for the protection of benthic animals are based on metal concentrations in one of these two compartments, then it should first be demonstrated that the majority of the metal taken up by these animals comes from the given compartment. To determine whether benthic animals take up the majority of their cadmium (Cd) from the sediment compartment, we created a Cd gradient in lake sediment and compared Cd accumulation by the invertebrates colonizing these sediments with Cd concentrations in the sediment compartment. On the basis of this relationship and using a bioaccumulation model, we estimate that indigenous benthic invertebrates take up the majority of their Cd from the water column compartment. The results of our experiment are similar to those from a previous study conducted on a different benthic community in a larger lake. Taxa common to both lakes obtained similar proportions of their Cd from the water column compartment, suggesting that Cd accumulation by the same species will be constant across lakes of differing size and chemistry. Our results strengthen the argument that the protection of benthic communities from metal pollution should consider metal in both the water column and sediment compartments. In this regard, the AVS model, which considers only sedimentary metals, was more effective in predicting Cd concentrations in pore waters than those in most animal taxa. We suggest that measurements of vertical chemical heterogeneity in sediments and of animal behavior would aid in predicting the bioaccumulation and effects of sedimentary pollutants.

River Water Quality Model no. 1 (RWQM1): Case study I. Compartmentalisation approach applied to oxygen balances in the River Lahn (Germany)

A case study on the application of the River Water Quality Model No. 1 (RWQM1) is presented in order to illustrate the importance of modelling a sediment compartment for an ecologically meaningful assessment of the impact of wastewater effluents and combined sewer overflows. The focus of this case study is on the compartmentalisation approach of the RWQM1 that makes such a description possible. In contrast to this, a strongly simplified biochemical submodel is used that considers only oxygen and dissolved substrate. The object of the case study is the River Lahn, a moderately polluted 5th order stream in Germany, for which the connectivity of surface/subsurface flows and mass fluxes within river sediments have been intensively investigated. The hyporheic flow between a downwelling and upwelling zone of a riffle-pool sequence has been studied with the aid of tracer experiments and continuous records of water constituents. High diurnal fluctuations of oxygen travelled to considerable depth of the sediment and oxygen in the interstitial water decreased considerably while travelling through the riffle. Starting with the implementation of a strongly simplified version of the biochemical part of the RWQM1, but with the consideration of a sediment pore water compartment in addition to the water column compartment, a calibration procedure is performed using tracer data from the water column and the sediment. The calibrated model is then used to study the system response to wastewater treatment plant effluent and combined sewer overflow emissions. The modelling approach makes it possible to quantify the sediment oxygen demand and the spatial and temporal extent of sediment zones with oxygen depletion. However, the spatially averaged approach does not account for inhomogeneities in the sediment. It is shown that for this river with its alluvial coarse sediments even moderate emissions from sewerage systems may be high enough to drop sediment oxygen concentrations to low levels while those in the surface flow remain close to saturation. Similarly, it is demonstrated that combined sewer overflows may cause anoxic sediment oxygen conditions for extended time periods. The implications for ecologically sound river water quality modelling and for specific quality objectives are discussed.

Environmental toxicity assessment in the Paraná river delta (Argentina): simultaneous evaluation of selected pollutants and mortality rates of Corbicula fluminea (Bivalvia) early juveniles

Water and sediment samples were collected in the lower Paraná delta at four sites with different levels of exposure to pollution to evaluate the anthropogenic impact through chemical analyses and mortality bioassays. Individual polychlorinated biphenyls, chlorinated pesticides, aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, and heavy metals were measured in waters, porewaters and sediments. The same three phases were also subjected to toxicity assays with straight-hinged juveniles of Corbicula fluminea. Concentrations of several pollutants were above levels recommended for the protection of aquatic life: in waters, Zn, Cu and Cr were 1.6–4.9 times higher, whereas in the sediments Cr was 1.8–3.6, and benzo(a)pyrene was 2.8–5.6 times higher. Pollutant concentrations followed a clear geographic pattern with highest values in the densely populated area of the Reconquista and Luján rivers, lower levels in the San Antonio, and lowest loadings in the remote Paraná de las Palmas. This gradient was adequately matched by the pattern of mortality rates of C. fluminea early juveniles, which were highest in the Reconquista-Luján (40–93%) and lowest (and not significantly different from the control) in the Paraná (3.3–23%). Mortality rates also increased from surface waters (3.3–53%), to porewaters (12–73%), to sediments (23–93%). Although toxicity was probably mainly due to dissolved contaminants, agreement between chemical and biological evidence of pollution was best for the sediment compartment, whereas porewater and surface water showed a higher degree of variability.

Composition of biofouling communities on suspended oyster cultures: an in situ study of their interactions with the water column

This study was based on in situ experiments conducted in a shellfish lagoon (Thau, France) to determine the interactions between suspended oyster Crassostrea gigas Thunberg cul- tures and their environment at a seasonal scale. Three groups of descriptors were considered simul- taneously during the course of the study: the taxonomic composition of an oyster culture unit (OCU), the nutrient and oxygen exchanges at the OCU-water interface (fluxes) and the water-column char- acteristics (water). We used a multiple factorial analysis (MFA) to assess in a single reference frame: (1) the seasonal organisation of the taxonomic groups which compose the OCU and (2) the interac- tions between this multispecific assemblage and the water column. We found strong seasonal vari- ability in the OCU composition and the OCU interactions with the water column. Taxonomic richness of the biofouling was at a maximum in spring and minimum in August after a bottom anoxia. Among biofouling species, ascidians always constituted the dominant group. The annual pattern of coexis- tence observed between ascidians and oysters might be explained by resource partitioning, and also by a beneficial trophic interaction between these 2 groups. The structure of the OCU was based on a complex system of interactions, particularly in spring. During this season, the development on the OCU of macrofauna of soft substratum such as polychaetes reflected the presence of a suspended sediment compartment. The influence of the OCU on the water column was maximum in summer and minimum in winter. In the shellfish zone, the OCU plays a central role in nutrient renewal, particu- larly in summer when benthic fluxes are insufficient to satisfy phytoplankton nitrogen requirements. We estimate that the multispecific assemblages occurring on the oyster ropes (oysters and biofouling) have a potential annual DIN production of 2 × 10 7 mol yr -1 , and thus oyster cultures could have a determining influence on nitrogen recycling in the water column in the Thau lagoon.