Dioxin compounds in the deep-sea rose shrimp Aristeus antennatus (Risso, 1816) throughout the Mediterranean Sea

Polychlorodibenzo-p-dioxins (PCDDs) and polychlorodibenzofurans (PCDFs) are among the more toxic anthropogenic contaminants. They are fat-soluble and accumulate in animal tissues. Exposure to PCDD/Fs can cause several endocrine, reproductive and developmental problems in animals, including human beings. Several studies have demonstrated that fish and invertebrates living in association with sediments are exposed to and accumulate contaminants, but to date there have been no studies of PCDD/Fs contamination in deep-sea regions. Specimens of Aristeus antennatus (Risso, 1816) were collected from depths of 600–2500 m at different points in the Mediterranean Sea, from the western basin off the coast of Barcelona to the central basin off the Peloponnesian Peninsula, with otter trawl gear. Amounts of PCDD/Fs were measured in different animal tissues by high resolution gas chromatography coupled to high resolution mass spectrometry (HRGC–HRMS). This is the first study to report the presence of PCDD/Fs in deep-sea organisms dwelling at depths below 600 m. A. antennatus presented levels of PCDD/Fs of the same order of magnitude, or slightly higher, as those found in shallow-water species (Melicertus kerathurus) with respect to land-generated contamination. This highlights the widespread distribution of these pollutants and the potential threat posed to the biodiversity of fragile and vulnerable ecosystems such as the deep-sea. PCDD/F levels detected in the edible parts (muscle) of the commercial shrimp A. antennatus were clearly below the toxic limit value established by European legislation. Levels followed the trend muscle<ovary<hepatopancreas, according to the lipid content of the organs in question. The specific profile of toxic PCDD/F congeners in the studied Penaeid shrimp specimens was characterized mainly by octachlorodibenzo-p-dioxin (OCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF) and 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) congeners. Moreover, non 2,3,7,8-substituted congeners bioaccumulated in this species, as observed in shallow-water crustaceans. PCDD/Fs were found in the shrimp A. antennatus throughout the Mediterranean Sea. Total PCDD/Fs burdens were higher in shrimps caught in the western Mediterranean than in those caught at eastern Mediterranean sites. There was a tendency for higher levels of PCDD/F contamination in samples obtained from deeper (2500 m) than from shallower sites (600 m).     

Distribution and retention of Petrolisthes armatus in a coastal plain estuary: The role of vertical movement in larval transport

Since transport of planktonic larvae is essential to the maintenance and expansion of many marine species, we examined the spatial and temporal distribution of green porcelain crab Petrolisthes armatus (Gibbes, 1850) larvae and the possible underlying physical and behavioral mechanisms using a combination of field observations and numerical modeling. The field study consisted of observations of larval abundance and distribution as well as hydrographic surveys of the Satilla River estuary on the east coast of the USA in August 2006. Larvae were found throughout the water column within the tributaries but primarily at depth in the main river. A numerical model was used to examine the effect of flow and possible larval behavior responsible for the observed distribution and the consequences for larval retention in the estuary. Model results that included downward larval movement are consistent with the field observations, supporting the hypothesis that P. armatus larvae vertically migrate within the water column, which aids in their retention within the estuary.     

Annual variation in the nocturnal nekton assemblage of a tropical estuary

A nocturnal demersal nekton assemblage was sampled fortnightly for two years at five sites in the Labu estuary using a 3 m beam trawl with a 3·2 cm mesh net. Forty-eight species were caught, totalling 31 458 individuals with the five most abundant species comprising over 95% of the catch. Using multiple regression techniques with Fourier transformations, the mean number of species, S, the mean abundance, N, and mean weight, W, were found to conform to a regular annual cycle with maxima in April and May. Seven of the 11 most abundant species demonstrated regular annual cycles of abundance. S, N and W were greatest in the wider, middle sites and lowest in a shallow, stagnant side branch of the estuary. Catch weights and abundances were significantly correlated with physical data.Salinity and temperature values in the estuary exhibited an annual cycle with maxima occurring in February/March. The annual thermal variation of surface water outside the estuary followed a similar cycle. The salinity at the mouth of the Markham River is lowest during January/February, which corresponds with the rainy season in the Markham River catchment. Significant annual variation existed between years in estuarine bottom salinity and salinity values in Labu Bay.Several species exhibited a greater variation in abundance and mass between years than within years. This supports the hypothesis that in the tropics between-year variation in coastal marine biotic communities is greater than within-year variation.     

Reduced rates of primary production in the Columbia River Estuary following the eruption of Mt Saint Helens on 18 May 1980

The eruption of Mt Saint Helens on 18 May 1980 resulted in a massive increase in suspended particulate material in the Columbia River Estuary, producing a substantial increase in light attenuation. Since photosynthesis in the estuarine water column is partly controlled by the depth of light penetration in the water, photosynthesis was reduced by about 75% during the period of increased turbidity. It took about five weeks for the estuary to clear. Although primary production within the estuarine water column was greatly diminished during this period, the flux of particulate carbon through the estuary was high, and the total production we estimate to have been lost amounted to only about 2% of the total particulate carbon flux just after the eruption. That the high levels of turbidity in the estuary did not induce a severe fall in the phytoplankton population is evidence that phytoplankton biomass concentrations in the Columbia River Estuary are mostly a function of import from the Columbia River, rather than a function of in situ production.     

Three dimensional water quality modeling of a shallow subtropical estuary

Knowledge of estuarine hydrodynamics and water quality comes mostly from studies of large estuarine systems. The processes affecting algae, nutrients, and dissolved oxygen (DO) in small and shallow subtropical estuaries are relatively less studied. This paper documents the development, calibration, and verification of a three dimensional (3D) water quality model for the St. Lucie Estuary (SLE), a small and shallow estuary located on the east coast of south Florida. The water quality model is calibrated and verified using two years of measured data. Statistical analyses indicate that the model is capable of reproducing key water quality characteristics of the estuary within an acceptable range of accuracy. The calibrated model is further applied to study hydrodynamic and eutrophication processes in the estuary. Modeling results reveal that high algae concentrations in the estuary are likely caused by excessive nutrient and algae supplies in freshwater inflows. While algal blooms may lead to reduced DO concentrations near the bottom of the waterbody, this study indicates that stratification and circulation induced by freshwater inflows may also contribute significantly to bottom water hypoxia in the estuary. It is also found that high freshwater inflows from one of the tributaries can change the circulation pattern and nutrient loading, thereby impacting water quality conditions of the entire estuary. Restoration plans for the SLE ecosystem need to consider both a reduction of nutrient loading and regulation of the freshwater discharge pattern.     

The effect of a storm surge event on the macrophytes of a temporarily open/closed estuary,South Africa

Temporarily open/closed estuaries typically open to the sea due to freshwater inflow coupled with storm surge events. In September 2008, in the absence of freshwater inflow, the mouth of the East Kleinemonde Estuary breached in response to a storm surge. The mouth of the estuary closed the following day at a high level. Marine overwash events following the breach introduced large volumes of saline water into the estuary and raised the water level by 0.07–0.33 m. Salinity was significantly higher in the 15 month closed phase after the breach (31 ± 0.9) compared to 21.9 ± 0.9 in the closed brackish phase before the breach. The historical average salinity for the estuary during a closed period is 23–25. The increase in salinity has reduced submerged macrophytes Ruppia cirrhosa and Chara vulgaris cover by 38.1%. Macroalgal cover of species such as Dictyota dichotoma, Caulacanthus ustulatus, Codium tenue and Ulva spp. have increased by 7.9%. The saline high water levels have also significantly reduced supratidal salt marsh cover by 15.2%, and reed and sedge cover by 19.7%. Loss of these habitats may result in bank destabilisation and erosion. This is the first record of an extended saline period in the 15 years the estuary has been monitored. Sea level rise in association with climate change, together with localised freshwater inflow reduction is likely to result in an increase in marine overwash events. The frequency and duration of closed saline periods are likely to increase in this type of estuary. A loss of submerged macrophytes may have significant impacts on faunal composition and abundance and on the subsequent functioning of temporarily open/closed estuaries. This has serious ecological implications since these estuaries represent 70% of the different types of estuaries found in South Africa.     

Salt-wedge propagation in a Mediterranean micro-tidal river mouth

The dynamics of a seasonally formed salt-wedge propagating along the micro-tidal channel of Strymon River estuary, Northern Greece, and its consequences on river water quality, are thoroughly studied through intensive sampling campaigns. The wedge is developed at the downstream river part, under the summer limited freshwater discharge conditions (Q < 30 m³/s). The geometric features of the wedge (length and thickness) appeared directly related to Strymon River discharge. A maximum intrusion length of 4.7 km along Strymon River estuary was observed under minimum river discharge of almost 6 m³/s. Relations produced from in situ data illustrate that limited river flow expands the wedge horizontally, reducing its vertical dimension, while higher flows lead to increased wedge thickness. Estuarine flushing time ranges between 0.2 and 1.5 days, exponentially dependent on Strymon River discharge. Wedge velocities depicted tidal asymmetry between tidal phases, with consistent inward motion, even under the ebb tidal stage. Strong vertical stratification prevails throughout the tidal cycle, proving the limited vertical mixing between the two layers, although higher interfacial stresses are produced in ebb. Bottom topography plays an interesting role in wedge propagation, as the presence of an underwater sill either prevents saline intrusion during flood or isolates the front of the wedge from its core at the ebb. Ecological consequences of salt-wedge propagation in Strymon River estuary are the frequent evidence of bottom hypoxic conditions and the increased TSS levels, leading to the occurrence of a turbidity maximum at the tip of the salt-wedge. Higher BOD and ammonium levels were mostly observed at the river end, associated to point and non-point pollution sources. Nitrates and silicates were found associated with freshwater fluxes, while ammonia levels were related to saline intrusions. The reduced phosphorus freshwater fluxes, resulting from phosphorus uptake at the upstream reservoir (Kerkini Lake) and the increased bottom turbidity induced by the salt-wedge seem responsible for the limited chlorophyll-a levels along Strymon River estuary.     

Flow and geochemistry of groundwater beneath a back-barrier lagoon: The subterranean estuary at Chincoteague Bay,Maryland, USA

To better understand large-scale interactions between fresh and saline groundwater beneath an Atlantic coastal estuary, an offshore drilling and sampling study was performed in a large barrier-bounded lagoon, Chincoteague Bay, Maryland, USA. Groundwater that was significantly fresher than overlying bay water was found in shallow plumes up to 8 m thick extending more than 1700 m offshore. Groundwater saltier than bay surface water was found locally beneath the lagoon and the barrier island, indicating recharge by saline water concentrated by evaporation prior to infiltration. Steep salinity and nutrient gradients occur within a few meters of the sediment surface in most locations studied, with buried peats and estuarine muds acting as confining units. Groundwater ages were generally more than 50 years in both fresh and brackish waters as deep as 23 m below the bay bottom. Water chemistry and isotopic data indicate that freshened plumes beneath the estuary are mixtures of water originally recharged on land and varying amounts of estuarine surface water that circulated through the bay floor, possibly at some distance from the sampling location. Ammonium is the dominant fixed nitrogen species in saline groundwater beneath the estuary at the locations sampled. Isotopic and dissolved-gas data from one location indicate that denitrification within the subsurface flow system removed terrestrial nitrate from fresh groundwater prior to discharge along the western side of the estuary. Similar situations, with one or more shallow semi-confined flow systems where groundwater geochemistry is strongly influenced by circulation of surface estuary water through organic-rich sediments, may be common on the Atlantic margin and elsewhere.     

Annual variation in the nocturnal nekton assemblage of a tropical estuary

A nocturnal demersal nekton assemblage was sampled fortnightly for two years at five sites in the Labu estuary using a 3-m beam trawl with a 3·2-cm mesh net. Forty species were caught, totalling 31458 individuals with the five most abundant species comprising over 95% of the catch. Using multiple regression techniques with Fourier transformations, the mean number of species (S), the mean abundance (N), and mean weight (W) were found to conform to a regular annual cycle with maxima in April and May. Seven of the 11 most abundant species demonstrated regular annual cycles of abundance. S, N and W were greatest in the wider, middle sites and lowest in a shallow, stagnant side branch of the estuary. Catch weights and abundances were significantly correlated with physical data.Salinity and temperature values in the estuary exhibited an annual cycle with maxima occurring in February-March. The annual thermal variation of surface water outside the estuary followed a similar cycle. The salinity at the mouth of the Markham River is lowest during January-February, which corresponds with the rainy season in the Markham River catchment. Significant annual variation existed between years in estuarine bottom salinity values in Labu Bay.Several species exhibited a greater variation in abundance and mass between years than within years. This supports the hypothesis that in the tropics between-year variation in coastal marine biotic communities is greater than within-year variation.     

Transport of suspended matter in a bar-built Danish estuary

The river Varde Å discharges into the bay of Ho Bugt on the western coast of Jutland forming a small, bar-built estuary. This paper deals with tidal fluxes of water and sediment in the Varde Å estuary.The inflowing water at flood tide is part of a turbidity maximum in the northern part of the bay. At high tide slack water the suspended material deposits inside the estuary. During ebb-tide it is resuspended, and the estuary bottom is washed clean coinciding with the influx of relatively pure freshwater from the drainage area.From one station in the estuary mouth, current velocities and concentrations of suspended material have been measured during 10 tidal periods covering all four seasons. It is shown how these data can be used in a quantitative calculation of the transport of water and suspended material through the cross-section of study.A model has been formulated which—based on half-tidal periods—quantifies the transport of water and suspended material through the estuary mouth.The model is calibrated on the basis of measurements made during the above-mentioned 10 tidal periods. The rather small number of measurements is to some extent compensated for by a carefully pre-arranged selection of tidal periods.The model is discussed in relation to the prediction of net suspended transport through the estuary mouth in different weather and tidal situations.     

Prediction of salinity intrusion in the sheltered estuary of Terengganu River in Malaysia using 1-D empirical intrusion model

Generally one dimensional(1-D) empirical salinity intrusion model is limited to natural alluvial estuary. However,this study attempts to investigate its ability to model a sheltered alluvial estuary of the Terengganu River in Malaysia. The constructed breakwater at the mouth of the river shelters the estuary from direct influence of the open sea. The salinity density along the estuary was collected during the wet and dry seasons for scenarios before and after the constructed breakwater. Moreover, the freshwater discharges, tidal elevations and bathymetry data were also measured as model inputs. A good fit was demonstrated between simulated and observed variables,namely salinity distribution and intrusion length for both scenarios. Thus, the results show that 1-D empirical salinity model can be utilized for sheltered estuarine condition at the Terengganu Estuary, but with an appropriate determination of an initial point. Furthermore, it was observed that the salinity intrusion in the study area is largely dependent on the freshwater discharge rather than tidal elevation fluctuations. The scale of the salinity intrusion length in the study area is proportional to the river discharge of the –1/2 power. It was appeared that the two lines of the 1-D empirical salinity model and discharge power based equation fitted well to each other, with the average predicted minimum freshwater discharge of 150 m3/s is going to be required to maintain acceptable salinity levels during high water slack(HWS) near the water intake station, which is located at 10.63 km from river mouth.     

Mass balance of lead through a small macrotidal estuary: The Morlaix River estuary (Brittany,France)

Dissolved and particulate lead were measured over an annual cycle (12 surveys between February 1998 and January 1999) in the Morlaix River estuary (Brittany, France). The concentrations were investigated in both the water column and the sediment of the river bottom in relation to hydrological conditions. In the water column, dissolved and particulate lead concentrations ranged from 0.1 to 4.4 nM and from 0.04 to 1.9 μmol g^− 1, respectively. Lead concentrations in surface sediment varied from 0.04 to 0.19 μmol g^− 1 and concentrations in the sediment pore water of the estuary were below the detection limit. Compared with the ranges known for pristine estuaries, concentrations of Pb in the water column of the Morlaix River estuary were found to be much higher. Concentrations of Pb also exceeded the lower range of those known for industrialized estuaries. Extensive agricultural activities in the drainage basin may be responsible for Pb levels above pristine conditions. Furthermore, the sediment appeared not to be contaminated. A mass balance was constructed quantifying all known sources and sinks for the Pb in the estuary. Riverine input accounts for most of the total annual metal flux. Burial in sediments was the major sink within the estuary, which acts as a trap especially for the particulate lead. The mass balance shows that the metal accumulation ranged between 414.6 and 446.0 kg year^− 1.     

Estuarine nitrification: A naturally occurring fluidized bed reaction?

The rates of nitrification in the water column of the Tamar river estuary, southwest England have been measured using the incorporation of H¹⁴CO₃ in samples with and without the inhibitor of nitrification, 2-chloro-6-(trichloromethyl) pyridine (N-Serve). N-Serve proved successful in totally inhibiting NH₄-oxidizing bacteria but the activity of NO₂-oxidizing bacteria was inhibited by only 30%; other organisms were only slightly affected. Measurements of the nitrification rate made over the entire salinity range of the estuary (0–30‰) showed that maximum nitrification always coincided with the turbidity maximum. The field data suggest that the organisms responsible for nitrification were associated with periodically resuspended particulate material and that the turbidity maximum acts in a manner similar to a fluidized bed reactor. A dispersion model has been used to demonstrate that nitrification in the water column can account for 100% of the NO₂ maximum which is apparent down estuary from the turbidity maximum.     

Trophic functioning of the St. Lucia estuarine lake during a drought phase assessed using stable isotopes

The St. Lucia Estuary is Africa’s largest estuarine system and is currently experiencing the stress of prolonged freshwater deprivation, manifested by extremely low water levels and hypersalinity. These unprecedented conditions have raised questions regarding the trophic functioning of the ecosystem. Despite the substantial amount of research previously undertaken within this system, no studies of food web structure and function have yet been documented. This study therefore aimed to examine the food web structure of the St. Lucia estuary system through the use of carbon and nitrogen stable isotope analysis. Analysis of carbon isotope ratios indicates that benthic carbon sources are most utilised at sites with low water levels and generally higher salinity (Catalina Bay, Charter’s Creek). Conversely, the estuarine region of the mouth and Narrows, with its elevated water levels and lower salinity, still sustains a viable pelagic food web. Analysis of δ¹⁵N ratios indicates that the number of trophic transfers (food chain length) might be related to water levels. Overall, the study provides a greater understanding of the ecological processes of this complex estuarine lake, which may allow for future comparisons of trophic functioning under drought and normal/wet conditions to be made.     

Calibration studies concerning a one-dimensional numerical tidal model with particular reference to resistance coefficients

Since the hydrodynamics of flow in most shallow estuaries is dominated by the boundary resistance, the selection of appropriate values for the roughness coefficients in any numerical tidal model is important. The paper describes studies involving the intra-tidal variation of resistance coefficients in twelve reaches of the Conwy estuary in North Wales. Resistance coefficients are evaluated for six water level data sets covering the neap to spring tidal variation by cubature based and parameter estimation techniques. Results from these two inverse methods are shown to agree well despite somewhat different schematizations for the estuary. A direct check on the intra-tidal variation of resistance coefficients at one particular reach is made possible by specifically gathered field data described in an earlier paper by Knight. In general the resistance coefficients exhibit three basic trends—a strong stage dependence, a flow directional dependence due to flood or ebb dominated bed forms, and a high variability with location along the estuary, especially where sand banks are oxposed at low water. The optimized resistance coefficients are then used in a one-dimensional numerical tidal model in order to ascertain the effect of particular modes of calibration. Estuary mean errors in water levels are of the order of 2,5 and 16 cm for the three calibrations tested.     

High partial pressure of CO2 and its maintaining mechanism in a subtropical estuary: the Pearl River estuary,China

We investigated distributions of surface water CO₂ partial pressure (pCO₂), dissolved oxygen (DO) and associated carbonate parameters in the Pearl River estuary, a large subtropical estuary under increasingly anthropogenic pressure in China, in the summer of 2000 and late spring of 2001. pCO₂ levels, measured underway using a continuous measurement system, were high during both seasons, with levels of >4000 μatm at salinity <0.5. pCO₂ distribution overall mirrored DO across the salinity gradient. Using the linear relationship between excess CO₂ and apparent oxygen utilization (AOU) in surface water, we conclude that aerobic respiration is the most important process in maintaining such high pCO₂ measured upstream. The material being respired is likely in a close association with the organic pollutants discharged into the system. Based on the measured excess CO₂ vs. AOU plots, we estimate that the upper limit of pCO₂ should be ∼7000 μatm in the Pearl River estuary assuming that CO₂ was produced solely by aerobic respiration.     

Seasonal physical–chemical structure and acoustic Doppler current profiler flow patterns over multiple years in a shallow, stratified estuary, with implications for lateral variability

The overall goal of this study was to strengthen understanding of the hydrographic structure in shallow estuaries as influenced by seasonal and depth-dependent variability, and by variability from extreme meteorological events. The mesohaline Neuse Estuary, North Carolina, U.S.A., which was the focus, receives surface inputs from upriver and tributary freshwater sources and bottom inputs from downriver high-salinity sound water sources, resulting in varying degrees of stratification. To assess depth-dependent, estuary-wide changes in salinity, a multiple time series was created using data from four discrete depths (surface and 1, 2, and 3 m±0.25 m). The database was developed from weekly to biweekly sampling of the entire water column, and included side-channel as well as mid-channel data. We characterized seasonal differences in halocline depth affecting the hydrographic structure of the mesohaline estuary and site-specific variation in nutrient concentrations, based on a comprehensive eight-year physical/chemical database. The first two years of the record showed an expected seasonal signal and included events that impacted the surface layer from freshwater inputs. Remaining years had greater variability over seasons and depths, with freshening events that affected all depths. Halocline depth was compared at specific locations, and a “snapshot” view was provided of the relative depth of these water masses within the estuary by season. We also examined flow patterns at the same cross-estuary sites over a three-year period, using a boat-mounted acoustic Doppler current profiler (ADCP) with bottom-tracking capability. Composite visualizations constructed with single-transect ADCP data revealed a classical estuarine circulation pattern of outflow at the surface/southern shore and inflow at the bottom/northern shore. Although this pattern deviated under extreme climatological events and was sometimes variable, the estuary generally exhibited a high probability of direction of flow. Wind fields, hurricanes, and small-scale, high-precipitation events represented significant forcing variables.     

A modelling study of residence time in a macro-tidal estuary

This paper outlines a numerical modelling study to predict the average residence time of a conservative tracer in a macro-tidal estuary, namely the Mersey Estuary, UK. An integrated hydrodynamic-dispersion model was used to predict the average residence time in the estuary for various tidal level and freshwater discharge conditions. The numerical model was verified against six sets of field measured hydrodynamic data, with the model-predicted water elevations and salinity levels generally agreeing well with the field measurements. The numerical model results show that in the Mersey Estuary both the tidal level and river discharge affect significantly the predicted average residence time. The value of the average residence time is also shown to be closely linked to the intensity of the residual tidal current. This is due to the fact that a large proportion of the Upper and Inner Estuary dries out during low tides, thus a significant amount of the tracer material is transported through the deep channels. An increase in the freshwater discharge causes a considerable increase in the intensity of the residual current along the main channels and thus a reduction in the average residence time. The predicted overall tracer residence time for the whole estuary is relatively short for a relatively large estuary, ranging from less than 1 day to 4 days for various tidal level and freshwater flow combinations. When the tidal range and freshwater discharge are both small, then the local tracer residence time in the upper part of the estuary can be significantly longer than the values predicted for the middle and lower reaches of the estuary.     

The artisanal fishery of Cynoscion guatucupa in Argentina: Exploring the possible causes of the collapse in Bahía Blanca estuary

Cynoscion guatucupa Cuvier 1829 is a migratory pelagic fish species, which has a wide geographical distribution. It is the most important fishing resource for local communities in Bahía Blanca estuary and has been captured by artisanal fishermen since the 1900s. The industrial fleet has been fishing this species in the coastal area of Buenos Aires province since the 1950s, and, since 1970, landings have increased sharply. Between 2000 and 2004, the artisanal fishery in the estuarine waters of Bahía Blanca collapsed. Variations in total landings of the artisanal fleet might have arisen from the environmental variables within the estuary, fishing activity in the surrounding sea region, local pressure within the estuary and/or several other variables. Our results suggest that neither oceanographic parameters nor local pressure seem to have influenced the artisanal fishery of C. guatucupa in the estuarine region. Instead, this fishery seems to have been partially influenced by the increasing fishing pressure exerted by the industrial fishing fleet operating in open waters around the estuary. This study emphasizes the need to take into account fisheries data from both the estuarine environment and the surrounding sea region, particularly when designing management plans for the sustainable use of migrating fish resources.     

The nematodes of the Thames estuary: Assemblage structure and biodiversity, with a test of Attrill's linear model

Although many studies of Nematoda have been undertaken in estuarine systems, there are relatively few studies which have analysed the distribution of fauna across the entire salinity range from marine to freshwater conditions. The Thames estuary has a long history of anthropogenic impact and recovery, since it was described as “azoic” in the 1950s, which has been monitored primarily through studies of water quality and fish stocks, with less emphasis on macroinfauna and very little information on meiofaunal organisms. This study aimed to describe the nematode fauna at eight stations along the estuary from marine to freshwater conditions in order to assess patterns of density, diversity and species assemblage structure. Nematode density and diversity were generally lower in the middle reaches of the estuary, associated with the region of greatest salinity range, a pattern which was found to be in agreement with Attrill's [2002. A testable linear model for diversity trends in estuaries. Journal of Animal Ecology 71, 262–269] linear model. Multivariate analysis confirmed that each station supported a distinct nematode fauna, which could be used to identify five zones along the estuary related to salinity regime. Although alpha diversity at each station was relatively low, species turnover along the estuary resulted in relatively high gamma diversity (153 spp.) similar to that found in a number of European estuaries. The results of this study did not suggest that the nematode fauna was under significant stress from the lower levels of pollution currently found in the system. The potential routes for the recovery and re-colonization of the estuary since it most polluted days are discussed.