Medway Estuary Research Articles

Microplastics pollution in sediments of the Thames and Medway estuaries, UK: Organic matter associations and predominance of polyethylene

Microplastics at 10 sites along a 77 km transect of the river Thames estuary (UK) and 5 sites along 29 km of the Medway estuary were separated from sediment and analysed by ATR-FTIR spectroscopy. Microplastics were observed at all sites. Highest Thames concentrations were in urban London between Chelsea and West Thurrock (average 170.80 particles kg−1 ± 46.64, 3.36 mg kg−1 ± 1.79 by mass), mid-outer estuary sites were two to three times lower. Microplastics were slightly dominated by particles (54 %) over fibres (45 %), including polymer types ranked: polyethylene > PET > polypropylene > polyamide. Medway microplastics decreased seaward, with one urban-municipal site impacted by a combined-sewer-overflow containing a high proportion of fibres (Rochester, 484 particles kg−1, 7.39 mg kg−1 by mass). Microplastic abundance was correlated to organic carbon (TOC %) (R2 of 0.71 Thames and 0.96 Medway), but not sediment particle size. Sedimentary microplastics accumulation in the Thames was controlled by urbanisation-distance, and site hydrodynamics.

Diversity of dimethylsulfide-degrading methanogens and sulfate-reducing bacteria in anoxic sediments along the Medway Estuary, UK.

Methane is a powerful greenhouse gas but the microbial diversity mediating methylotrophic methanogenesis is not well-characterized. One overlooked route to methane is via the degradation of dimethylsulfide (DMS), an abundant organosulfur compound in the environment. Methanogens and sulfate-reducing bacteria (SRB) can degrade DMS in anoxic sediments depending on sulfate availability. However, we know little about the underlying microbial community and how sulfate availability affects DMS degradation in anoxic sediments. We studied DMS-dependent methane production along the salinity gradient of the Medway Estuary (UK) and characterized, for the first time, the DMS-degrading methanogens and SRB using cultivation-independent tools. DMS metabolism resulted in high methane yield (39%-42% of the theoretical methane yield) in anoxic sediments regardless of their sulfate content. Methanomethylovorans, Methanolobus and Methanococcoides were dominant methanogens in freshwater, brackish and marine incubations respectively, suggesting niche-partitioning of the methanogens likely driven by DMS amendment and sulfate concentrations. Adding DMS also led to significant changes in SRB composition and abundance in the sediments. Increases in the abundance of Sulfurimonas and SRB suggest cryptic sulfur cycling coupled to DMS degradation. Our study highlights a potentially important pathway to methane production in sediments with contrasting sulfate content and sheds light on the diversity of DMS degraders.

The Long-term sedimentary regime of the outer Medway Estuary

Estuary evolution over the last 300 years is evaluated, based on pronouncements by some that the recent erosional phase was a temporary response to human intervention and is now shifting back in favour of accretion. In contrast, a painting from 1698, 1700's surveys and accurate surveys from 1800 and 1865 confirm that the erosional phase commenced naturally around 1700. From the early 1840s the saltmarshes were heavily exploited for their clay by the brick and cement industries. One or more storm surges in 1875, 1877, 1881 and 1897 breached embankments of many “inned” (reclaimed) marsh pastures. In deference to this exploitative industry these were never reclaimed. Natural loss due to sea level rise-induced erosion was thus exacerbated by saltmarsh digging. Fine sediment budget measurements show saltmarsh cliff, creek and tidal flat surface losses greatly exceed gains on vegetated salt-marsh surfaces. Stoke Marshes is losing about 160,000 tonnes/yr and loss for the entire intertidal zone is thought to lie close to 1 M tonnes/yr. The exceptionally low-lying tidal flats are highly vulnerable to sea level rise. Relative sea level will rise due to 3 overlapping factors and is suggested to be accelerating. 50 cm increments of relative rise would induce losses to 28, 44, 72 and 83% respectively. In respect of the major tributary and estuary channels, in a short period coupled embankment breaching together with mud digging greatly increased the tidal volume passing Sheerness. Tidal velocities rose significantly. North Sea Pilot extracts, plus tidal cycle measurements at major creek entrances, reveal a strong ebb-dominance, which is further confirmed by a fluorescent tracer test on labelled muddy dredge material. Over a 132 year period the estuary bed has self-deepened and a further increase in tidal volume has arisen. With accelerating sea level rise in a starved ebb-dominated system there is no known mechanism which could shift the Medway back in the direction of accretion.

The depth-specific significance and relative abundance of anaerobic ammonium-oxidizing bacteria in estuarine sediments (Medway Estuary, UK)

Variations in the overall and depth-specific significance of anammox were measured using (15) N isotope experiments in both bioirrigated and undisturbed sediments of the Medway Estuary, UK. This was performed over two surveys, alongside FISH experiments, to identify and track shifts in the relative abundance of anammox organisms with depth. In Survey 1 (initially screening for the presence of anammox), the potential for anammox (ra) decreased from 32% upstream to 6% downstream. In Survey 2, depth-specific values of ra varied between a maximum of 37% upstream and a minimum of 4% downstream. This was linked to a small population of anammox organisms accounting for <1-8% of total bacteria with depth in Survey 1 and <1-3% in Survey 2. The relationship between the relative abundance of anammox cells and the potential contribution of anammox to total N(2) production did not however correlate. In Survey 2, infaunal disruption of the sediment substrata, and concomitant fluctuations of O(2) over depth, did not appear to inhibit the potential for anammox, even at the most bioturbated site. Moreover, deficits detected in the retrieval of (15) N gas from denitrification in Survey 2 may imply potential links between dissimilatory nitrate reduction to ammonium and anammox in estuarine sediments.

Abundance, Diversity and Activity of Sulfate-Reducing Prokaryotes in Heavy Metal-Contaminated Sediment from a Salt Marsh in the Medway Estuary (UK)

We investigated the diversity and activity of sulfate-reducing prokaryotes (SRP) in a 3.5-m sediment core taken from a heavy metal-contaminated site in the Medway Estuary, UK. The abundance of SRPs was quantified by qPCR of the dissimilatory sulfite reductase gene β-subunit (dsrB) and taking into account DNA extraction efficiency. This showed that SRPs were abundant throughout the core with maximum values in the top 50cm of the sediment core making up 22.4% of the total bacterial community and were 13.6% at 250cm deep. Gene libraries for dsrA (dissimilatory sulfite reductase α-subunit) were constructed from the heavily contaminated (heavy metals) surface sediment (top 20cm) and from the less contaminated and sulfate-depleted, deeper zone (250cm). Certain cloned sequences were similar to dsrA found in members of the Syntrophobacteraceae, Desulfobacteraceae and Desulfovibrionaceae as well as a large fraction (60%) of novel sequences that formed a deep branching dsrA lineage. Phylogenetic analysis of metabolically active SRPs was performed by reverse transcription PCR and single strand conformational polymorphism analysis (RT-PCR-SSCP) of dsrA genes derived from extracted sediment RNA. Subsequent comparative sequence analysis of excised SSCP bands revealed a high transcriptional activity of dsrA belonging to Desulfovibrio species in the surface sediment. These results may suggest that members of the Desulfovibrionaceae are more active than other SRP groups in heavy metal-contaminated surface sediments.

Modelling the distribution and growth of ‘problem’ green seaweed in the Medway estuary, UK

The results of a modelling study to investigate the mechanisms controlling macroalgal growth within the Medway estuary, UK, are presented. Intertidal zone bathymetry, tidal dynamics and turbidity control the time available for nutrient uptake and photosynthesis, and were used as a basis for predicting areas where macroalgae will grow. Tidal bed stress was also considered as a controlling factor for the presence of the less robust green macroalgae species. Two approaches to predicting macroalgal distributions were applied: (1) a simple ‘suitability index’ method based on tidal flooding and drying, taking account of the conflict between time available for nutrient uptake and for photosynthesis; and (2) a biological macroalgal growth model that includes a detailed treatment of nutrient uptake and plant growth. The former approach assigns a value between zero and one for the suitability of a location for macroalgal occurrence, while the latter predicts the full macroalgal growth dynamics over an annual cycle. Tidal bed stress was included in both approaches as an independent modifier of macroalgal occurrence/growth. Results were compared with aerial survey maps of observed vegetation cover and time series of measured biomass density. Both approaches gave good predictions of non-species-specific vegetation cover in the intertidal zone of the Medway. Tidal bed stress was found to be a strong predictor of the specific occurrence of Enteromorpha spp. and Ulva spp., with these species favouring areas of low tidal energy. It was concluded that light and a lack of suitable regions with low tidal bed stress, rather than nutrients, were the main factors limiting excessive growth of Enteromorpha spp. and Ulva spp. in the estuary. Although this study was focussed on the Medway, the results are likely to be applicable to a broad range of relatively turbid, meso- and macro-tidal estuaries.

An improved method for the simultaneous analysis of phenolic and steroidal estrogens in water and sediment

This paper describes an improved method for the extraction and analysis of seven endocrine disrupting chemicals with wide-ranging polarities from water and sediments using gas chromatography–tandem mass spectrometry (GC–MS/MS). The analytes were 4- tert-octylphenol, 4-nonylphenol, bisphenol A, estrone, 17β-estradiol, 17α-ethynylestradiol and 16α-hydroxyestrone. The optimised GC–MS/MS method produces increased selectivity and sensitivity compared to GC–MS, with limit of detection ranging from 0.01 to 0.49 ng L −1 in water and from 0.05 to 0.14 ng g −1 in sediment. Extraction from aqueous samples was performed by solid-phase extraction (SPE) and from sediment samples by microwave-assisted extraction (MAE). The improved method for the clean-up of sediment extracts carried out by SPE enhanced EDC recovery (86–102%) while reducing matrix interference and sample drying time. Derivatisation of final sample extracts was achieved using N, O-bis(trimethylsilyl)trifluoroacetamide and pyridine, and their stability was enhanced by reconstituting the derivatised extracts with hexane. The method was validated by spiking experiments which showed good recovery and reproducibility. The method was applied to samples taken from the Medway estuary in Kent, UK, where non-conservative behaviour of EDCs was demonstrated.

A comparative study of mercury distribution and methylation in mudflats from two macrotidal estuaries: The Seine (France) and the Medway (United Kingdom)

Abstract Mercury is a contaminant of great concern in the marine environment, particularly in coastal environments where the formation of methylmercury (MeHg) in aquatic sediment, and its subsequent bioaccumulation in edible aquatic organisms (mainly fish), presents a major pathway for human exposure to MeHg. Consequently, it is important to determine the factors controlling MeHg production in sediment, especially in contaminated environments. This study investigates some geochemical factors affecting the speciation and distribution of Hg in estuarine sediment from two highly industrialized macrotidal salt marsh/mudflat systems in the Seine estuary, France, and in the Medway estuary, UK. Obtained data revealed that the entire sediment core from a rapidly accreting mudflat in the Seine estuary (170 cm vertical accretion over the last 10–50 a) was contaminated with Hg at concentrations which are 10–50 times higher than the Hg background for sediments of the Seine basin. In the Medway mudflat, characterized by a slow sedimentation rate (400 cm over approximately the last 800 a), near-surface sediment was significantly more contaminated (10–40 times) than sediment at greater depths, which were characterized by pre-industrial Hg concentrations. Geochemical conditions in the surface sediment of the Medway mudflat are characterized by stable anoxic redox conditions (about −200 mV), which are generally favourable for Hg methylation, whereas near-surface sediment of the Seine mudflat is characterized by more oxidizing redox conditions (about +100 mV), which are generally less conducive to Hg methylation. Consequently, MeHg concentration in the upper 10 cm of the sediment column was about four times higher in the Medway than in the Seine mudflat, in spite of similar total Hg concentrations. In surface sediment, where Hg is actively methylated, MeHg variability was associated with the activity of SO 4 2 - -reducing microorganisms (SRM) and the presence of acid volatile sulphides (AVS). A strong correlation was observed between MeHg and AVS in sediments from these mudflats, which may be a consequence of the common origin of AVS and MeHg (both produced by microorganism activity), but also can be derived from the ability of Fe monosulfides to adsorb, and thus stabilize, solid phase MeHg.

Sediment transfer and accumulation in two contrasting salt marsh/mudflat systems: the Seine estuary (France) and the Medway estuary (UK)

Understanding the dynamics of fine sediment transport across the upper intertidal zone is critical in managing the erosion and accretion of intertidal areas, and in managed realignment/estuarine habitat recreation strategies. This paper examines the transfer of sediments between salt marsh and mudflat environments in two contrasting macrotidal estuaries: the Seine (France) and the Medway (UK), using data collected during two joint field seasons undertaken by the Anglo-French RIMEW project (Rives-Manche Estuary Watch). High-resolution ADCP, Altimeter, OBS and ASM measurements from mudflat and marsh surface environments have been combined with sediment trap data to examine short-term sediment transport processes under spring tide and storm flow conditions. In addition, the longer-term accumulation of sediment in each salt marsh system has been examined via radiometric dating of sediment cores. In the Seine, rapid sediment accumulation and expansion of salt marsh areas, and subsequent loss of open intertidal mudflats, is a major problem, and the data collected here indicate a distinct net landward flux of sediments into the marsh interior. Suspended sediment fluxes are much higher than in the Medway estuary (averaging 0.09 g/m3/s), and vertical accumulation rates at the salt marsh/mudflat boundary exceed 3 cm/y. Suspended sediment data collected during storm surge conditions indicate that significant in-wash of fine sediments into the marsh interior can occur during (and following) these high-magnitude events. In contrast to the Seine, the Medway is undergoing erosion and general loss of salt marsh areas. Suspended sediment fluxes are of the order of 0.03 g/m3/s, and the marsh system here has much lower rates of vertical accretion (sediment accumulation rates are ca. 4 mm/y). Current velocity data for the Medway site indicate higher velocities on the ebb tide than occur on the flood tide, which may be sufficient to remobilise sediments deposited on the previous tide and so force net removal of material from the marsh.

Sedimentation on intertidal mudflats in the lower part of macrotidal estuaries: Sedimentation rhythms and their preservation

The objective of this study is to propose an original approach to the analysis of the formation of intertidal rhythmites, their preservation, and the evaluation of sedimentation rates on estuarine mudflats. Three mudflats, one from each of three estuaries, were analysed using a combination of long-term (> a year), high-frequency (1 burst/10 or 20 min), and high-resolution (0.2 cm) altimeter datasets and X-ray images of sediment cores collected during topographic surveys. The results highlight the roles played by sediment supply, hydrodynamics, and morphology of the lower estuaries on the sedimentation rhythms. While the sediment-starved Medway estuary (Kent, UK) remains stable at different time-scales, the annual sedimentation rates on the sheltered Authie mudflat (Pas-de-Calais, France) and the open Seine mudflat (Normandy, France) are relatively high at 18 and 15 cm yr − 1 , respectively. On the Authie mudflat, sedimentation rhythms correspond to the semi-lunar cycle, with a good correlation between tidal range and deposit thickness. Sedimentation occurs at the beginning of the recovery (mean value of 0.25 cm per semi-diurnal cycle), and is not disturbed by wind-induced waves. In the lower Seine estuary, semi-diurnal mechanisms of deposition occur mainly when the mudflat is covered by a minimal water height (tidal range threshold value = 7.1 m). Sedimentation rhythms are discontinuous and deposition occurs only during the highest spring tides. Mean deposit thickness is 0.6 cm per semi-diurnal cycle, controlled by the turbidity maximum and the long high tide slack (2–3 h). The fluid mud is sensitive to wind-waves in this open funnel-shaped estuary, which undergoes wind-induced erosion (0.2 to 2 cm) about 10 times per year. Comparison of altimeter datasets and lithology of the sediments cored at the same points provided improved understanding of the sediment record rhythms and the sedimentation events. On the sediment-starved Medway mudflat, the result of sediment processes is a single superficial lamina. The elementary deposit in the Authie bay corresponds to a semi-lunar-linked layer, typical of sheltered environments. On the lower Seine mudflat, sedimentation rhythms are linked to the highest spring tides (i.e., the lunar cycle), resulting from increased sediment availability related to the high suspended matter concentration in the turbidity maximum. These results underline the complex response of intertidal mudflats to hydrodynamics and sediment supply conditions, from the semi-diurnal to the annual scales.

Heavy metal distribution and accumulation in two Spartina sp.-dominated macrotidal salt marshes from the Seine estuary (France) and the Medway estuary (UK)

The upper intertidal zone, and salt marshes in particular, have been shown by numerous authors to be effective medium to long-term storage areas for a range of contaminants discharged or transported into the estuarine environment. A detailed understanding of the specific controls on the trapping and storage of contaminants, however, is absent for many estuarine systems. This paper examines heavy metal distribution and accumulation in two contrasting Spartina sp.-dominated macrotidal salt marsh systems – a rapidly prograding, relatively young marsh system at the Vasiere Nord, near the mouth of the Seine estuary, France, and a more mature, less extensive marsh system in the Medway estuary, UK. The spatial distribution of the heavy metals Zn, Cu, Pb, Ni and Co is assessed and compared in both systems via detailed surface sampling and analysis, while the longer-term accumulation of these metals and its temporal variability is compared via analysis of dated sediment cores. Of the two sites studied, the more extensive marsh system at the Vasiere Nord in the Seine estuary shows a clear differentiation of heavy metals across the marsh and fronting mudflat, with highest metal concentrations found in surface sediments from the more elevated, interior marsh areas. At Horrid Hill in the Medway estuary, the spatial distribution of heavy metals in surface sediments is more irregular, and there is no clear relationship between heavy metal concentration and site elevation, with average concentrations similar in the marsh and fronting mudflats. Sediment core data indicate that the more recent near-surface sediments at Horrid Hill are clearly more contaminated than those at greater depth, with most heavy metal contamination confined to the upper 20 cm of the sediment column (with peak metal input in the late 1960s/early 1970s). In contrast, due to extremely rapid sediment accretion at the mouth of the Seine, heavy metal distribution with depth at the Vasiere Nord site is relatively erratic, with metal concentrations showing a general increase with depth. These sediments provide little information on temporal trends in heavy metal loading to the Seine estuary. Overall, heavy metal concentrations at both sites are within typical ranges reported for other industrialised estuaries in NW Europe.

Heavy metal distribution and early-diagenesis in salt marsh sediments from the Medway Estuary, Kent, UK

Salt marsh cores are increasingly being used to study metal pollution chronologies. Salt marshes in macro-tidal estuaries, however, tend to retain a time-integrated or ‘smoothed’ signal rather than a record of discrete pollutant inputs, due to extensive sediment reworking. More generally, an accurate chronology of metal input to salt marsh sediments can be difficult to assess because of the potential early-diagenetic mobility of both the radionuclides used for dating and the contaminants of interest. A dated salt marsh core from the macro-tidal Medway Estuary, southeast England, was assessed using both total sediment metal data and partitioning data. These data indicate that both Mn and Fe have been significantly remobilised and that these diagenetic processes have slightly modified the vertical distributions of Cu, Pb and Zn. Zinc is the most diagenetically reactive followed by Cu and then Pb. However, general trends in pollutant loading can still be identified with maximum inputs occurring between ca. 1900 and 1950, decreasing towards the present day.

Spatial variability of metals in the inter-tidal sediments of the Medway Estuary, Kent, UK

Concentrations of major and trace metals were determined in eight sediment cores collected from the inter-tidal zone of the Medway Estuary, Kent, UK. Metal associations and potential sources have been investigated using principal component analysis. These data provide the first detailed geochemical survey of recent sediments in the Medway Estuary. Metal concentrations in surface sediments lie in the mid to lower range for UK estuarine sediments indicating that the Medway receives low but appreciable contaminant inputs. Vertical metal distributions reveal variable redox zonation across the estuary and historically elevated anthropogenic inputs. Peak concentrations of Cu, Pb and Zn can be traced laterally across the estuary and their positions indicate periods of past erosion and/or non-deposition. However, low rates of sediment accumulation do not allow these sub surface maxima to be used as accurate geochemical marker horizons. The salt marshes and inter-tidal mud flats in the Medway Estuary are experiencing erosion, however the erosion of historically contaminated sediments is unlikely to re-release significant amounts of heavy metals to the estuarine system.

The influence of a power station on the survival of juvenile sea bass in an estuarine nursery area

Trawl surveys in the estuary of the River Medway in the autumns of 1987–1991 have shown that the distribution of juvenile sea bass Dicentrarchus labrax is strongly associated with the warm‐water outflow from Kingsnorth power station. In years of low abundance of first‐year bass, very few were caught outside the warm‐water discharge channel, whereas proportionately more fish of abundant year classes occupied the main river. About 15% of the available juvenile bass population died on the cooling‐water intake screens at Kingsworth in the autumn and winter of 1987 and 1988. However, growth and survival of first‐year bass in the Medway Estuary may be enhanced by the power station's warm‐water effluent, such that overwinter mortality due to inadequate nutritional reserves and low temperatures may be considerably reduced.

Managed retreat: a natural analogue from the Medway estuary, UK

A comparison between open marshes (un-reclaimed) and breached reclaimed land is made for sites within the Medway estuary, UK, in order to determine the effectiveness of these natural retreat sites in protecting the hinterland, and to use these findings as an analogue for modern managed retreat schemes. Two sites, who’s defences were breached at the turn of the century are studied to show rates of erosion experienced by the post-breach marsh, and compared to open marshes in the same system. This has shown that the rates of loss are greater in the open marshes over the same period of time, and that as an analogue for contemporary managed retreat sites, the breached sites have provided significant coastal protection to the hinterland for over 71 years, and, it could be argued, have survived natural erosive processes more effectively than open marshes.

Utilization and degradation of an ester-based synthetic lubricant by Acinetobacter lwoffi.

An oil-degrading bacterium, Acinetobacter lwoffi, isolated by elective culture from the Medway estuary, utilized an ester-based synthetic lubricating oil EMKARATE DE 155 as sole carbon and energy source. Analysis of culture supernatants by gas chromatography showed the accumulation of a nondegradable metabolite 1,1,1 Tris (hydroxymethyl)propane in addition to two metabolizable fatty acids, octanoic and decanoic acids as products of the synthetic oil degradation. Esterase activities were subsequently demonstrated in oil and acetate-grown cells. The synthetic oil therefore appears to be partially biodegradable in the environment.

The sediment budget of the erosional intertidal zone of the Medway Estuary, Kent

A prolonged erosion of the broad salt marshes of the Lower Medway began around 1700 A.D. This trend accelerated rapidly between about 1860 and 1910 A.D. when the marsh clay was exploited by the brick and cement industries. In the late 1880's several enclosed marshes were breached by the tide and have remained derelict since. Recent measurements show that accretion on the shrinking areas of salt marsh surfaces is still proceeding. In contrast, the salt marsh creeks and cliffs, and the increasing areas of tidal flat surface are all eroding. Enough measurements have been made to confirm that the changes are proceeding in the same direction throughout the estuary, though at varying rates. Only for Stoke Marshes has sufficient data been assembled to compile a sediment budget, and this shows an annual deficit of about 160,000 tonnes. The deficit for the entire intertidal zone probably approaches 1,000,000 tonnes/yr. There is no evidence that this sediment is redeposited subtidally within the estuary. Erosional intertidal zones such as those in the Medway, Severn and Poole Harbour provide many contrasts with more widely described accretionary regimes, such as The Wash, Dee and Solway.

The environmental impact of paper mill waste discharges to the Upper Medway Estuary, Kent, England

The Upper Medway Estuary is considered as the narrow, funnel-shaped channel from the tidal limit at Allington downstream to Lower Upnor. Mathematical models are used to predict the depletion of dissolved oxygen resulting from organic waste inputs to the upper estuary. Surveys of the soft sediment fauna together with sediment redox potentials, organic carbon and cellulose content showed that conditions were generally poor and that faunal diversity was low, although a few surviving oligochaetes proliferated in the absence of predation and competition, with numbers in excess of 1·0 × 10 6 individuals per m 2 of sediment being recorded. The bottom sediments were relatively mobile, although a combination of sediment organic content and redox potential profiles clearly demonstrated the occurrence of reduced sediment conditions at depths below 6 cm at some sites in the upper estuary, with the greatest effect at sites downstream of the paper mill discharges. A significant reduction in the amount of organic waste discharged to the upper estuary should increase faunal diversity, although high species richness is considered unlikely given the naturally harsh conditions.

The benthos of a coastal power station thermal discharge canal

Kingsnorth Power Station, on the River Medway Estuary, Kent, discharges cooling water into a canalcomprising a 4 km creek system. A comprehensive investigation of the sublittoral benthic fauna of the discharge system was undertaken from January 1979 to September 1981. The benthic macrofauna was sampled monthly at five stations along the system and a sixth in the River Medway adjacent to the creek mouth (DC4); salinity, sea-bed temperature, sediment particle size, sediment redox potential, residual chlorine and meiofaunal numbers were measured. The macrofauna is significantly suppressed at sites along the discharge canal, representing a community with half the number of species as at station DC4, comprising dense populations of a few dominant opportunistic species tolerant of thermal stress (e.g.Tubificoides, Cauleriella), and not those characteristic of organic pollution stress communities. The latter are regular summer immigrants in the creek, but persist only in low numbers if at all in the winter (e.g.Polydora ciliata). This suppression is the result of a 10°C temperature front between the heated discharge water and ambient estuarine water, passing over the sea bed with the ebbing and flooding tide four times each day. The residual gradient of mean temperature along the discharge canal causes some changes in the dominance of those species not eliminated by the temperature front. These temperature effects are expected to be localized, mainly in the area of sea-bed impingement of the discharge water; they are predicted to occur at any thermal discharge in tidal waters where the heated effluent contacts the sea bed.

Observations on the fish populations of an east coast Estuary

Monthly samples of fish were collected from the cooling-water intake screens at Kingsnorth Power Station on the Medway Estuary. A total of 26 372 fish, comprising 41 species, was recorded between April 1981 and August 1983. The seasonal distribution of both species numbers and abundance of fish was similar to earlier studies, although the community structure has altered in recent years. The regular occurrence and increased population size of Osmerus eperlanus, the smelt, and Clupea harengus, the herring, were the most notable changes. The results are compared with previous studies, and the indirect effects of enhanced water quality conditions in the tidal Thames and the subsequent recolonization by fish, which were previously absent for many years, are discussed.