SW England Research Articles

Aspects of the Petrology and Geochemistry of Greenstones: with special reference to SW England and Wales

Aspects of the Petrology and Geochemistry of Greenstones: with special reference to SW England and Wales

Grazing dynamics in intertidal rockpools: Connectivity of microhabitats

Differences between rockpool and emergent rock communities are often attributed to their contrasting physical conditions. However, differences in grazing pressure between rockpools and open rock could also exert an important structuring role. Greater densities and/or the lack of tidal constraints on foraging may allow grazing intensity to be greater in rockpools. Here, wax discs were deployed to compare grazing intensity between rockpool and emergent rock habitats at each of three tidal heights on a moderately exposed shore in SW England. Grazing intensity was then examined in relation to herbivore density. Grazing intensity in pools was twice that on emergent rock, despite a lower density of herbivores in the rockpools. Of these herbivores, patellid limpets are the dominant grazers on rocky shores throughout the NE Atlantic and are recognised to have a major role in structuring intertidal communities. Thus, subsequent experiments focussed on the influence of limpets in determining the differences in consumer pressure between rockpools and emergent rock. Three alternative explanations were considered: (1) the effect of continuous immersion on grazing intensity in rockpools; (2) differences in limpet species abundance between the two habitats; (3) movement of limpets from emergent rock into pools to feed. The level of grazing pressure exerted by Patella ulyssiponensis (Gmelin), the predominant species living constantly immersed in rockpools, was similar to that of P. vulgata (Linnaeus) which is predominantly found on emergent rock. P. vulgata were observed moving from emergent rock into rockpools during high tide. Manipulative experiments confirmed that these foraging excursions resulted in a 2-fold increase in grazing intensity in the pools. Grazing activity of P. vulgata in rockpools was not consistent between sites and may be influenced by differences in wave exposure and/or the abundance of microbial resources. Elevated consumer pressure in rockpools may be an important factor influencing algal assemblages and probably explains the predominance of grazer resistant-species in these pools.

Dissolved organic phosphorus speciation in the waters of the Tamar estuary (SW England)

The speciation of dissolved organic phosphorus (DOP) in the temperate Tamar estuary of SW England is described. Eight stations from the riverine to marine end-members were sampled during four seasonal campaigns in 2007 and the DOP pool in the water column and sediment porewater was characterized and quantified using a flow injection manifold after sequential enzymatic hydrolysis. This enabled the enzymatically hydrolysable phosphorus (EHP) fraction and its component labile monoester phosphates, diester phosphates and a phytase-hydrolysable fraction that includes myo-inositol hexakisphosphate (phytic acid), to be determined and compared with the total DOP, dissolved reactive phosphorus (DRP) and total dissolved phosphorus (TDP) pools. The results showed that the DOP pool in the water column varied temporally and spatially within the estuary (1.1–22 μg L −1) and constituted 6–40% of TDP. The EHP fraction of DOP ranged from 1.1–15 μg L −1 and represented a significant and potentially bioavailable phosphorus fraction. Furthermore the spatial profiles of the three components of the EHP pool generally showed non-conservative behavior along the salinity gradient, with apparent internal estuarine sources. Porewater profiles followed broadly similar trends but were notably higher at the marine station throughout the year. In contrast to soil organic phosphorus profiles, the labile monoester phosphate fraction was the largest component, with diester phosphates also prevalent. Phytic acid concentrations were higher in the lower estuary, possibly due to salinity induced desorption processes. The EHP fraction is not commonly determined in aquatic systems due to the lack of a suitable measurement technique and the Tamar results reported here have important implications for phosphorus biogeochemistry, estuarine ecology and the development of efficient strategies for limiting the effects of phosphorus on water quality.

Do Eurasian otters Lutra lutra (L.) in the Somerset Levels prey preferentially on non-native fish species?

Eurasian otters Lutra lutra (L.) are known to be opportunistic, exploiting the most abundant prey available, however, there have been few studies of otter diet in lowland areas of Britain. The Somerset Levels and Moors, a coastal flood plain and marsh area of 64 000 ha in SW England, is an otter stronghold where a number of introduced, non-native fishes (common carp Cyprinus carpio, pumpkinseed Lepomis gibbosus, sunbleak Leucaspius delineatus) have been introduced, with the latter species occurring locally in high abundances. The present study aimed to determine whether otters of the Somerset Levels are preying on, and thus represent 'biological resistance' to, these non-native fishes. In 2004-2005, otter spraints (feces) were collected seasonally in areas of the Levels also subjected to fisheries surveys, and the data were used to determine diet composition and preferences. Otter diet was dominated by European eel Anguilla anguilla and threespine stickleback Gasterosteus aculeatus, and non-native species occurred relatively rarely (carp, sunbleak) or not at all (pumpkinseed) in the otter spraints. Seasonal variations in otter diet were observed, with birds notable for summer and sticklebacks for winter. Despite their relative prominence in the Somerset Levels, non-native fish species represented minor components of the otter's diet, a pattern also observed in Spain. In particular, the popular angling species (carp) was taken infrequently, but the species of high conservation interest (eel) was a preferred and common item in otter diet.

Geometry and formation of gypsum veins in mudstones at Watchet, Somerset, SW England

AbstractGypsum veins and faults were studied in red mudstones of the Upper Triassic Mercia Mudstone Group, in the Bristol Channel Basin, exposed in E–W-trending cliffs at Watchet on the Somerset Coast (SW England). In nodular gypsum horizons, individual gypsum nodules are connected by subhorizontal gypsum veins. In evaporite-free mudstone layers, however, dense anastomosing networks of gypsum veins occur. In a 300 m long profile dissected by 28 (mostly) normal faults with small displacements, 24 faults have veins following them, indicating palaeofluid transport along the fault planes. Ninety-seven cross-cutting relationships and mostly perpendicular vein fibres indicate that the veins are primarily extension fractures. The thickest veins in the vein network are subhorizontal (160 measurements), indicating a vertical orientation of the minimum principal compressive stress (horizontal basin compression). Such a stress state may have existed during basin inversion associated with Alpine compression (late Cretaceous to early Tertiary). I propose that the gypsum veins are the result of hydrofracturing. In the gypsum nodules, then presumably consisting of anhydrite, overpressure was generated related to the hydration of anhydrite to gypsum. Stress concentration around the nodules led to rupturing and injection of thin subhorizontal hydrofractures. Some of the calcium-sulphate saturated fluids were then transported upwards along the faults and gained access to evaporite-free mudstone layers where dense anastomosing vein networks developed. Most veins were arrested during their propagation by layers with contrasting mechanical properties (stress barriers). Some veins, however, propagated through the barriers along faults to shallower levels. The dense networks of mineral veins observed in Watchet indicate that hydrofractures can generate a very high temporary permeability in fluid reservoirs.

An investigation of the impacts on biodiversity and ecosystem functioning of soft sediments by the non-native polychaete Sternaspis scutata (Polychaeta: Sternaspidae)

In recent years the non-native polychaete Sternaspis scutata has rapidly expanded its range in inshore muddy sediments along the coast of SW England (UK). To determine the impact the arrival of this moderately large infaunal deposit feeding polychaete could have on benthic biodiversity and ecosystem function, a mesocosm experiment has been conducted. Sternapsis scutata were added to undisturbed sediment cores containing natural fauna. Individuals of S. scutata were added at densities 2x and 10x the current abundance of this species at the collection site in Plymouth Sound. Nutrient fluxes and macrobenthic diversity were assessed after a period of fifty days. Despite the very large increase in biomass caused by the addition of S. scutata to the sediment cores there was no significant difference in the diversity of the resident benthic biota in treated cores. It is concluded that despite its size, the relative immobility of S. scutata dictates that its presence has little impact on the species around it. The addition of this species did significantly reduce the release of nitrite (NO 2 -) and increase the release of ammonium (NH 4 +) from the sediment. These findings support the assertion that the relationship between biodiversity and ecosystem functioning is idiosyncratic.

Methane seeps on an Early Jurassic dysoxic seafloor

The rhythmically bedded limestone–marl–shale succession of the Blue Lias Formation (Lias Group, Early Jurassic age) of Kilve in Somerset (SW England) preserves a suite of large conical concretions that formed around methane seeps. These are 1–2 m high, and elliptical in plan (axes 2–4 m), with an outer limestone shell forming the flanks of the cone. The cone flank is composed of micritic carbonate (20–30 cm thick), which locally includes sheets and pods of intraclasts and bioclasts. The cycle-forming limestone beds of the host strata are composed of dark grey micrite with carbon-isotope values ( δ 13C = 0.6 to 0.8‰) consistent with carbon sourced from a mixture of seawater and by sulphate reduction, and oxygen-isotope values ( δ 18O = − 6‰) suggesting some degree of later diagenesis. The pale grey micrite that forms the sides of the mounds includes three-dimensional ammonites and intraclasts, and thus cemented close to the sediment–water interface prior to compaction. The mound-forming carbonate is markedly isotopically light with respect to carbon, but not with respect to oxygen ( δ 13C = − 24.3 to − 26.4; δ 18O = − 2 to − 3.5‰). The isotope signature indicates that cements were probably derived from a mixture of sources that included biogenic methane. The intraclasts within the limestone suggest that syn-depositional physical brecciation and mixing of cements had occurred, and thus mixing of methane rich-fluids with the overlying surface waters is likely also to have occurred. The relatively heavy oxygen-isotope values may be indicative of anaerobic oxidation of methane. The mound-bearing interval of the Blue Lias Formation is benthos-poor and comprises predominantly laminated black shales, characteristic of poor bottom water oxygenation. The largest of the mounds is however, capped with fossiliferous breccias. Thus the mounds either formed benthic islands that elevated the biota into an oxic zone or, alternatively, they may have supported a chemotrophic community. Although cold seep deposits have been documented previously they are still comparatively rare. This example is one of the oldest in Europe, and is unusual amongst described ancient seeps in preserving relief that extended above the ancient seafloor.

Micro-Fourier Transform Infrared (FT-IR) and δD value investigation of hydrothermal vein quartz: Interpretation of fluid inclusion δD values in hydrothermal systems

Some recent studies have suggested that the hydrogen isotopic composition ( δD) of hydrothermal fluids, released in vacuo by thermal decrepitation of quartz, are not always accurately revealed. We report the results of a step-heating δD value study of vein quartz, hosted by Lower Palaeozoic rocks in SW England, which was analyzed by micro-FT-IR for hydrogen speciation, before and after fluid extraction at temperatures between 750 and 1500 °C. The δD values of individual aliquots of released water vary between −3‰ and −208‰, with the lowest values generally corresponding to the highest temperature fractions and samples of relatively low yield. The data show significant departures from geologically reasonable δD. Micro-FT-IR analyses show that a variety of OH species are present within the vein quartz, with significant intra and inter sample variation. Typically a broad absorption due to molecular water, in the region 3400 cm −1 is observed, along with bands attributed to Li–OH and Al–OH. On heating, the broad absorption due to molecular water is reduced, accompanied by a measurable loss of Li–OH species. The latter becomes more pronounced in the higher temperature fractions (>750 °C). These data support earlier studies which indicated that contributions from the contrasting OH reservoirs in quartz can significantly influence the reported δD values. These new data also suggest that the incorporation of OH released from Li–OH sites in the quartz may be the most important factor in the generation of the anomalous values for these samples.

Distributions and seasonal variability of dissolved organic nitrogen in two estuaries in SW England

Nitrogen loadings to coastal waters have increased over the last century, resulting in deterioration in water quality. In this study we investigated the distributions and seasonality of dissolved organic nitrogen (DON), and its relationship to total dissolved nitrogen (TDN), for two anthropogenically influenced estuarine systems in southwest England. Concentrations of DON in both estuaries were generally < 80 μM. DON showed non-conservative distributions, resulting from external and internal inputs and in situ reactivity. DON contributed 38 ± 22% (range 4–79%, Yealm) and 36 ± 17% (range 4–84%, Plym) to the TDN pool, with lower values generally observed in the fresher samples relative to the more saline samples. DON was a larger fraction of the TDN pool during the summer and autumn relative to winter and spring, indicating the influence of bacterioplankton release on nitrogen cycling in the estuaries. Ammonification and nitrification were observed in the estuaries, processes which were reproduced in incubation experiments using bioreactors. The bioreactor experiments showed that 12% h − 1 of the DON flux from the River Plym may be available to bacteria, indicating significant removal of DON during the residence time of the water in the estuary (a few days). The bioavailable nature of the DON means that this N fraction significantly adds to the eutrophication burden of the receiving coastal waters, and therefore cannot be ignored in environmental assessments.

Incorporating ecological functioning into the designation and management of marine protected areas

Marine protected areas are generally designed and managed on the basis of the presence and extent of specific habitat types or the habitats of important species. However, it has become clear that in addition to including these ‘structural’ elements of marine systems, management strategies should incorporate a consideration of the functional aspects of the ecosystems. Biological traits analysis (BTA) has been successfully used to describe ecological functioning in marine benthic systems. BTA uses a number of biological characteristics expressed by the taxa present as indicators of key ecosystem functions. Two expert workshops were used to examine the potential for the application of BTA in the designation and management of MPAs. They concluded that BTA represented the best tool currently available for quantifying ecological functioning and agreed on 10-key ecological functions delivered by marine benthic communities. Twenty-four biological traits were also identified by the workshops as indices of these ten functions. In order to demonstrate the practical utility of the approach, BTA using these traits, was applied to a dataset covering benthos from within and around the proposed Eddystone Special Area of Conservation (SW England). The case study demonstrated that with the type of data normally available from conservation assessment type surveys, and a knowledge of the relevant biological traits, it is possible to use a consideration of ecological functioning to set boundaries for the MPA and to inform the site management objectives. The use of structure and function information to inform the designation process and subsequent management of marine protected areas is discussed.

Conservation of Britain's biodiversity: the case of Hieracium cyathis (Asteraceae) and its relation to other apomictic taxa

Hieracium cyathis (Ley) W. R. Linton (Asteraceae), Chalice Hawkweed, is a rare species endemic to Britain. It is a distinct member of Hieracium Section Oreadea, characterized by a short acladium and out-curved tips to the phyllaries. Historic records and current field survey indicate it has occurred in two localities in SW England and seven in South Wales. It is still present in all sites except for one Welsh locality, is significantly threatened at Cheddar Gorge, and two Welsh sites have only three plants each. Comparative morphology, cultivation experiments and amplified fragment length polymorphism (AFLP) analysis do not support the reported differences between material from SW England and Wales. AFLP analysis showed that every plant in the field was genetically similar but different, and minor genetic variation was found in one progeny array. It is a triploid (2n = 27) with apomictic seed production, and 26% potentially viable pollen. It qualifies as ‘Near Threatened’ under the 2001 IUCN threat criteria. General guidelines for conservation of microspecies of Hieracium are discussed. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 156, 669–680.

Zostera marina seagrass beds enhance the attachment of the invasive alga Sargassum muticum in soft sediments

The first reports of Sargassum muticum occurring in British waters coincided with a warning that this invasive alga may replace seagrass beds, particularly those of Zostera marina. Initial fears were allayed when subsequent research suggested that both species required different attachment substrata, and, therefore, S. muticum would be unable to replace Zostera beds on soft sediments. Here, however, we present evidence supporting the original warning: contrary to previous research we found that S. muticum is able to successfully colonise soft sediments and, furthermore, that the presence of Z. marina may aid attachment. The majority of S. muticum plants within Zostera beds studied at Salcombe, SW England, were found attached within the soft sediment; outside of the seagrass bed, however, this occurred in greatly reduced numbers. Therefore, we suggest that the presence of Z. marina may actually enhance S. muticum colonization of soft sediments, trapping drifting fragments and allowing viable algae to settle on the seagrass matrix in an otherwise unfavorable environment. The consequences for the invasion of seagrass beds by S. muticum may therefore be more severe than previously thought.

Dissolved and total organic and inorganic carbon in some British rivers

Rivers transport both organic and inorganic carbon from their sources to the sea. Results of ~800 organic and inorganic analyses from various British rivers of contrasting size and land use are presented here: (1) the headwater River Tern, a rural river of 852 km2 catchment; (2) the Ouseburn, a small urban 55 km2 catchment; (3) the River Tyne, a larger river system of ~3000 km2 catchment; (4) a spatial survey from 205 sample sites on ~60 rivers from SW England. We found that, with the exception of peat‐rich headwaters, DIC concentration is always greater than DOC. DIC is primarily in the form HCO3 − , with DIC concentrations highest in highly urbanised catchments, typically greater than those observed in catchments with carbonate bedrock, demonstrating a significant and previously unrecognised anthropogenic inorganic carbon input to urban rivers.

An assessment of the potential impact of dredging activity on the Tamar Estuary over the last century: II. Ecological changes and potential drivers

The Tamar estuary is a Special Area of Conservation (SAC) under the EU Habitats and Species Directive and a Special Protection Area under the Wild Birds Directive (1979). The lower Tamar is also the site of the Devonport naval dockyard which requires annual maintenance dredging, as well as occasional capital dredging for new installations. The main objective of this study was to investigate whether there is any evidence of significant temporal changes in key species (intertidal macrofauna, fishes, birds) and habitats (intertidal mudflats and saltmarsh) that could be related to dredging activities in the lower Tamar. Other physical variables, such as Tamar river flow and the North Atlantic Oscillation (NAO) index, representing potential drivers of changes in the abundance of biota, were also examined. Spatial and temporal changes in the abundance of intertidal macrofauna (between 1939 and 2000) were analysed but there was insufficient comparable data to enable us to draw any conclusions about long-term changes in the Tamar estuary. Commercially and ecologically important fish species (salmon and sea trout) showed a steady decline in numbers caught in the Tamar since the 1970’s. There was a significant correlation between the number of salmon caught by rod and the Tamar river flow. The sea trout abundance was significantly negatively correlated with the NAO index, suggesting that sea trout may be adversely affected by mild winters, which have been a feature of the late 1980’s and 1990’s. There were also significant correlations between the number of salmon caught in the Tamar and other rivers of the SW of England. Ten species of wildfowl and wader birds were analysed. There were no significant correlations between over-wintering numbers and dredging activity, but there were significant declines in teal and wigeon over 30 years. These species also showed a negative correlation with the NAO index suggesting the declines were related to the milder winters; possibly reducing their need to migrate south as far as SW England. Aerial photographs of the Tamar showed that the Egypt salt marsh, creeks and mudflat maintained a remarkably consistent structure over a period of >50 years. However, there was evidence of gradual erosion (0.23 m y−1) of the saltmarsh’s front cliff edge that may be related to sea level rise (~1.5 mm y−1 in SW England). The study concluded that there was no evidence of ecological changes related to the dredging activity in the Tamar. However, there were significant changes in salmon and sea trout catches, and the number of over-wintering teal and wigeon, over many decades and these changes appear to be related to large scale climatic events rather than anthropogenic factors within the Tamar estuary.

Multiple stable isotope (18O, 13C, 15N and 34S) analysis of human hair to identify the recent migrants in a rural community in SW England

Relationships between recent migration and hair delta(18)O values were examined for 40 people living in a rural community in SW England. The isotopic contents of 35 'local' hair samples were compared with those of 5 recently arrived individuals (from Australia, Canada, Chile, Germany and the USA). The hair delta(18)O values of these 'visitors' were +7.9 (Omaha, USA), +11.2 (Jena, Germany), +12.1 (Osorno, Chile), +12.6 (Montreal, Canada) and +14.3 per thousand (Adelaide, Australia). The hair value for the USA visitor (+7.9 per thousand) fell outside the range for the 33 local adult residents, +10.5 to +14.3 per thousand (+12.7 +/- 0.8 per thousand). Hair delta(18)O values did not identify the individuals from Adelaide, Montreal and Osorno as 'visitors', but hair delta(13)C or delta(34)S data did. Combining the hair delta(18)O, delta(13)C and delta(34)S values using principal components analysis (two components explained 89% of the overall variation among the 40 subjects) helped to more clearly distinguish European from non-European individuals, indicating the existence of global overall isotope (geo-origin) relationships.

Bioaccessibility of arsenic in mine waste-contaminated soils: A case study from an abandoned arsenic mine in SW England (UK)

This study characterises the total As concentrations and As bioaccessibility in 109 soils from Devon Great Consols Mine, an abandoned Cu-As mine in Devon, SW England, UK and discusses the soil and mineralogical factors that influence the bioaccessibility of this element. These data provide the basis for developing more accurate exposure estimates for use in human health risk assessments. The median value of the percent bioaccesible As of 15% for these As rich soils contaminated by mining activities indicated that relatively little of the total As is present in a bioaccessible form. Spatial variability of As bioaccesibility in the soils was also recognised throughout the mine site as a function of mineralogy. Multivariate statistical analysis identified a sulphide component responsible for the reduced As bioaccessibility of one cluster of soils. In the larger cluster of acidic mine soils covered by woodland As is mainly hosted in Fe oxyhydroxides whose partial dissolution is responsible for the bioaccessible As fraction. It was highlighted that the degree of Fe oxyhydroxide crystallinity might represent an important factor influencing arsenic bioaccessibility. Mine soils from Devon Great Consols Mine showed overall higher As bioaccessibility (15%) than other mineralised soils not affected by mining activities and background soils within the Tamar Catchment whose percent bioaccessible As median values were 9%.

Diseases affect cold-water corals too: Eunicella verrucosa (Cnidaria: Gorgonacea) necrosis in SW England

The first recorded incidence of cold-water coral disease was noted in Eunicella verrucosa, a coral on the international 'red list' of threatened species, at a marine protected area in SW England in 2002. Video surveys of 634 separate colonies at 13 sites revealed that disease outbreaks were widespread in SW England from 2003 to 2006. Coenchyme became necrotic in diseased specimens, leading to tissue sloughing and exposing skeletal gorgonin to settlement by fouling organisms. Sites where necrosis was found had significantly higher incidences of fouling. No fungi were isolated from diseased or healthy tissue, but significantly higher concentrations of bacteria occurred in diseased specimens. Of 21 distinct bacteria isolated from diseased tissues, 19 were Vibrionaceae, 15 were strains of Vibrio splendidus and 2 others closely matched Vibrio tasmaniensis. Vibrios isolated from E. verrucosa did not induce disease at 15 degrees C, but, at 20 degrees C, controls remained healthy and test gorgonians became diseased, regardless of whether vibrios were isolated from diseased or healthy colonies. Bacteria associated with diseased tissue produced proteolytic and cytolytic enzymes that damaged E. verrucosa tissue and may be responsible for the necrosis observed. Monitoring at the site where the disease was first noted showed new gorgonian recruitment from 2003 to 2006; some individuals had died and become completely overgrown, whereas others had continued to grow around a dead central area.

Seepage forces, important factors in the formation of horizontal hydraulic fractures and bedding‐parallel fibrous veins (‘beef’ and ‘cone‐in‐cone’)

AbstractBedding‐parallel fibrous veins (‘beef’ and ‘cone‐in‐cone’) are common to a number of sedimentary basins, especially those containing black shale. The type locality is SW England. The commonest mineral in the fibres is calcite. The fibres indicate vertical opening, against the force of gravity. In the past, this has been attributed to fluid overpressure. However, a simple analysis, based on Von Terzaghi's concepts, leads to the conclusion that, for the fractures to be horizontal, either the rock must be anisotropic, or it must be subject to horizontal compression. By means of a more complete analysis, supported by physical models, we show that horizontal fractures are to be expected, even if the rock is isotropic and there are no tectonic stresses. Upward fluid flow, in response to an overpressure gradient, imparts seepage forces to all elements of the solid framework. The seepage forces counteract the weight of the rock, and even surpass it, generating a tensile effective stress. The process may lead, either to tensile hydraulic fracturing, or to dilatant shear failure. We suggest that these two failure modes, and the availability of suitable solutes, explain the frequent occurrence of ‘beef’ and ‘cone‐in‐cone’ respectively.

Pluton shapes in the Cornubian Batholith: new perspectives from gravity modelling

Gravity modelling of the Dartmoor, Bodmin, St. Austell and Carnmenellis plutons of the Cornubian Batholith of SW England strongly supports a tabular form for the exposed granites. Modelled thicknesses for the individual bodies, with the exception of Dartmoor, are consistent with those derived from empirical relationships for plutons and laccoliths. In the case of the Dartmoor, Bodmin and St. Austell plutons the deepest parts are located near their southern margins, suggesting a steep conduit with northerly and outward direct flows. Modelling of the Carnmenellis granite suggests a more centrally located feeder. In the case of Dartmoor, at least, the granite appears to have exploited the Devonian–Carboniferous interface during its emplacement.

Effect of Cerastoderma edule density on near-bed hydrodynamics and stability of cohesive muddy sediments

Flume experiments were performed to investigate the impact of cockle density ( Cerastoderma edule — 0, 47, 141 and 312 individuals m − 2 ) on near-bed hydrodynamics and sediment erodability. Undisturbed cohesive muddy sediment was collected by coring at an intertidal site in the Tamar estuary (SW England) and placed in the annular flumes. Cockles at the required density were added to the surface sediments, allowed to bury, and were then exposed for 24 h to sinusoidal cycles of currents (3 to 18 cm s − 1 ) which simulated 6-h tidal cycles comparable to the Tamar site. During this period the cockles adopted their usual behaviour of burrowing, suspension feeding and shell valve adductions. After 24 h, current speeds were increased from 5 to 50 cm s − 1 in 11 steps, and current velocities, turbulent kinetic energy (TKE), and suspended sediment concentration (SSC) were measured. Current speeds decreased and turbulence increased within 3 cm of the sediment surface as a function of cockle density. Sediment resuspension increased with increasing cockle density: SSC at 0.5 m s − 1 = 156, 574, 1045 and 2253 mg L − 1 for 0, 47, 141 and 312 animals m − 2 respectively. Enhanced sediment erosion was due to increased bioturbation and bed roughness. Critical erosion velocity decreased from 26 to 8 cm s − 1 with increasing cockle density. Shear stress, measured in terms of TKE, increased at 0.5 and 1 cm above the bed for 141 and 312 animals m − 2 and up to 2 cm above the bed for 312 animals m − 2 , reflecting the increased bed roughness due to bioturbation. However, the critical erosion shear stress was relatively independent of cockle density (0.225 and 0.151 N m − 2 for 0 and 312 animals m − 2 respectively). The valve adduction frequency of the cockles was also measured in response to increasing SSC. The frequency increased from 1.2 to 16.3 adductions h − 1 with increasing SSC from 13 to 308 mg L − 1 . It represents a significant behavioural response, creating positive feedback with increased SSC further enhancing sediment disturbance and resuspension.