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Globally, habitat loss in coastal marine systems is a major driver of species decline, and estuaries are particularly susceptible to loss. Along the United States Pacific coast, monospecific eelgrass (Zostera marina) beds form the major estuarine vegetated habitat. In Morro Bay, California, eelgrass experienced an unprecedented decline of > 95%, from 139 ha in 2007 to < 6 ha by 2017. Fish populations were compared before and after the eelgrass decline using trawl surveys. Beach seines surveys were also conducted during the post-decline period to characterize species within and outside of remnant eelgrass beds. While the estuary-wide loss of eelgrass did not result in fewer fish or less biomass, it led to changes in species composition. The post-eelgrass decline period was characterized by increases in flatfish (mainly Citharichthys stigmaeus) and staghorn sculpin (Leptocottus armatus), and decreases in habitat specialists including bay pipefish (Syngnathus leptorhynchus) and shiner perch (Cymatogaster aggregata). There were similar trends inside and outside of remnant eelgrass patches. These findings support evidence across multiple ecosystems suggesting that the predominance of habitat-specialists predicts whether or not habitat loss leads to an overall decline in fish abundance. In addition, loss of critical habitats across seascapes can restrict population connectivity and lead to range contraction. For bay pipefish, the loss of eelgrass in Morro Bay is likely to create a population biogeographic divide. Currently, Morro Bay is dominated by flatfish and sculpins, and the longevity of this new ecosystem state will depend on future eelgrass recovery dynamics supported by ecosystem-based management approaches.

The flux of terrigenous organic carbon through estuaries is an important and changing, yet poorly understood, component of the global carbon cycle. Using dissolved organic carbon (DOC) and fluorescence data from 13 British estuaries draining catchments with highly variable land uses, we show that land use strongly influences the fate of DOC across the land ocean transition via its influence on the composition and lability of the constituent dissolved organic matter (DOM). In estuaries draining peatland-dominated catchments, DOC was highly correlated with biologically refractory “humic-like” terrigenous material which tended to be conservatively transported along the salinity gradient. In contrast, there was a weaker correlation between DOC and DOM components within estuaries draining catchments with a high degree of human impact, that is, relatively larger percentage of arable and (sub)urban land uses. These arable and (sub)urban estuaries contain a high fraction of bioavailable “protein-like” material that behaved nonconservatively, with both DOC removals and additions occurring. In general, estuaries draining catchments with a high percentage of peatland (≥18%) have higher area-specific estuarine exports of DOC (>13 g C m−2 yr−1) compared to those estuaries draining catchments with a high percentage (≥46%) of arable and (sub)urban land uses (

The Lynher Estuary in Southwest England is a small, strongly tidal sub-estuary of the Tamar Estuary. It is a Site of Special Scientific Interest (SSSI), a Marine Conservation Zone (MCZ), a part of the Plymouth Sound and Estuaries Special Area of Conservation (SAC) and a Special Protection Area (SPA). Management of the Lynher SSSI and MCZ stipulates that good water quality and sediment quality should be maintained; as such, a good understanding of its responses to influences such as climate change and changes in agricultural practices within its catchment area is required. Observations of salinity, temperature, suspended particulate matter (SPM) concentrations, estuarine turbidity maximum (ETM) behaviour, and chlorophyll-a are presented for the Lynher over a 1-y period. The dataset provides important baseline information with which to identify future changes and guide management of the SSSI and MCZ as well as adding to our knowledge of estuarine systems. Salt intrusion is largely controlled by tides and runoff. A persistent ETM occurs that is closely associated with the freshwater-saltwater interface at high water (HW) and with a minimum in dissolved oxygen concentrations. HW depth-averaged ETM magnitudes are relatively low, less than 60 mg l−1 and typically 30 mg l−1 over the observation period. Larger tides and stronger flood-tide wind speeds lead to a stronger ETM. Tidal river HW SPM concentrations are intrinsically small (8 ± 8 mg l−1 during the observation period). Surface chlorophyll-a concentrations are low during winter (when they often peak near the ETM) and are much higher during spring and summer.

Anthropogenically-induced global warming is expected to decrease primary productivity in the subtropical oceans by strengthening stratification of the water column and reducing the flux of nutrients from deep-waters to the sunlit surface layers. Identification of such changes is hindered by a paucity of long-term, spatially-resolved, biological time-series data at the basin scale. This paper exploits Atlantic Meridional Transect (AMT) data on physical and biogeochemical properties (1995–2014) in synergy with a wide range of remote-sensing (RS) observations from ocean colour, Sea Surface Temperature (SST), Sea Surface Salinity (SSS) and altimetry (surface currents), combined with different modelling approaches (both empirical and a coupled 1-D Ecosystem model), to produce a synthesis of the seasonal functioning of the North and South Atlantic Sub-Tropical Gyres (STGs), and assess their response to longer-term changes in climate. We explore definitive characteristics of the STGs using data of physical (SST, SSS and peripheral current systems) and biogeochemical variables (chlorophyll and nitrate), with inherent criteria (permanent thermal stratification and oligotrophy), and define the gyre boundary from a sharp gradient in these physical and biogeochemical properties. From RS data, the seasonal cycles for the period 1998–2012 show significant relationships between physical properties (SST and PAR) and gyre area. In contrast to expectations, the surface layer chlorophyll concentration from RS data (CHL) shows an upward trend for the mean values in both subtropical gyres. Furthermore, trends in physical properties (SST, PAR, gyre area) differ between the North and South STGs, suggesting the processes responsible for an upward trend in CHL may vary between gyres. There are significant anomalies in CHL and SST that are associated with El Nino events. These conclusions are drawn cautiously considering the short length of the time-series (1998–2012), emphasising the need to sustain spatially-extensive surveys such as AMT and integrate such observations with models, autonomous observations and RS data, to help address fundamental questions about how our planet is responding to climate change. A small number of dedicated AMT cruises in the keystone months of January and July would complement our understanding of seasonal cycles in the STGs.

This paper describes a near-surface ocean profiler, which has been designed to precisely measure vertical gradients in the top 10 m of the ocean. Variations in the depth of seawater collection are minimized when using the profiler compared to conventional CTD/rosette deployments. The profiler consists of a remotely operated winch mounted on a tethered yet free-floating buoy, which is used to raise and lower a small frame housing sensors and inlet tubing. Seawater at the inlet depth is pumped back to the ship for analysis. The profiler can be used to make continuous vertical profiles or to target a series of discrete depths. The profiler has been successfully deployed during wind speeds up to 10 m s−1 and significant wave heights up to 2 m. We demonstrate the potential of the profiler by presenting measured vertical profiles of the trace gases carbon dioxide and dimethylsulfide. Trace gas measurements use an efficient microporous membrane equilibrator to minimize the system response time. The example profiles show vertical gradients in the upper 5 m for temperature, carbon dioxide and dimethylsulfide of 0.15 °C, 4 µatm and 0.4 nM respectively.

Analyses of consistently measured survey data and published hydrological data for the estuaries of southwest England show that: (1) freshwater runoff and estuary length are strongly related to catchment area; (2) salt intrusion length is always greater than 74% of the estuary length during low runoff, spring-tide summer conditions and, on average, is more than 90% – there is a statistically significant relationship between salt intrusion length and tidal range (increased intrusion) and freshwater runoff (decreased intrusion); (3) salinity stratification is strongly, positively correlated with the Simmons’ Ratio – a measure of the relative balance of freshwater- and tidally-induced current speeds; (4) residence times are strongly correlated with tidal range and tidal length and somewhat less strongly correlated with tidal prism – longer estuaries with smaller tides have longer residence times than shorter estuaries with stronger tides; (5) maximum surface suspended particulate matter (SPM) concentrations vary from 2 to 20,000 mg l −1 ; (6) maximum surface SPM concentrations are strongly related both to tidal range and tidal length and less strongly correlated with tidal prism – long, strongly tidal estuaries have greater concentrations; (7) loss-on-ignition (LOI) of SPM is strongly related to SPM concentration – low concentrations (less than a few mg l −1 ) typically have LOI > 50% and high concentrations (greater than a few hundred mg l −1 ) typically have LOI ∼ 10%; (8) LOI of the freshwater SPM is greater than that in the area of maximum estuarine SPM concentration, so that SPM within the high-turbidity reaches of southwest estuaries is significantly different from that entering the estuaries in their freshwater inflows; (9) an observed scaling relationship between estuary tidal length and a prescribed function both of depth at the mouth and tidal range is similar to that given by theoretical work (Prandle, 2004; Prandle et al., 2006), as is an observed scaling relationship between depth at the mouth and mean annual runoff. To our knowledge, the measurements and results presented here for the estuaries of southwest England are, in many cases, the first that quantify their important in-water physical characteristics and behaviour. Apart from presenting new information, the synthesis has some important implications for estuarine management, and may aid the straightforward estimation of some key physical variables for these systems.

Whitsand Bay and Plymouth Sound are located in the southwest of England. The Bay and Sound are separated by the ∼2–3 km-wide Rame Peninsula and connected by ∼10–20 m-deep English Channel waters. Results are presented from measurements of waves and currents, drogue tracking, surveys of salinity, temperature and turbidity during stratified and unstratified conditions, and bed sediment surveys. 2D and 3D hydrodynamic models are used to explore the generation of tidally- and wind-driven residual currents, flow separation and the formation of the Rame eddy, and the coupling between the Bay and the Sound. Tidal currents flow around the Rame Peninsula from the Sound to the Bay between approximately 3 h before to 2 h after low water and form a transport path between them that conveys lower salinity, higher turbidity waters from the Sound to the Bay. These waters are then transported into the Bay as part of the Bay-mouth limb of the Rame eddy and subsequently conveyed to the near-shore, east-going limb and re-circulated back towards Rame Head. The Simpson–Hunter stratification parameter indicates that much of the Sound and Bay are likely to stratify thermally during summer months. Temperature stratification in both is pronounced during summer and is largely determined by coastal, deeper-water stratification offshore. Small tidal stresses in the Bay are unable to move bed sediment of the observed sizes. However, the Bay and Sound are subjected to large waves that are capable of driving a substantial bed-load sediment transport. Measurements show relatively low levels of turbidity, but these respond rapidly to, and have a strong correlation with, wave height.

Observations are presented of estuarine infragravity oscillations in a very small estuary, the Ria de Santiuste, northern Spain, that discharges into a bay. Time-scale calculations and measurements indicate that the most likely source of amplified infragravity waves within the estuary are coastal edge waves, which appear to be resonating between the headlands of the bay, and onshore-offshore seiching of waters that may be resonating between the beach and the bay's seaward limit. Infragravity waves in the estuary have a narrow, power-dominant periodicity range of 4.3–4.8 min. Generally, only very weak waves, and at other periodicities, occur within the tidal river when it is unaffected by the tide. Although infragravity water-level amplitudes are small in the tidal river, typically ∼0.01 m during high water, the corresponding velocity amplitudes are significant, typically ∼0.1 m s −1 , despite the occurrence of high runoff conditions during these measurements. Within the estuary and close to the mouth, near-bed salinity shows considerable variability that is a consequence of salt wedge oscillations with periodicities that are within the range of 4.3–4.8 min. Model results for simulated neap to spring high-water water levels show that wave periodicities in the range 3.5–4.5 min correspond approximately to 3 λ /4 resonances. The model also indicates that an effect of low runoff is to greatly enhance resonant behaviour within the estuary, such that water level amplitudes of 0.02 m at the mouth can produce wave currents as fast as 0.4 m s −1 within the estuary, compared with ∼0.1 m s −1 during high runoff.

Observations are presented of water levels, currents, salinity, turbidity, sediment grain sizes and sediment transport in the Devonshire Avon Estuary, UK, in order to improve knowledge of freshwater, wave and tidal influences on small, strongly tidal ria estuaries. A large reduction in tidal range occurred progressing from the coastal zone to the upper estuary that was mainly a consequence of rising bed and river water levels. The spring-neap cycle also had an influence on the reduction in tidal range along the length of the estuary. Surface gravity waves were completely dissipated propagating into the estuarine channel from the coastal zone, and despite strong wave-induced resuspension, suspended sediment was not transported into the lower estuary in observable amounts during the ensuing flood tide, indicating that the wave-suspended material was too coarse to remain in suspension once transported away from the surf zone. Turbidity in the lower estuary was relatively low during low runoff summer conditions and had largest values over low water, when turbid waters from farther up-estuary had been transported there. Strong resuspension events occurred at peak currents in the upper estuary during summer, reflecting the presence of finer-grained sediment sources. Turbidity was similar but greater in the lower estuary during high runoff winter conditions and strong resuspension occurred at peak currents, indicating an easily erodible, nearby sediment source, due to down-estuary movement and relocation of finer sediment over the winter. A large shoal in the lower estuary exhibited a consistent pattern of accretion/erosion during the high runoff months of late autumn and winter to spring that also was qualitatively consistent with sediment transport modelling and implied: (a), erosion from the up-estuary limit of the shoal with (b), down-estuary bed-load and suspended-load transport that accreted the centre and down-estuary limit of the shoal until (c), a diminished supply led to erosion via continued down-estuary transport from the shoal centre.

Management strategies for freshwater drainage basins often do not take into account the effects of human interventions (such as water abstractions, i.e. water extractions) on their receiving estuaries (e.g. EA, 2007 ; Wolanski et al., 2004 ). However, it is necessary to know how such interventions are likely to influence estuarine hydrodynamics and sediment transport processes in order to then assess important management aspects such as the likely effects to an estuary's ecology and sedimentation; for example, potential accretion and dredging requirements. Following Schuttelaars et al. (in this issue) we consider a tiered approach, but in which simplistic models (rather than analytical models) are used initially to determine whether modelling of greater complexity, effort and expense is required to assess the intervention. To this end, we have linked a section-averaged, along-estuary hydrodynamic and salt-balance model with a depth-averaged, across-estuary hydrodynamic model to simulate currents and tidal behaviour that can be used in a sediment-transport model to investigate, as a first step, intervention scenarios for shallow, strongly tidal estuaries quickly, easily and at low cost. The essential element is a sensitivity analysis in which simulations for before-and-after intervention scenarios are compared and percentage differences tabulated. The methodology is applied to the predicted consequences on the hydrodynamics and sediment transport in the Devonshire Avon Estuary, UK, of water abstractions due to an up-stream dam and reservoir. Although the freshwater abstractions generally are small, the results nevertheless indicate an increased potential for up-estuary transport and retention of sediment in the central and upper reaches of the estuary.

A simple estimation method for estuarine turbidity that may be helpful for management purposes is used to predict SPM concentrations for the strongly tidal estuaries of the northeast Irish Sea. It demonstrates a systematic tendency for the longer, more slowly flushed estuaries to have greater ‘intrinsic’ SPM concentrations within their high turbidity regions than the shorter estuaries. Predicted maximum SPM concentrations for the mainland estuaries vary from ∼10 mg l −1 (Bladnoch) to ∼10 g l −1 (Solway, Mersey and Dee). The Isle of Man estuaries are predicted to have maximum SPM concentrations that are less than 10 mg l −1 . These small predicted concentrations are largely a consequence of their short lengths and short residence times. Geomorphology is the ultimate reason for this difference in Isle of Man and mainland estuarine turbidity. The Isle of Man has steep slopes and small rivers that flow rapidly to sea via short estuaries, whereas the mainland has a number of large, long, coastal plain estuaries that allow strong tidal currents to develop and provide storage for large quantities of fine sediment. Survey data for the Solway Esk and the Forth and Clyde Estuaries are used to illustrate the effects of uncertainty in the definition of an estuary's tidal length, which is a problem both for estuarine science and estuarine management. Despite the dearth of published, good quality field data for the estuaries described herein, there is nevertheless a good qualitative agreement between the predicted values of SPM and the majority of these observations.

Measurements over an annual cycle of longitudinal and vertical salinity distributions in a small sub-estuary, the Tavy Estuary, UK, are used to illustrate the dependence of salt intrusion and stratification on environmental variables. The interpretations are aided by vertical profiling and near-bed data recorded in the main channel and on the mudflats. Generally, high water (HW) salt intrusion at the bed is close to the tidal limit and is dominated by runoff and winds, with decreasing salt intrusion associated with increasing runoff and increasing up-estuary winds (or vice versa). Tidal effects are not statistically significant because of two compensating processes: the long tidal excursion, which is comparable with the sub-estuary length for all but the smallest neap tides, and the enhanced, near-bed, buoyancy-driven salt transport that occurs at small neap tides close to the limit of saline intrusion. The effect of wind on HW surface salt intrusion in the main channel is not statistically significant, partly because it is obscured by the opposing local and estuary-wide effects of an up-estuary or down-estuary wind stress. These processes are investigated using a simple tidal model that incorporates lateral, channel–mudflat bathymetry and reproduces, approximately, observed channel and mudflat velocities. Surface salinity at HW increases with tidal range because of enhanced spring-tide vertical mixing—a process that also reduces salinity stratification. Stratification increases with runoff because of increased buoyancy inputs and decreases with up-estuary winds because of reduced near-bed salt intrusion. Stratification and plume formation are interpreted in terms of the bulk and estuarine Richardson Numbers, and processes at the confluence of the sub-estuary and main estuary are described.

A buoy system has been developed to continually monitor the operationally demanding coastal and open-shelf environment of the western English Channel. The buoys measure a range of physical and biogeochemical parameters on an hourly basis at two established long-term monitoring sites and the data are relayed to shore in near–real time using radio communications. This paper describes the technological challenges faced in such long-term marine deployments including the mooring design, warning systems, command and control, and radio communications, and how each were overcome. The fine temporal frequency data are used within an operational oceanography context, will underpin the long-term sustained observations in the western English Channel, and will form the basis of improvements to finescale ecosystem modeling to better predict any changes in the U.K. shelf seas.

Sub-estuaries, i.e. tidal creeks and also larger estuaries that branch off the stem of their main estuary, are commonplace in many estuarine systems. Their physical behaviour is affected not only by tributary inflows, winds and tides, but also by the properties and behaviour of their main estuary. Measurements extending over more than an annual cycle are presented for the Tavy Estuary, a sub-estuary of the Tamar Estuary, UK. Generally, waves are small in the Tavy because of the short wind fetch. A several-hour period of up-estuary winds, blowing at speeds of between 7 and 10 m s −1 , generates waves with significant wave heights of 0.25 m and a wave periodicity of 1.7 s that are capable of eroding the bed over the shallow, ca. 1.5 m-deep mudflats. Waves also influence sedimentation within and near salt marsh areas. An estuarine turbidity maximum (ETM) occurs in the Tavy's main channel, close to the limit of salt intrusion at HW. Suspended particulate matter (SPM) concentrations typically are less than 40 mg l −1 at HW, although concentrations can exceed 80 mg l −1 when tides and winds are strong. Flood-tide SPM inputs to the Tavy from the Tamar are greater during high runoff events in the River Tamar and also at spring tides, when the Tamar has a high-concentration ETM. Higher SPM concentrations are experienced on the mudflats following initial inundation. Without wave resuspension, this is followed by a rapid decrease in SPM for most of the tide, indicating that the mudflats are depositional at those times. SPM concentrations on the mudflats again increase sharply prior to uncovering. Peak ebb tidal speeds at 0.15 m above the mudflat bed can exceed 0.26 m s −1 at spring tides and 0.4 m s −1 following high runoff events, which are sufficient to cause resuspension. Time-series measurements of sediment bed levels show strong seasonal variability. Higher and lower freshwater flows are associated with estimated, monthly-mean sediment transport that is directed out of, or into, the upper sub-estuary, respectively. Seasonal sediment transfers between the estuary and its sub-estuary are discussed.

Vertical profiles of the critical erosion threshold (τcrit) in sediment have been measured at 11 stations along the axis of the Tamar Estuary and at a single station in a tributary of the Tamar at St. John's Ford. The τcrit of surface sediment increased from 0.04 Pa in the upper, brackish estuary to 0.09 Pa in the lower estuary. In the upper estuary τcrit only increased slightly with depth whereas in the marine estuary τcrit increased rapidly from 0.09 Pa at the surface to 0.25 Pa at 15 cm below the sediment surface. The results showed that the relationship between τcrit and bulk density (ρb) obtained previously for surface sediment was also applicable to sediments from depths of 10–15 cm and probably deeper. Profiles of ρb were measured to depths of 70 cm using a corer. In the lower (marine) estuary ρb increased with depth in the sediment from 1580 kg m−3 at the surface to 1720 kg m−3 at 70 cm. In the upper estuary ρb values were lower at 1170–1200 kg m−3 and profiles were almost homogeneous indicating that consolidation was not occurring. The mid-estuary was transitional between these two situations. These results are consistent with the seasonal accumulation and loss of ‘mobile’ sediment observed previously in the upper estuary with changes in river flow, and with the apparent stability of intertidal mud in the lower marine estuary deduced from historical bathymetric survey records. The slopes of the intertidal mud banks ranged from 1–2% in the lower estuary to 20–25% in mid-estuary but, instead of continuing to increase in steepness towards the head as the estuary became narrower, the measured slopes actually decreased. It is speculated that the lack of consolidation through continual mobilisation and settlement cycles combined with an increase in silt content in the upper estuary resulted in sediment that lacked the mechanical strength to maintain steep slopes.

Despite its importance to ocean–climate interactions, the metabolic state of the oligotrophic ocean has remained controversial for >15 years. Positions in the debate are that it is either hetero- or autotrophic, which suggests either substantial unaccounted for organic matter inputs, or that all available photosynthesis (P) estimations (including 14C) are biased. Here we show the existence of systematic differences in the metabolic state of the North (heterotrophic) and South (autotrophic) Atlantic oligotrophic gyres, resulting from differences in both P and respiration (R). The oligotrophic ocean is neither auto- nor heterotrophic, but functionally diverse. Our results show that the scaling of plankton metabolism by generalized P:R relationships that has sustained the debate is biased, and indicate that the variability of R, and not only of P, needs to be considered in regional estimations of the ocean's metabolic state., Whether the oligotrophic ocean is net heterotrophic or net autotrophic has been a matter of debate for many years. Here Serret et al. show that, rather than being one or the other, the oligotrophic ocean is functionally diverse, with different metabolic states in different gyres.

Results are presented from longitudinal and vertical measurements of salinity and turbidity made at spring and neap tides throughout the length of the upper Humber and Ouse Estuary, UK, during a 1-year period that experienced strong freshwater runoff during winter and a long period of very low runoff during summer. These data were complemented by tidal-cycle profiling measurements at a site in the lower Ouse. Despite the strong suspended particulate matter (SPM) stratification, salinity was fairly well mixed vertically except in the presence of high concentration, near-bed muddy layers. The data consistently exhibited a strong estuarine turbidity maximum (ETM) in the low salinity reaches of the estuary. The geographical positions of the freshwater–saltwater interface (FSI), X S , and the up-estuary intrusion of the ETM (the ETM nose, X P ) measured from the tidal limit, exhibited a pronounced seasonal cycle. A multiple regression for X P in terms of the monthly-averaged runoff, Q 31 (averaged over 31 days preceding the survey) and the tidal range for the surveyed tide, TR, ‘explained’ 97% of the variance in X P . The ETM nose was located in the lower estuary during high runoff winter conditions, more than 50 km from the tidal limit, and in the upper estuary during very low runoff summer conditions, within 2 km of the tidal limit. According to the regression, the ETM nose was shifted up-estuary by 15 km between mean neap tides and mean spring tides. The location of the FSI also depended strongly on runoff. The regression for X S in terms of TR and Q 31 explained 88% of the variance. The FSI was located in the lower estuary during winter conditions and in the upper estuary during summer conditions and according to the regression there was an up-estuary displacement of 6 km between mean neap tides and mean spring tides. The separation between the ETM nose and FSI, ( X P − X S ), tended to be positive (ETM nose down-estuary of the FSI) when the nose was located more than about 55 km down-estuary from the tidal limit, but was otherwise negative (ETM nose up-estuary of the FSI). When the nose was located within about 30 km of the tidal limit, it tended also to be about 10–30 km up-estuary of the FSI. A multiple regression of ( X P − X S ) in terms of X P and TR explained 78% of the variance. The magnitude of the maximum, depth-averaged SPM concentration within the ETM, P , was most closely related to the location of the ETM nose. The logarithm of P decreased approximately linearly with X P and P was reduced from 35 g l − 1 during summer, when X P was less than 2 km, to 3.5 g l − 1 when X P was 58 km. A regression explained 94% of the variance. Tidal-cycle profiling illustrated the seasonal ETM movements via the presence of very high turbidity waters in the lower estuary at high water during winter and spring, at low water during summer and early autumn and at mid-ebb through to mid-flood in the late autumn. This seasonal movement of the ETM also was accompanied by a seasonal variability in sand-sized subtidal and intertidal bed-sediment distributions within the upper Humber and Ouse Estuary in which very fine sand was transported down-estuary over the high runoff winter period.

Critical erosion thresholds of intertidal sediments have been measured throughout the Tamar Estuary, SW England, between March 2004 and August 2005. Erosion measurements were made with a miniature, portable annular flume, which could be deployed in situ or on large cores transferred to a research vessel or to the laboratory. Parallel measurements were made of sediment bulk properties (silt content, water content, wet bulk density, chlorophyll and carbohydrate). The data set included sediments with bulk densities ranging from 1555 kg m −3 in the lower, marine estuary where silt and water contents were low (45% and 43%, respectively) to 1138 kg m −3 in the upper estuary, at the upper limit of the estuarine turbidity maximum, where silt and water contents were higher (75% and 84%, respectively). For this data set, spanning the region from almost fully marine to the summer, low river flow, limit of the turbidity maximum, critical erosion thresholds ranged from 0.245 to 0.025 Pa and were highly correlated with sediment wet bulk density. There were no significant correlations with chlorophyll or carbohydrate. This is interpreted, at the whole-estuary scale, as reflecting an underlying physical control of sediment properties. The control is exerted through the combined influence of tidal pumping of fine material up-estuary to form the turbidity maximum, resulting in a reduction in sand content with distance up-estuary, and the generation of increasingly wet, low-density sediments in the upper estuary through repeated tidal mobilisation and deposition. It is speculated that adverse environmental conditions of low and fluctuating salinity, combined with regular tidal sediment disturbance in the upper estuary, prevent the benthic stabilisers (mainly algae) from colonising the sediment to the same extent as they do in the lower, marine estuary. The implication is that the distribution of benthic species in macro-tidal estuaries is determined by a combination of physical influences that (a) control the nature of the sediment along the estuary and, (b) result in a hostile, fluctuating salinity in the low-salinity region associated with variable river flows. Conversely, because of the less variable conditions in the marine end of the estuary, elements of the biota are able to colonise the mud flats where they can modify sediment properties. In these physically dominated, macro-tidal estuaries, seasonal (biological) effects on sediment stability contribute to the variability of the erosion–bulk density relationship.

Measurements are presented of the properties of suspended particulate matter (SPM) in the estuarine turbidity maximum (ETM) of the upper Humber and Ouse estuaries during transient, relatively low freshwater inflow conditions of September 1995. Very high concentrations of near-bed SPM (more than 100 g l−1) were observed in the low-salinity (less than 1), upper reaches. SPM within the ETM consisted largely of fine sediment (silt and clay) that existed as microfloc and macrofloc aggregates and individual particles. Primary sediment particles were very fine grained, and typically, about 20–30% was clay-sized at high water. The clay mineralogy was dominated by chlorite and illite. There was a pronounced increase in particle size in the tidal river, up-estuary of the ETM. The mean specific surface area (SSA) of near-bed SPM within the ETM was 22 m2 g−1 on a spring tide and 24 m2 g−1 on a neap tide. A tidal cycle of measurements within a near-bed, high concentration SPM layer during a very small neap tide gave a mean SSA of 26 m2 g−1. The percentage of silt and clay in surficial bed sediments along the main channel of the estuary varied strongly. The relatively low silt and clay percentage of surficial bed sediments (about 10–35%) within the ETM’s region of highest near-bed SPM concentrations and their low SSA values were in marked contrast to the overlying SPM. The loss on ignition (LOI) of near-bed SPM in the turbid reaches of the estuary was about 10%, compared with about 12% for surface SPM and more than 40% in the very low turbidity waters up-estuary of the ETM. Settling velocities of Humber–Ouse SPM, sampled in situ and measured using a settling column, maximized at 1.5 mm s−1 and exhibited hindered settling at higher SPM concentrations.

The macrotidal Humber system, comprising estuaries of the Humber, Trent and Ouse, and their various tributaries, is one of the largest and most turbid in the British Isles. This paper presents detailed spatial and temporal data on the estuarine turbidity maximum (ETM) within the Ouse Estuary of the Humber system during quiescent summer conditions, when freshwater runoff was very low and approximately steady. Under these conditions, an extremely turbid ETM existed in the low salinity reaches of the upper Humber, within the Trent and Ouse Estuaries. Longitudinal surveys of salinity, temperature and turbidity were obtained at approximately local high water (HW) or low water (LW) between the tidal limit of the Ouse and the upper Humber. Tidal-cycle stations were worked between the upper Ouse and the coastal zone. In situ median floc sizes were measured at some stations. Tidal water levels were very asymmetric and currents were flood dominant in the upper estuary, especially at spring tides. Frictional drag on the currents was approximately balanced by water-level slope forcing, which led to a large reduction in tidal amplitude as the tide propagated into the estuary. A tidal bore, 0.1–0.2 m high, formed at spring tides in the upper estuary, but did not cause suspension of fine sediment at locations up-estuary of the ETM. Generally, salinity was fairly well mixed vertically, despite strong SPM stratification in the ETM region. However, large salinity inversions did occur in the presence of underlying, stationary sediment suspensions (∼90 g l−1). The ETM core region, in which near-bed SPM concentrations exceeded 16 g l−1, extended over a longitudinal distance of 35 km at HW, both at spring and at neap tides. It was separated by nose and tail regions from much lower turbidity waters. The nose was much sharper than the tail and was located 15 km into the tidal river at spring tides, where salinity was less than 1. Except at very small neap tides, when fluid mud layers and stationary suspensions formed in the tail region of the ETM, maximum near-bed SPM concentrations (∼50 g l−1) occurred close to the nose in the upper core region. The ETM was displaced down-estuary by ca. 12 km during the transition from spring to neap tides. It also was displaced down-estuary between HW and LW. Floc settling led to pronounced SPM stratification over the HW, HW-slack and early ebb period. Estimates of settling velocity, corrected for hindered settling, ranged from 1.2 to 2.1 mm s−1. At HW slack, hindered settling prevented appreciable deposition of flocs to the bed in the main channel of the nose and upper core regions and at LW of spring tides there was little time for deposition. In the water column, in situ floc sizes maximised around mid-depth at HW slack. Therefore, slack water and low current shears led to increased flocculation, greater sizes and enhanced settling. These maximum median sizes typically were 300–500 μm, whereas sizes during other states of the tide were in the range 70–300 μm. A strong, negative correlation existed between depth-averaged median floc size and bulk vertical current shear in the water column for flocs that were greater than a station-dependent size. The largest median floc sizes occurred within the near-bed stationary suspensions at neap tides, where floc sizes could exceed 1 mm. Entrainment of these flocs led to floc breakage, which reduced their median sizes to less than 200 μm.

Results are presented from a 1-year campaign to measure the seasonal variability of some key physical and biological properties of intertidal mudflats over a section of the central Tamar Estuary and to relate these to the physical environment. Seasonal variations in “physical” mudflat properties, such as grain size, density and moisture content were relatively small. With the exception of the particulate organic carbon content in the upper 0.002 m of surface sediment, biological variations were large. Redox potential exhibited considerable seasonal variation and showed that the sediments were less reduced in winter and more reduced in summer. Chlorophyll a and extracellular polymeric substances (EPS) within the surface 0.002 m of sediment (due to the presence of benthic diatoms) were strongly correlated and exhibited a pronounced seasonal pattern, with smallest values during winter and greatest values during late summer and early autumn. EPS had a dominating influence on sediment erosion, as determined from annular flume measurements. Velocity measurements and velocity modelling indicated that during the flood, and for much of the time during benthic diatom “bloom” conditions of high chlorophyll a and EPS sediment contents, the stresses exerted by tidal currents were too small to cause significant suspension of sediments over much of the middle and upper mudflats. Suspended fine sediment in the turbidity maximum zone was transported down-estuary and deposited in the main channel at low-water (LW) slack. Some of this sediment, in the form of relatively large aggregates, was subsequently transported onto the mudflats during the flooding tide, where slack currents and fast settling velocities may have enhanced sediment deposition there.

Do artificial reefs serve as productive marine fish habitats (sources) or do fish assemblages of such reefs contribute little to the gene pool of succeeding generations (sinks)? Using data from a 24-year study of the breakwater at King Harbor (California, USA), annual densities of reef fish larvae were compared with densities observed elsewhere in the Southern California Bight. Larval production at King Harbor has decreased over the study period, as has the size of the fish assemblage. Both declines may be related to recorded increases in water temperature. Larval densities from similar water depths (0-15 m) throughout the bight vary widely between sites and years. However, the mean percentage represented by reef fish among all larvae at King Harbor was 51% during those 5 years for which bight-wide samples were available (35% for all years) compared to 5% in the bight-wide samples. Utilizing larvae of 12 genera of reef fish species common to the samples, five were significantly more abundant in King Harbor samples. Results indicate that the breakwater represents a mature artificial reef and contributes to the reef fish larval pool of the bight, acting as a source rather than a sink. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

It is shown that there is a marked tendency for long, strongly tidal estuaries to have greater suspended particulate matter (SPM) concentrations within their high-turbidity regions than shorter estuaries with comparable tidal ranges at their mouths, or weakly tidal estuaries. Using consistently derived data from 44 estuaries in Europe and the Americas, contours of the logarithm of maximum estuarine SPM concentration are shown to be reasonably smooth when plotted against the logarithm of mean spring tidal range (at the estuary mouth) and the logarithm of estuarine tidal length. Predictions from the plot are compared with published observations made in the Delaware, Scheldt, Rio de la Plata, Gironde, Bay of Fundy, Changjiang (Yangtze), Amazon, Patos Lagoon and the Hawkesbury Estuary and it is shown that, qualitatively, there are no serious discrepancies. Short, weakly tidal estuaries are predicted to have very low ‘intrinsic’ SPM concentrations. High SPM concentrations in these estuaries would most likely be the result of either locally generated wave resuspension, high freshwater sediment loads due to freshets, or intruding seawater carrying suspended sediments derived from wave activity in the coastal zone. Application of a generic tidal model demonstrates that longer estuaries possess faster tidal currents for a given tidal range at their mouth and, in the presence of a supply of erodable fine sediment, therefore (by implication) produce greater concentrations of SPM that can be accumulated within a turbidity maximum. The same is true if the tidal range is increased for estuaries of a given length. These features are illustrated by comparing surveys of SPM data from two large estuaries possessing greatly different tidal ranges (the microtidal, medium turbidity Potomac and the macrotidal, highly turbid Humber-Ouse) and a third, much smaller but strongly tidal estuary (the low-turbidity Tweed). It is demonstrated that longer estuaries tend to have longer flushing times for solutes than shorter systems and that larger tides tend to reduce flushing times, although the tidal influence is secondary. Short, rapidly flushed estuaries quickly lose their erodable fine sediment to the coastal zone during freshets and during the ebbing currents of spring tides. Turbidity is therefore small during low runoff, low wave activity conditions. Very long, very slowly flushed estuaries are unlikely to lose a significant fraction of their resuspended sediments during freshets or individual ebb tides and are therefore able to accumulate large and increasing amounts of fine sediment in the long-term. Turbidity within them is therefore high during the fast currents of large spring tides.

Longitudinal and depth profiles of salinity and temperature were measured in the Tamar Estuary, southwest England, during 1985. During late autumn, winter and early spring, the observed temperature-salinity scatter plots illustrated either near-conservative behaviour or heat loss from the estuary to the atmosphere, whereas the rest of the year they illustrated heat gain from the atmosphere. Observed maximum and minimum freshwater temperatures occurred during late July (19.6°C) and mid January (3.1°C). Maximum and minimum observed coastal-water temperatures occurred during early October (15.1°C) and late February (6.2°C). Similar coastal (15.1°C) and offshore (15.4°C) surface temperatures were observed in summer as a consequence of stratified conditions offshore and the existence of a near-surface, mixed layer there. Air temperature, freshwater (zero salinity) temperature and coastal water temperature showed a strong correlation with coastal soil temperatures measured at 0.05, 0.3 and 1 m beneath ground, respectively. Simulated temperatures were derived from a model that solved the heat budget equations for 1985 on an hourly time-scale, using hourly measurements of local meteorological variables. The yearly average and standard deviation of temperatures in mid estuary were 11.6°C and 4.3°C, respectively. Seasonal temperature amplitude exceeded tidal and diurnal amplitudes by almost an order of magnitude. Temperatures exhibited strongly tidal, predominantly semidiurnal signals, as well as non-tidal, diurnal signals. The daily averaged rates of horizontal heat transport to the coastal waters from the estuary and its rivers during 1985 were typically between 1–2 GW during the spring and summer months. Heat transport was generally reversed during late autumn, winter and spring. The estuary then generally received heat from the coastal zone, with maximum rates of heat transport that were typically 3 GW. Because estuary residence times varied from less than a few days in winter to less than two weeks in summer, there was a close coupling (on seasonal time-scales) between estuarine heat fluxes and coastal seawater and terrestrial freshwater temperature differences.

Results are presented from a study of the velocity, salinity, temperature and turbidity behaviour on intertidal mudflats bounding a 400 m wide cross-section in the central reaches of a small, macrotidal, ria estuary (the Tavy Estuary, Southwest England, UK). Measurements were made during low freshwater inflow, summer spring tides. The mudflats comprised a muddy mixture of predominantly silt and clay, with bulk densities typically in the range 1.2–1.4 g ml −1 . Bed sediment in the main channel, and on the upper shores of both banks, comprised a mixture of predominantly coarse, non-cohesive sediment, with very small fractions of silt and clay. Two near-bed instrument packages were continuously deployed during the observational period, one on each of the intertidal mudflats located on opposite sides of the estuarine main channel. The instruments recorded water level, velocity, temperature, salinity and turbidity at 0.25 m above the bed. Tidal-cycle measurements of these variables were additionally made throughout the water column in the cross-section's main channel. Longitudinal and vertical surveys of temperature, salinity and turbidity were made throughout the estuary in order to aid interpretation. Maximum bed shear stresses were flood dominant in the main channel and strongly ebb dominant on the upper mudflats. Currents over the intertidal mudflats had much slower peak speeds than those in the main channel. The data presented here indicate that vertically mixed, relatively high-salinity, high-turbidity waters flooded onto the upper mudflats during spring tides, and that the suspended particulate matter largely settled to the bed there, both during the flood and over high-water slack. With one exception, there was little evidence of any subsequent strong sediment resuspension during the salinity-stratified ebb (at least when depths >0.25 m). These results are consistent with the long-term depositional environment associated with rapidly rising sea levels of the early Holocene, and with the present, relatively slower rate of sea-level rise.

A strong turbidity maximum (TM) of suspended particulate matter (SPM) was observed in the upper Humber and lower Ouse during both spring and neap tides of May 1994. Near-bed concentrations within the TM sometimes exceeded fluid mud levels following slack water periods. SPM within the TM comprised very fine-grained material. Its low organic content demonstrated that the SPM was essentially mineral, clastic sediment. Generally, tidal advection of SPM was the dominant flux mechanism and the pronounced flood-ebb asymmetry in the tidal currents was reflected in these fluxes. However, the presence of fluid mud near the down-estuary margins of the TM on the early ebb resulted in a strong, up-estuary shear flux that opposed the ebb-directed advection. This mechanism therefore acted to maintain fine sediment in the TM region. SPM concentrations in the inflowing fresh water at Naburn Weir were much less than those observed within the TM region ( 70 000 mg l -1 ). The estimated mean SPM transport into the tidal Ouse across Naburn Weir was about 4 kg s -1 (>2 and

Measurements are presented of salinity, temperature and dissolved inorganic nutrient concentrations from an intensive survey of the near-mouth region of the Humber Estuary and coastal zone during a tidal cycle on 6 June 1995. The tidal movement of water in this area is illustrated with model results and used to interpret the observations. Variable–salinity plots showed that while estuarine-derived nitrate and silicate mixed conservatively with seawater, nitrite and phosphate showed evidence of sources within the mouth region of the estuary. A temperature–salinity scatter plot showed elevated temperatures within the shallow, estuarine waters due to solar heating. Despite the relatively small range of salinities encountered, from 34.2 to 29.6 progressing from the coastal zone into the estuary mouth, the suspended particulate matter concentration increased from 2 to 80 mg l −1 and the temperature from 10°C to 14°C. Nitrate concentration increased from 1.4 to 106 μM; nitrite, 0.1–0.4 μM; phosphate, 0.3–1.7 μM and silicate, 0.3–11.3 μM. The residence time of the Humber was approximately 60 days during summer 1995. Using freshwater inflows averaged over the residence time gave estimates of the seaward transport rates for nitrate–nitrogen, phosphate–phosphorus and silicate–silicon of 9.9 × 10 3 , 73 and 1.1 × 10 3 k mol d −1 . However, significantly greater nutrient fluxes from the Humber system are likely to occur during winter and spring, high freshwater inflow periods.

Results are presented of turbidity measurements made in the Tweed Estuary during 14–29 September 1993. The observations covered a spring-neap period of very strong and relatively weak tides, which included strong wave activity at the mouth of the estuary, the aftermath of a strong freshwater inflow event and a minor inflow event which coincided with neap tides. Turbidity levels between the mouth and the limit of saline intrusion during this period were observed to lie in the range 2–30 ppm. Temperature–salinity relationships, based on rapid sampling throughout the estuary, often indicated conservative mixing between riverine and coastal waters. Turbidity–salinity relationships throughout the estuary were approximately linear for most of the lower salinity range (

Results are presented from an unusually detailed survey of the temporal and spatial behaviour of an estuarine tidal intrusion front (Simpson, J.H., Nunes, R.A., 1981. Estuar. Coast. Shelf Sci. 13, 257–266). The front was observed as it propagated through the inlet and moved into the main channel of a small estuary (Tweed Estuary, UK) during a flooding spring tide. High salinity coastal waters plunged beneath low salinity estuarine waters, the plunge lines defining a “V” shaped front for part of the flood. The data were obtained using aircraft and sea-truth measurements as part of a two-day field programme in September 1994. Hydrographic surveys undertaken during similar environmental conditions in September 1993 also are used to aid interpretation. A convex-seaward front was observed in the seaward section of the inlet at LW + 2.1 h (LW = low-water). By LW + 2.7 h, approaching maximum flood currents at mid-water, a “V” shaped tidal intrusion front was observed at the neck of the inlet, with the “V” pointing up-estuary. These data are consistent with measured salinity in the inlet at LW + 2.1 h and the occurrence of a critical inflow Froude number in the inlet neck at LW + 2.7 h. At this time a tongue of colour-difference waters was observed which extended about 400 m into the estuary and emanated from the intrusion front inside the inlet. The front moved through the neck as flood currents increased, entered the estuary main channel at LW + 3.1 h and collapsed within the channel at LW + 3.3 h. The plunge point then transferred to the tip of the colour-demarcation tongue, which subsequently developed a “V” head and migrated up-estuary with an apparent speed that was approximately equal to the estimated difference between tidal and buoyancy current speeds (0.2 m s−1). Measured salinity showed the horizontal, near-surface structure of the intrusion front when it was located approximately 1 km inside the estuary at LW + 3.9 h. A lens of less saline surface water was observed slightly up-estuary of this front, effectively trapped between the main channel and shoals. The front then remained effectively stationary until LW + 4.6 h, at which time it dissipated. Pronounced frontal structures that defined the boundary of the low salinity lens, on the channel margins and over the inundated shoals, remained however, and were surveyed at LW + 4.8 h. These frontal structures, and a diminished lens of fresher waters, were observed remotely by aircraft at LW + 5.6 h, within 30 min of high-water. The effects of mixing, basal current circulations and variable channel depths, especially the occurrences of particularly deep areas, are discussed. Centrifugal effects are shown to be important.

The Tamar (England) and Weser (Germany) Estuaries exhibit strong turbidity maxima in their low-salinity reaches. Whereas their morphologies and freshwater inflows are different, the estuaries have similar mean tidal ranges and mean tidal current speeds. Despite the differences, the turbidity maxima show similar intratidal, spring–neap and seasonal behaviour for the two estuaries. On an intratidal time scale, strong suspended particulate matter (SPM) variability occurs in both turbidity maxima regions due to deposition over slack-water periods, subsequent re-suspension and depletion of easily erodible bed-source sediment during the following flood or ebb, and transport by tidal currents. Over a spring–neap cycle, the spring-tide SPM levels are greater than those during neap tides owing to enhanced re-suspension in the stronger spring-tide currents. Both estuaries exhibit a hysteresis in SPM during the falling and rising spring–neap cycle. On a seasonal basis, the occurrences of river spates in both the Tamar and the Weser move both the limits of saline intrusion and the turbidity maxima further down-estuary. Following the return to normal freshwater inflows, the saline intrusion moves rapidly up-estuary, whereas the turbidity maximum exhibits a time lag.

Results are presented from a 2-week field programme in the Tweed Estuary, U.K. Maximum values of the empirically based Estuarine Richardson Number, RiE, occurred during neap tides, and minimum values occurred during spring tides. Estimated values of RiEvaried between 0·3 and 2·3, suggesting the occurrence of partially mixed to stratified conditions, depending on tidal state and freshwater inflow. These relatively large values of RiEwere consistent with both observed strong salinity stratification and large salt fluxes due to vertical shear transport. Low values ( 0·5) values in the halocline. A velocity maximum occurred within the halocline during the early flood. Wave-like spatial oscillations of the halocline occurred on the ebb. The oscillation troughs were situated above deep holes located just down-estuary of the rail and old road bridges. There was an indication that the constricted flow between the bridges’ arches resulted in enhanced mixing of near-surface waters and a thickening of the halocline. It is also possible that these wave-like structures were stationary, near-critical internal lee waves, triggered by the deep holes. Trapping of high-salinity waters occurred on the ebb. Saline pools were isolated within a deep hole or deeper section of bed by the falling halocline. When the salt wedge moved further down-estuary, the ‘ trapped ’ waters were subjected to strongly ebbing, overlying freshwater, and were subsequently entrained and flushed. The salinity intrusion was a strong function of spring–neap tidal state and a weaker function of freshwater inflow. The estimated salinity intrusion varied from about 4·7 to 7·6 km during the fieldwork period. The strong dependence on tidal range followed from the comparable lengths of the tidal excursion and salinity intrusion. Long excursion lengths were also partly responsible for the short residence (or flushing) times and their strong dependence on spring–neap tidal state. For typical summer freshwater inflows of 40 m3s−1, the estimated residence times varied from one tide at large spring tides (removal on one ebb) to two tides (removal over two ebbs) at neap tides. Computed residence times were shorter for typical winter inflows.

Measurements describing the intratidal, spring-neap and seasonal variations of fine, cohesive, suspended particulate matter (SPM) concentrations at two sites (CK and HQ) within the upper reaches of the Tamar Estuary, U.K. are presented. The data were obtained using two, near-bed instrument packages. Correlations between daily-averaged SPM concentrations and runoff and tidal range during the separate deployments often showed a significant dependence of SPM on these variables. Where statistically significant, increasing tidal range led to enhanced SPM levels because of resuspension of bed sediments. There was an asymmetry between the daily-averaged SPM concentrations on the rising and falling 15 day spring-neap cycle. On the rising cycle, the SPM levels were significantly lower than on the falling cycle for the same tidal range. This asymmetry was a feature of all deployments. The daily-averaged SPM during 3·8 m tidal-range rising spring tides at stations CK and HQ was approximately 0·3 kg m -3 in July and November 1988 and then fell to 0·01 kg m -3 during December 1988. This reduction in SPM was a manifestation of the winter period of bed-sediment depletion. Sediment was scoured from the upper estuary by strong ebb currents, which were enhanced by high and increasing runoff. SPM levels remained low during March 1989 (0·08 kg m -3 ) at the end of the high runoff, winter period. The daily-averaged SPM at HQ during April was much higher (0·77 kg m -3 ) than that further up-estuary at CK (0·11 kg m -3 ), which was comparable with the March levels. This indicates that the bed-sediment supply had moved up-estuary to the vicinity of HQ, so that local resuspension was very important there at that time, but had not reached the vicinity of CK. Fine sediment continued to move into the upper reaches during the spring to summer period. By May 1989 the daily-averaged SPM concentrations at CK were much higher than those further down-estuary at HQ(0·98 as opposed to 0·22 kg m -3 ). For the June and (both) July 1989 deployments, SPM levels at CK exceeded those at HQ, although at both stations the levels were lower in July. As the limited amount of fine sediment moved further up-estuary, through CK and closer to the head, the amount of bed-source sediment available for local erosion at CK and HQ was reduced.

Two synoptic, spring-tide, multi-station experiments in the 31 km-long Tamar Estuary were analysed for suspended particulate matter (SPM) concentrations and transport. SPM concentrations were very low in the seaward reaches and were largely associated with the flushing of fresh water from the estuary. Depth-averaged SPM concentrations in the upper 8·5 km of estuary were of the order of 103 higher than those near the mouth under similar, low-runoff, spring-tide, summer conditions. Local resuspension and advection of locally resuspended sediment from nearby bed-sources in the upper 8·5 km of estuary dominated the SPM loads during the flood. Highest SPM concentrations occurred during peak flood currents in the fresh water at 4·5 km from the head. At high-water, this location was in the vicinity of the freshwater-saltwater interface (FSI). During the ebb, SPM maxima occurred in fresh water throughout the upper 8·5 km of estuary. Near-bed measurements and longitudinal-transect data in the upper estuary showed that the FSI had an important influence on intratidal SPM variations.

Measurements are presented of the bulk and mineralogical properties of intertidal sediments along the axis of the Tamar Estuary, southwest England. Bulk and dry density data indicate a significant increase in consolidation of the surface layer of the intertidal mudflats progressing down-estuary from the turbidity maximum region to the mouth. There are very significant trends of both density and estimated critical eroison shear stress with distance. Cross-sectionally averaged, bed shear stresses due to tidal currents are computed using a hydrodynamical model. These tidal stresses generally increase along the axis of the estuary from mouth to head and reach a maximum in the upper reaches. The increasing consolidation towards the mouth appears to result from the small bed shear stresses due to tidal currents in the lower reaches and the increasing proportion of coarse material there. Bed shear stresses are large in the upper reaches and regular intratidal resuspension and transport occur during spring tides, with little time available for consolidation following deposition during slack-water periods. The silt and clay fraction of the intertidal sediments increases from the mouth to the turbidity maximum region near the head (60 to > 99% dry weight). The particulate organic carbon (POC) content, assuming this to be approximated by loss on ignition, similarly increases from about 2% of dry weight near the mouth to about 8% in the turbidity maximum region. The POC content of a sample is largely dependent on the proportion of fine sediment within the sample, regardless of its position. The lower estuary is associated with intertidal sediments having relatively low silt and clay content and low associated organic material. It appears to be a fairly stable zone between the marine and estuarine environments. The upper estuary, in the turbidity maximum region, appears to be almost homogeneous in both POC (8.1 ±0.6%) and silt content (95±5%) during summer. The central region is an area of great variability in the size fraction, sediment type and POC content.

The physical processes determining the path of the Gulf Stream as it leaves the continentalshelf of the USA are complex and involved. Understanding these processes is important toglobal climate studies because the Gulf Stream plays a major rôle in the distribution of heatin the northern hemisphere. Here we show that during the last 3 decades, the latitude of theGulf Stream has been clearly correlated with the North Atlantic Oscillation (NAO); high valuesof the NAO index, which correspond to stronger westerly and trade winds, favouring morenortherly paths of the Stream about 2 years later. The time-delay may be associated with theadjustment time of this part of the ocean circulation. If allowance is made for persistence ofthe Gulf Stream position from one year to the next, more than half the interannual variationin the position can be predicted.DOI: 10.1034/j.1600-0870.1998.00010.x

Data are presented from several experiments in the freshwater-saltwater interface (FSI) region of the Tamar Estuary. Longitudinal surveys of salinity and suspended particulate matter (SPM) at high water showed that the location of the FSI could be predicted in terms of a power-law regression with freshwater runoff. Longitudinal transects also were surveyed over periods of several hours. The FSI was observed to advect into the region on the flood with strong vertical mixing. After high water, stratification became intense as fresher water ebbed in the surface layers. The near-bed water in the stratified region began to ebb between 2 h and 3 h before low water. A model of the vertical structure of longitudinal currents showed that the enhanced stratification on the ebb, coupled with the longitudinal density gradient, partly produced this long period of slack, near-bed currents following high water. A strong turbidity maximum (TM) occurred during spring tides and was located slightly up-estuary of the FSI at high water. Longitudinal transects during a period of low freshwater runoff and large neap tide showed that at the start of the flood the TM was associated with the FSI region. As the FSI advected up-estuary on the flood there was considerable resuspension of sediment at the FSI. Some of this SPM moved with the FSI and reached the limit of saline intrusion, where it formed a slowly-eroding TM as particles settled during the long, high-water slack period. As the near-bed currents increased on the ebb and the FSI moved down-estuary, strong vertical mixing and resuspension of recently deposited sediment occurred in the unstratified water behind the FSI and the associated TM advected down-estuary. Additional effects were present with stronger tides and increased runoff.

Measurements of velocity, salinity, and suspended solids concentration have been used to investigate the intra-tidal variation of vertical and transverse shear-induced dispersion. For the study research the interaction of the longitudinal density gradient and vertical shear during the early part of the ebb tide accounted for much of the net longitudinal dispersion of solute landward. The same mechanism also is shown to lead to a net particulate transport landward. The landward flux, however, takes place during the flood tide. The field data are also used to elucidate the tidally averaged tidal pumping mechanism.

A one-dimensional, hydrodynamical model of the Tamar Estuary shows good agreement with measured tidal elevations and currents. Computed currents are used to drive a one-dimensional moving-element model of the salt balance. The moving-element model overcomes the numerical difficulties associated with strong tidal advection. Axial distributions of salinity at high water, computed using the moving-element model, compare well with measurements. The modelled and observed high water salinity distributions in this macrotidal estuary show little dependence on tidal range. The major variability in salinity is due to runoff. This strong and rapid dependence on runoff is a consequence of short residence (or flushing) times. Typically, residence times are less than one day throughout the year in the upper 10 km of estuary. The residence times maximize in summer, reaching 14 d for the whole estuary. During high runoff winter periods residence times are less than 5 d. Mixing coefficients for the moving-element salinity model are deduced from salinity measurements. Dispersion coefficients at fixed locations along the estuary are deduced from solutions of the salinity model. The spatially-averaged coefficients at mean spring and neap tides are 180 and 240 m2 s−1, respectively, for average runoff. Therefore, spring-neap variations in dispersion are fairly small and show a negative correlation with tidal range. The spatially-averaged dispersion coefficients at mean tides vary from 150 to 300 m2 s−1 for typical summer and winter runoff, respectively. The increase in dispersion with runoff and the decrease with tidal range implies that buoyancy-driven currents generate an important component of the shear dispersion in this estuary.

A salt marsh estuary which has received large discharges of inorganic mercury for several years was studied to evaluate the occurrence and production of methylmercury. The methyl form was not detected (

Elevated Hg concentrations (

A linear decrease in dissolved organic carbon and nitrogen with increasing salinity offshore from the Georgia coast suggests that organic nitrogen compounds contributed to coastal waters by rivers are stable during the period (2–3 months) of their transfer over the continental shelf. While the C/N ratio decreased with distance from shore, total dissolved organic nitrogen (DON), total amino nitrogen, and primary amino nitrogen showed similar relative decreases, suggesting that nitrogen is associated with refractory organic compounds. Measured amino nitrogen accounted for about 20% of the total DON, leaving about 80% of the organic nitrogen undefined.

A turbidity maximum is consistently observed in the upper reaches of the Tamar estuary at high water. Neap tide suspended particulate matter (SPM) concentrations are typically 20 ppm at high water; spring tide concentrations are typically an order of magnitude greater. The location of the turbidity maximum is usually associated with the freshwater-saltwater interface but occasionally occurs further up-estuary. At neap tides the maximum is less defined and can be masked by freshwater SPM concentrations. The spring tide maximum is generally small during winter periods, when it is located down-estuary, and reaches its largest concentrations during summer when it is close to the head. About 65% of the variance in the magnitude of the maximum at spring tides can be explained in terms of its distance from the head. The position of the maximum is determined mainly by freshwater runoff with a logarithmic regression explaining about 80% of the variance. Only 30–35% of the variance in the turbidity maximum magnitude and location at neap tides can be explained in terms of tidal range and runoff. A tidal resuspension model is considered which ignores density effects but which has a spatially independent, runoff dependent (but otherwise time independent) erodibility constant as a single “free” parameter. The model provides a reasonable description of the magnitude and location of the turbidity maximum in the Tamar, both at spring and neap tides, despite the fact that the maximum usually occurs near the freshwater-saltwater interface (which in itself would strongly suggest that gravitational circulation and intratidal stability effects have a strong influence on the location of the maximum). It is possible that all three mechanisms act together to produce a much narrower maximum than we have simulated, with resuspension being enhanced in the strongly mixed fresh water behind the interface, and tidal pumping and gravitational circulation accumulating sediment close to the interface.