A.J. Bale

In situ measurement of particle size in estuarine waters

A novel approach to particle sizing in estuarine waters using a submersible laser diffraction instrument is described. In situ measurements with this instrument are rapid and avoid both the physical disruption of fragile aggregates with pumping and the settlement of larger particles in isolated samples which interfere with currently available sizing techniques. Measurements of the size distributions of suspended particle populations in the Tamar Estuary using this apparatus are compared with measurements carried out on discrete samples obtained from the same depth by pumping. The results confirm the fragile nature of estuarine aggregates and the necessity for in situ measurements in studies of estuarine suspended particle dynamics and fluxes.

The dynamics of estuarine manganese cycling

Seasonal observations of dissolved managnese in the Tamar Estuary,south-west England have demonstrated pronounced temporal and spatial variability in the distribution of this component. Simultaneous observations of physical and physico-chemical properties of the estuarine system, together with kinetic measurements of dissolved managenese behaviour in isolated samples, have demonstrated that the observed distributions are the instantaneous product of dynamic interactions involving external sources and internal input and removal processes. Dissolved manganese is mobilized internally from reduced sedimentsduring advective resuspension of bed material so that this input is regulated by temporal and spatial variations in tidal stress. Manganese is returned to the sediments by continuous uptake on to suspended particles. The rate of this process is responsive to changes in suspended particulate load, ionic strength pH and temperature. The reaction changes from zero-order kinetics in freshwater to first-order kinetics characteristic of the saline medium, with marked deceleration in effective, removal rate, following small increases in salt concentration. Relative changes in the first-order removal rate throughout the estuary are consistent, but there are considerable temporal variations in the absolute rates. This is partially attributable to variations in the quality, as well as the quantity, of the suspended particles. These results indicate that changes in the relative effectiveness of both the input and removal mechanisms, which lead to internal cycling of manganese and to variable distributions of dissolved managanese within the estuary, are closely coupled to the internal behaviour of particles and hence to the tidally-induced oscillations in bed stress. This observation has been confirmed by sequential recordings of dissolved manganese distributions through a spring tide to neap tide period. Furthermore, climatic events producing enhanced disturbance of bed sediment have been shown to induce a similar sequence of events to that arising from tidally induced net mobilization of bed material.

Nutrient distributions in an estuary: Evidence of chemical precipitation of dissolved silicate and phosphate

Continuous autoanalytical recordings of the axial distributions of dissolved nitrate, silicate and phosphate in the influent freshwater and saline waters of the Tamar Estuary, south-west England have been obtained. Short-term variability in the distributions was assessed by repetitive profiling at approximately 3-h intervals on a single day and seasonal comparisons were obtained from ten surveys carried out between June 1977 and August 1978. Whereas nitrate is always essentially conserved throughout the upper estuary, the silicate- and phosphate-salinity relationships consistently indicate a non-biological removal of these nutrients within the low (0–10%) salinity range. Attempts to quantify precisely the degree of removal and to correlate this with changes in environmental properties (pH, turbidity, chlorophyll fluorescence, salinity, freshwater composition) were mainly inconclusive due to short-term fluctuations in the riverine concentrations of silicate and phosphate advected into the reactive region and to the rapid changes in turbidity brought about by tidally-induced resuspension and deposition of bottom sediment.

Distributions and behaviour of dissolved Cu, Zn and Mn in the Tamar estuary

Abstract Concentrations of dissolved Cu, Zn and Mn have been recorded together with salinity, suspended particulate load, dissolved oxygen and pH, in twenty-two transects of the Tamar Estuary carried out between 1976 and 1985. The axial profiles of dissolved Mn accord with the results of previous studies. These showed persistent minima indicating varying degrees of removal within the low salinity, high turbidity region. Mid-estuarine maxima were also recorded. These were variable in magnitude and location within the salinity range 3–15‰. The pattern of Zn distributions was similar to that of Mn. Removal of Zn occurred in the same low salinity region but was generally less accentuated. The mid-estuarine Zn maxima fell within the salinity range 3–20‰. Cu was also removed within the low salinity region but the mid-estuarine maxima were confined to the 1–8‰ salinity range. The characteristics of removal within the low salinity zone for all three metals are compatible with rapid adsorption onto suspended particles forming the estuarine turbidity maximum. A comparison of metal distributions throughout the rest of the estuary using a statistical technique indicates that while direct pore water infusions contribute to the observed mid-estuarine maxima of dissolved Cu and Zn, desorption from tidally resuspended sediment particles also plays a significant role. Desorption from seaward fluxing particles of immediate riverine origin is probably largely confined to the very low salinity region where its effect is more than counterbalanced by the sorptive uptake process. It is concluded that the pronounced dissolved metal reactivity within this estuary is attributable to an enhancement of particle-water interactions by highly dynamic internal sediment mobility driven by high tidal energy inputs.

Chemical variability in the Tamar Estuary, south-west England

Procedures for the continuous in situ recording of salinity, temperature, dissolved oxygen concentration, pH and turbidity throughout an estuarine mixing profile have been developed. Application of these procedures in a study of the Tamar Estuary, south-west England has demonstrated the considerable temporal (short-term and seasonal) and geographical variability of these properties. The causes and interrelationships of this variability and their general implications with respect to field investigations of estuarine chemical interactions are discussed.

Dissolved aluminium in the Tamar Estuary, southwest England

Abstract Distributions of dissolved Al in the Tamar Estuary have been recorded through spring-neap tidal cycles in winter and summer. Dissolved Al is a highly reactive constituent in this estuary, undergoing net removal in the very low salinity region and net input to the mid-estuary. The extent of both the depletion and the augmentation varied systematically with tidal energy input, indicating that tidal sediment disturbance was the principal controlling agency. These field data, supported by the results of laboratory simulations, show that the removal is regulated by sorption onto resuspended sediment particles, which dominates kinetically over authigenic aluminosilicate formation. Removal of dissolved Al by flocculation of riverborne colloids is not significant. The mid-estuarine input is consistent with dissolution of authigenic aluminosilicate following net remobilization of estuarine sediment. The internal cycling of Al in the estuary, generated by these processes, alters both the time-course and the soluble-particulate balance of the riverine discharge of Al to the adjacent coastal water.

Laboratory simulation of chemical processes induced by estuarine mixing: The behaviour of iron and phosphate in estuaries

A series of well stirred tank reactors has been shown to provide an adaptable laboratory analogue of a one-dimensional estuarine mixing profile which can be applied dynamically to the study of the chemistry of estuarine mixing. Simulations of the behaviour of iron and phosphate in the low salinity region of an estuary have been achieved with this system. The well documented general features of iron removal, involving rapid aggregation of river-borne colloids, were reproduced. Phosphate removal is attributable in part to the coagulation process, although specific adsorption of phosphate by colloids also appears to be significant.

SEDIMENT TRAP PERFORMANCE IN TIDAL WATERS : COMPARISON OF CYLINDRICAL AND CONICAL COLLECTORS

Abstract A number of different sediment traps have been deployed at two sites which exhibited significantly different tidal streams. Measurements of suspended particle concentration and current velocities at each site during the deployments enabled horizontal particle fluxes to be calculated for periods corresponding to the sediment trap sampling intervals. These data have allowed the performance of the traps to be compared and the effect of the tidal environment on trap performance to be assessed under field conditions. In parallel deployments, different diameter cylindrical traps of similar aspect ratio (height/diameter) collected identical sample masses when corrected for the cross-section of trap aperture but conical traps collected only one-tenth of the material taken by the cylindrical traps. The sample collection rate of cylindrical traps was found to be positively related to the measured horizontal particle flux and this relationship was similar at both sites despite different tidal current strengths and a corresponding variation in the concentrations of suspended particles. Comparisons of the carbon content of ambient suspended particles with that of the samples collected by the traps indicated that the conical traps progressively under trapped carbon-rich particles as current velocity increased. These data confirm that the use of moored sediment traps in tidal waters, where horizontal particle fluxes greatly exceed vertical fluxes, is of limited value in the interpretation of suspended particle dynamics. However, using appropriate geometry for the collectors, sediments traps may provide representative samples of suspended particles for analysis.

Controls of the chemical composition of particle populations in a macrotidal estuary (Tamar Estuary, U.K.)

Abstract The chemical composition of suspended particulate material (SPM) in the Tamar Estuary and its response to tidal sediment resuspension at the turbidity maximum have been investigated. The results discriminate four regions: (1) the outer estuary where spatial changes in the composition of SPM reflect the dispersal of estuarine particles modified by in situ generation of organic particles, (2) the middle estuary where the composition of SPM is mainly determined by hydrodynamic mixing although there is some evidence of in situ chemical reactivity, (3) the upper estuary where sharply distinct and temporally variable SPM compositions are centred at the turbidity maximum; these are largely the result of particle-selective accumulation, sedimentation and resuspension processes rather than of local particle-water chemical exchanges, and (4) the fluvial estuary where the SPM composition reflects the influx of riverine particles.

Organic carbon in suspended particulate material in the North Sea: Effect of mixing resuspended and background particles

Abstract Intensive sampling of suspended particles was undertaken at two stations in the North Sea on three separate occasions during 1989 as part of a seasonal study of the influence of wave and tidal disturbances on bed sediments and suspended particle composition. Measurements of carbon content indicated that the composition of the particle population changed non-linearly with concentration. This is interpreted as mixing of bed sediment of relatively uniform and low carbon content with more permanently suspended, carbon-rich particles of biological origin, present at low concentration. A mixing model is used to describe the change in composition of the suspended particle population with concentration as bed material is mobilised into the water column. The model clearly shows that a two-component mixing scheme can account for the observed decrease of particulate carbon concentration with increasing suspended particle load. In addition, the model suggests that resuspended material is mobilised selectively since the mobilised material is richer in carbon than bulk surface sediment. The existence of a surficial layer of detrital ‘fluff’, similar to that observed on the deep ocean floor following the spring bloom, is postulated.