| Summary: | The majority of terrestrially derived suspended particulate matter (SPM) is transported to the open ocean by rivers, therefore the river estuary transition zone (RETZ) represents a globally significant boundary separating the riverine and coastal regimes. The RETZ is comprised of the Tidally-Influence River (TIR), found above the limit of salt intrusion and the upper part of the estuary including the Estuarine Turbidity Maximum (ETM). The fate of SPM in the RETZ depends on its physical properties which are likely to be extremely variable in the RETZ which is characterised by large temporal and spatial gradients in hydrodynamic properties. Therefore, quantifying SPM properties in relation to physical forcings is key to determining the transfer flux of SPM from the catchment to the coastal ocean. The aim of this study is to interrogate the relationship between floc properties and the turbulence regime in the RETZ of a tidally dominated estuary over tidal, lunar and seasonal temporal scales. Flocs are fragile in nature and their properties fluctuate on short spatial and temporal scales; therefore in situ optical instruments (LISST-IOO and transmissometer) were deployed in the RETZ to obtain volume and mass concentrations of SPM. Turbulence measurements were determined via acoustic methods; ADCP and ADVs were deployed near to the bed to estimate TKE dissipation rates relating to the floc measurements. Data have been collected over five field campaigns, each included; spatial surveys characterising vertical profiles of SPM properties from the mouth of the estuary to the TIR, anchor station surveys measuring the temporal variations in SPM properties, bed-mounted mooring deployments in the RETZ and river surveys of the five main tributaries. Diurnal and semi-diurnal signals in floc properties in the RETZ were observed: resuspension occurred at peak tidal flows, usually on the flood tide; the maximum floc sizes corresponded with minimum effective densities and largely corresponded with high and low waters, as a result of particle flocculation during low turbulence conditions. Turbulence dissipation did not simply scale on tidal current velocities due to the additional contribution of wind stress and direction to the turbulence field. The Kolmogorov turbulence microscale correlated significantly with floc size during periods of marine conditions (i.e. the flood and early ebb tides) but showed a variable relationship during the late ebb when the RETZ was dominated by fluvial conditions and particles. This was most evident in the ETM where marine influence was greater compared to the TIR where it occurred only on larger tides. Thus floc size was related to the turbulence microscale but differences between flood and ebb relationships were probably due to different floc strengths of marine and terrestrial particles. During the lunar cycle, the variations observed on springs were repeated on neaps except that the flocculation signal occurred late in the flood rather than at high water. These tidal and lunar variations of particle properties in the RETZ were observed at all seasons.
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