OPTICAL AND ACOUSTIC DETERMINATION OF THE PROPERTIES AND FLUX OF ESTUARINE SUSPENDED PARTICULATE MATTER UNDER WAVES AND TIDAL CURRENTS.

It is anticipated that climate change will impact water quality, biogeochemical cycling and flux, and ecosystem functioning and sustainability, in estuaries. All of these are significantly mediated by suspended particulate matter (SPM) since SPM determines turbidity (which constrains primary production), generates benthic fluff on the seabed (which modifies biogeochemical exchanges and benthic productivity), and carries important biogeochemical components (e.g. carbon, nitrogen, contaminants and pollutants). Quantification of the role of SPM is difficult because we have poor knowledge of the properties of SPM (i.e. particle size, density, settling velocity) and, in particular, the key processes of fine particle entrainment (i.e. pick-up from the bed) and sedimentation (i.e. settling to the bed) that depend on SPM properties. This is because most SPM in estuaries is in the form of flocs (i.e. aggregates of dead and living organic matter, cohesive inorganic matter, and water) that undergo aggregation and disaggregation, as well as settling and resuspension, on short time scales, particularly in response to the turbulence field. Flocs are not easily measured since they are dramatically modified by conventional sampling methods (they are easily ruptured and/or they may aggregate during sampling). We lack reliable and comprehensive information on key parameters such as pick-up functions and settling velocities of flocs, especially since floc properties change on a range of time scales: tidal (due to turbulence), lunar (the spring-neap cycle), and seasonal (due to storm resuspension and biological production of particulate matter). It is proposed to carry out an intensive field measurement campaign in the Dee estuary, in concert with an ongoing NERC-funded research programme (FORMOST), to win a unique dataset of SPM properties using a variety of optical and acoustic techniques. FORMOST is using state-of-the-art acoustic and optical technologies to characterise bed geometry, boundary layer dynamics, and suspended sediment characteristics in the near-bed (1 m above bed) region of the outer Dee estuary. FORMOST will provide new information on pick-up and transport rates of coarse and fine particles in relation to bed composition (a mixture of sand and mud) and geometry (rippled) in response to waves and currents. The research studentship will extend the study of SPM through the entire water column using cutting-edge technologies to determine floc sizes and concentrations (Laser In-Situ Scattering and Transmissometry (LISST), optical backscatter sensors (OBS), and transmissometers), and floc settling velocities (LISST-ST and settling velocity tubes, SVTs); the influence of phytoplankton on floc composition will be assessed using a Fast Repetition Rate Fluorometer (FRRF). Turbulence energy production through the water column will be determined using a seabed Acoustic Doppler Current Profiler (ADCP) using the proven variance method. The instrumentation, used in profiling and mooring deployments, will provide information on the interrelationships of floc size, density and settling velocity, and how these vary in response to changing SPM provenance and history. Measurements will be made over tidal and lunar cycles during 4 Prince Madog cruises in February and May (3 cruises are funded and Bangor University will provide 1 additional free cruise). The observations will be used to develop further an existing 1-D model, incorporating resuspension and aggregation, to investigate vertical flux of SPM in response to changing floc properties. The 1-D formulations will then be utilised in the POLCOMS area model for the Dee estuary to investigate how fine sediment fluxes in the estuary vary with changing SPM properties.