Impacts of hydrological variability on material transfers through the River-Estuary Transition Zone

Estuaries are biologically productive, susceptible to climatic variability, and vulnerable to nutrient enrichment. These characteristics are reinforced by their ability to trap, filter, and recycle particulate matter. The processing of particulate matter supplied to estuaries is therefore important and dependent on the major sources of material to estuaries. In this respect, suspended particulate matter (SPM) and its inherent chemical characteristics are instrumental in controlling the reactivity, transport, and biological impact of substances from river catchments to the coastal zone. During transport to the coast, the physical properties of SPM (particle size, density, settling velocity) change on short time and length scales, especially when terrestrial particles encounter the steep hydrodynamic, chemical, and biological gradients that characterise the River-Estuarine Transition Zone (RETZ), and enter the estuary. This is because SPM is largely in the form of flocs: aggregates of inorganic matter, organic matter, and water. Flocs grow to a much larger size than their individual inorganic components. Aggregation occurs when fresh water meets salt water in the RETZ and is further mediated by biological effects. Flocs are created when individual particles collide due to weak turbulence, and are ruptured by strong turbulence. Large flocs sink rapidly, thus transferring material to the sediment bed. So, the fate of terrestrial SPM (i.e., whether it is retained by rivers, transferred to estuaries, or exported to the coastal ocean) is strongly influenced by floc properties, which, in turn, are dependent on their provenance and genesis. Since flocs are fragile and ruptured by sampling, they are difficult to study. High resolution measurements are needed since flocs change their properties on short time and length scales. This has been difficult to achieve, but optical techniques are now available which allow high resolution in situ measurement of floc properties (size, volume concentration), which, coupled with measurements of mass concentration and settling velocity, provide assessment of floc strength. This project investigates the transfer from river catchments to estuaries of SPM and its particulate components (carbon, nitrogen, heavy metals). These terrestrial components are important for water quality and ecosystem functioning and health in the coastal zone, and are likely to be severely altered by changes to the fluvial regime - increased frequency of major flood events - driven by climate change. The aims are to determine (i) The varying provenance of riverine SPM, which changes seasonally and during episodic flood events, when material is mobilised, previously deposited on aerially exposed floodplains and high banks. We will use multiple geochemical proxies in the form of the C:N ratio, stable carbon isotopes, and the lignin-derived monomer yields of organic matter, as well as inorganic geochemical tracers in the form of heavy metals. (ii) How floc properties and geochemical composition evolve as they pass through the RETZ and the estuary. (iii) How flocs and material transfers respond to marine forcings (tidal, storm) and fluvial forcings, especially short duration (episodic) events. High resolution observational datasets from instrumented sampling sites in the catchment, RETZ, and estuary will be used to validate a 1-D vertical exchange model, which will be embedded in a 3-D hydrodynamic model for scenario testing of impacts of episodic river floods and storms on material transfers through the river-estuary system. The field study area is the macrotidal Dyfi system (West Wales), where there is a demonstration project set up by the Centre for Catchment and Coastal Research, which will provide a logistical infrastructure in support of this project. The project will be carried out with the participation of the Environment Agency and CEFAS, who will be important end users of the outputs.