Trapping and bypassing of suspended particulate matter, particulate nutrients and faecal indicator organisms in the river-estuary transition zone of a shallow macrotidal estuary

Abstract

Hydrodynamic controls of the transport of suspended particulate matter (SPM), particulate nutrients and faecal indicator organisms (FIOs) in the river-estuary transition zone (RETZ) of a shallow macrotidal estuary were studied on tidal and seasonal timescales. The RETZ was found to be a conduit for SPM rather than a zone of particle accumulation during spring tides, with complex size fractionation and biogeochemical exchanges of particulate nutrient/FIO compositions. The downstream RETZ was characterised by flood-dominant currents, but with ebb-dominant turbulence due to the suppression of flood tide turbulence by salinity stratification created by lateral convergence; this produced a net seaward mass transport of SPM. Without lateral convergence in the narrower upstream RETZ, flood-dominant currents and flood-dominant turbulence were experienced. Hence the RETZ exported SPM landwards from its upper end and seawards from its lower end — a process observed throughout the year during spring tides and low-to-mean river flows. During neap tides when SPM concentrations were reduced, the RETZ became a zone of minor particle accumulation as its lower end imported SPM landwards from the estuary and its upper end imported SPM seawards from the river. During a river flood event, net SPM flux was significantly increased and was seawards throughout the RETZ. SPM mass concentration and carbon, nitrogen, phosphorous, and FIO concentrations peaked due to local resuspension and advection of an ephemeral estuarine turbidity maximum (ETM). The ETM formed on the advancing flood tide due to entrainment of material from intertidal flats. Flocculation and settling occurred at high slack water. The ETM was reconstituted by entrainment on the ebb and was composed of larger flocs than on the flood. Particulate nutrients and FIOs were associated with flocs in the 10–200 μm range but not with smaller or larger flocs. SPM concentrations in the resuspension component and ETM exceeded microbial water quality standards, emphasising the need for monitoring practices that consider tidal dynamics. The results from this study showing periodic SPM export from, rather than prolonged accumulation in, the RETZ and the influence of particle size fractionation on biogeochemical fluxes in the RETZ, are likely to be transferable to many other embayment-type estuaries on macrotidal coasts.