Delivery, deposition and redistribution of fine sediments within macrotidal Fitzroy Estuary/Keppel Bay: Southern Great Barrier Reef, Australia

Abstract

The Fitzroy catchment in central Queensland, Australia is the largest catchment discharging to the Great Barrier Reef (GBR) Lagoon. Sediments and nutrients (both particulate and dissolved) originating in the catchment, are discharged from the Fitzroy River, into the Fitzroy Estuary (FE) and ultimately into Keppel Bay (KB). The flow in the river is characterised by summertime high-flow events typically of several weeks duration followed by virtually no flow for the remainder of the year. In this manuscript, we describe the hydrodynamics and fine-sediment dynamics in the coupled FE–KB system. Particular emphasis is placed on describing the temporal and spatial variability of these dynamics and on how the interactions between the temporal scales have important implications for system function. The greater bulk of the fresh water, sediments and nutrients are discharged into the FE in a relatively short time, but these materials are then redistributed throughout the FE–KB system and beyond by vigorous macrotidal currents during the rest of the year. Pronounced temporal variability in concentrations of suspended sediments also occurs at semi-diurnal and fortnightly timescales arising from the variation in the strength of tidal currents. The fluxes of fine sediments from the system to the lagoon of the Great Barrier Reef demonstrate pronounced variability at interannual, seasonal, fortnightly, and semi-diurnal timescales. Suspended sediments in the river discharge have very small sinking rates, but flocculation that occurs when they encounter salty water greatly increases their sinking rate and is an important agent for trapping these sediments near the estuary mouth. The large temporal variability of the hydrodynamics and fine-sediment dynamics of the FE–KB system also has important consequences for the biogeochemical function of the system and poses major challenges for the interpretation of moored, shipboard, and remote sensing measurements.