An evaluation of particulate phosphorus storage in an agricultural estuary
MetadataShow full item record
Knowledge of fine sediment delivery (both timing and loading) is fundamental to the assessment of non-point source pollution in estuarine environments. This study comprised three key components that led to the development of a fine sediment and particulate associated phosphorus budget in a typical agricultural estuary. Firstly, to explore catchment inputs, turbidity and flow were monitored continuously upstream of the freshwater/saline interface on the main stem channel of the south Devon River Avon, which drains a medium sized agricultural catchment (area 340 km2), in southwest UK. Thirty-five storms were studied in detail; and the hydrological and suspended sediment load response was observed to be highly variable. Suspended sediment concentrations (SSC) reached a maximum of 804 mg L-1 and sediment load varied from 3 to 227 t per hydrological event. Most sediment load was concentrated in winter months when competent flows occur frequently. Hydrological response was also variable in terms of lag, hydrograph shape and maximum discharge wherein the response to hydrological drivers was not consistent. Analysis of key storm parameters indicated that the hydrological response of the catchment was affected by the total amount of precipitation and antecedent rainfall history but the spatial pattern in rainfall across the catchment in relation to the spatial pattern of sediment sources was the key factor influencing total load. In the second component, examination of the sediment-associated phosphorus concentrations in the surface sediment in the Avon estuary was undertaken to evaluate spatial variation in concentration as influenced by the sediment storage dynamics of key geomorphological zones i.e. saltmarshes, intertidal flats and sandy shoals. Phosphorus concentrations ranged from 1524 to 68 mg kg-1 with higher concentrations found in saltmarsh. While there was no observed relationship between key sediment properties, particle size and total organic carbon within the different geomorphic units, a clear trend in particle size and particulate phosphorus concentration was observed longitudinally between mudflat zones linked to the sedimentation dynamics of the estuary. Furthermore, the relationship of particulate phosphorus concentration to organic matter content was modified by saltmarsh vegetation inputs to the sediment column. The final component of the work drew on evidence from a GIS and field-based survey to estimate (i) the total fine sediment and associated particulate phosphorus loading of the estuary and (ii), in conjunction with river flux data and literature evidence, the total fine sediment and PP storage and the annual sediment budget (inputs, storages and output) for the study estuary. The total amount of fine sediment stored in the estuary was ca. 99000 t which equated to 40 - 100 years of the annual sediment load of the river. Approximately 50% of all fine sediment that currently enters the estuary was estimated to be retained in storage supporting the important role of estuarine sediment sink zones in the attenuation of phosphorus. The total particulate phosphorus storage in estuary fine sediment was estimated to be 20 – 40 times the measured annual catchment particulate phosphorus input. Future changes in catchment sediment supply dynamics linked to catchment restoration programmes and soil conservation initiatives could destabilise estuarine sediment sinks and this has potentially important implications for future estuarine water quality. There is a need for further work on the potential bioavailability of estuarine sediment stored phosphorus.
The following license files are associated with this item: