AbstractThis study investigated the hypothesis that in a nutrient dominated catchment and estuary subject to extreme seasonal variation and large tidal inundations, physical processes play a substantial role in the nutrient status. This investigation also aimed to provide insight into the impact of high anthropogenic nutrient loads and the effect of runoff events, areas where limited research has been conducted in wet dry tropical, macrotidal systems. Buffalo Creek, Northern Territory, Australia, is a wet dry tropical, macrotidal system that has been subject to substantial anthropogenic nutrient inputs over a 36 year period. Levels of nutrients in the water column exceeded the Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand (ANZECC/ARMCANZ) guidelines under all seasonal and tidal conditions but high levels of nutrients were not reflected in the sediment. Buffalo Creek is well mixed with stratification only occurring during periods of minimal flow (neap tides).
The mixing plots revealed that different suites of nutrient processes occur under different conditions and that although biological transformations are the dominant mechanism for nitrogen on neap tides, physical processes play a key role in nutrient status during spring tides and wet seasons. The importance of physical processes was supported by high variations in turbidity and the large contribution of particulate nitrogen and phosphorus. The creek frequently experiences scouring by tide and/or flooding and the resuspension and transport of nutrients associated with particulate matter is a key factor in determining the status of nutrients in Buffalo Creek.
Interactions between sediment, porewater and overlying water were studied to elucidate key processes occurring in wet dry tropical, macrotidal systems. The first 8-10cm of the porewater profiles were characterised by low nutrient concentrations with the exception of nitrate, slightly acidic pH and oxic redox levels which are associated with resuspension by strong tidal currents, flood inundations during the wet season and bioturbation providing a pathway of diffusion for NH4 +-N and PO4 3-- P to overlying water. Another potential mechanism for the low nutrient concentrations is the uptake of nutrients by mangrove roots. Nitrification occurred at these depths as the sediment was aerated by mangrove roots, bioturbation and tidal currents supporting these two proposed mechanisms. Low phosphate levels are also attributed to adsorption to Fe and Al oxyhydroxides. Sequential extraction indicated that 80% of phosphorus was Fe and Al bound phosphate-P and polyphosphates and a statistically significant relationship between total phosphorus and total iron was observed in sediment cores. These results clearly show that at these depths physical processes not only provide a direct mechanism for nutrient release to the overlying water but also influence biological processes by aerating the sediments. Below 8- 10cm, NH4 +-N and PO4 3--P concentrations rapidly increased. The dominant process for nitrogen at depth was ammonification. Substantial increases in porewater phosphate corresponded to a decline in Eh, associated with the anaerobic conversion of Fe(III) to soluble Fe(II).
Experiments were conducted focusing on the influence of salinity and intense mixing on nutrient release from sediments. The mechanisms of nutrient release experienced when the sediment was intensely resuspended in seawater, representative of spring tide inundations, was an initial porewater exchange with the overlying water followed by competition between phosphate and seawater anions for exchange sites, ammonification and nitrification. The major processes occurring in sediment resuspended in oxic freshwater, representative of wet season flooding, included phosphate and ammonium release from porewater, ammonification, nitrification, continued slow release of phosphate for neutral or alkaline sediments and adsorption to iron oxyhydroxides for acidic sediments. The results indicate that mixing and salinity are key factors in the status of nutrients in wet dry tropical, macrotidal systems.
Lead-210 dating was conducted to provide a chronology of the sediment profile, to determine if urbanisation impacted the system and if this dating method is applicable to wet dry tropical, macrotidal systems. Dated sediment cores indicated that the sewage treatment plant and urbanisation have impacted on the creek. The sediment of the creek and floodplain contained a record of this impact despite surface mixing caused by macrotides and wet season inundations and bioturbation to a lesser extent. The results show that it is difficult, but not impossible, to establish a definitive date for sediments subjected to physical and biological mixing. The success of dating in these areas is dependent on careful site selection. Cores should be selected from sites that accumulate sediment regularly and are sheltered from disturbances. These results suggest that although it may not always be possible to achieve yearly resolution because of mixing it may still be possible to show substantial impacts on a decadal scale. Nutrients and PbIR were successfully used as chemostratigraphic markers in this system, despite the potential mobility of nutrients in sediments. These profiles also provide important information about mixing which is a key factor in nutrient status in wet dry tropical, macrotidal systems.
The results of this study clearly show that physical processes are of great importance, influencing nutrient status in wet dry tropical, macrotidal environments. The results of these studies have been used to produce a conceptual model for nutrient release in Buffalo Creek under different tidal and seasonal conditions.
|Date of Award
|David Parry (Supervisor)