Thesis
Hydrology, water quality, nitrogen and phosphorus transport, and mass budgets for the sub-tropical Richmond River catchment-estuary system, Australia
Southern Cross University, School of Resource Science and Management & centre for Plant Conservation Genetics
Doctor of Philosophy (PhD), Southern Cross University
1999
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Abstract
Dissolved and particulate, inorganic, and organic forms of nitrogen and phosphorus were studied from July 1994 to June 1996 in the sub-tropical Richmond River catchment and estuary, northern NSW, Australia. The aim was to gain a knowledge of nutrient processes in a linked catchment-estuarine system to better understand nutrient fluxes through a sub-tropical Australian landscape.
Consistent with relatively low population density and land use intensity, median total nitrogen and total phosphorus concentrations during stable flow conditions were low relative to other parts of the world (444 µg N L'' and 55 µg P L''). Nutrient concentrations reached a maximum during floods and decreased to a minimum within 2-3 months after a wet season rain event. Nutrient concentrations increased again as dilution of point-sources decreased in response to dry season river discharge. Inorganic nitrogen and phosphorus and particulate phosphorus forms dominated during the wet season, whereas organic nutrient forms dominated during the dry season. There were statistically significant relationships between geology and nutrient concentrations, pH, conductivity and turbidity; however, land use had the greatest influence on spatial water quality. Nutrient concentrations in rainfall also appeared to be correlated with human activity.
During stormflow, concentrations of all nutrient forms followed complex loop relationships with discharge and time attributed to the integration of floodwater from a spatially heterogeneous catchment. Annual exports were dominated by organic nitrogen (70-90%) and particulate phosphorus (33-69%) depending on the land use in each sub-catchment. Nutrient export was about 4 fold greater in the second year due to greater discharge. Flood discharge dominated the export of nutrients with >96% of export occurring in <6% of the time during the 1995/96 year. Annual nutrient exports varied across the catchment in response to runoff and land use. It was estimated that nutrient exports have increased in the Richmond River catchment by 2-3 times over the period of European settlement.
Input loadings varied between sub-catchments from 12 - 57 kg ha·' yr' for nitrogen and 0.25 - 6.6 kg ha·' yr' for phosphorus. Nitrogen fixation dominated the nitrogen inputs (47%), whereas fertiliser dominated the phosphorus inputs {66%). Produce exports accounted for 16% of the nitrogen and 32% of the phosphorus inputs to the catchment. Riverine exports accounted for 11% of the nitrogen inputs and 18% of the phosphorus inputs. When gaseous losses of nitrogen to the atmosphere where taken into account, nitrogen storage in soils or vegetation averaged 7.9 kg ha·' yr' and phosphorus storage was 1.4 kg ha·' yr' which is equivalent to 0.14% and 0.11% of the estimated nitrogen and phosphorus stores in the upper 30 cm of the soil profile.
During floods, the estuarine basin was completely flushed of brackish water and the majority of nutrient loads entering the estuary passed through the estuary and were deposited directly off shore. The nutrient load retained in the estuary during floods was inversely proportional to the flood magnitude and during the largest flood erosion occurred. During the dry season, the estuary was a net sink for nitrogen derived from the ocean whereas estuarine sediments remained a net source of phosphorus to the water column. Greater than 97% of the loads entering the estuary were from diffuse sources; loads from urban runoff, sewage, and precipitation directly entering the estuary were negligible. On an annual basis, less than 2.5% of the nitrogen and 5.4% of the phosphorus loads entering the estuary were stored in sediments. It was suggested that the flooding process is responsible for the maintenance of water quality on an annual basis and over the past 50 years. Potential nitrogen limitation also appears to be linked to the flood regime.
For the catchment-estuary system as a whole, 10% of the nitrogen loading and 18% of the phosphorus loading entering the Richmond system were deposited off shore during the two year study. Catchment storage accounted for 32% of the inputs of nitrogen and 50% of the inputs of phosphorus, whereas the estuary retained less the 1% of the nutrient inputs to the catchment-estuary system. In the Richmond system, it is the catchment, not the estuary, that buffers the adjacent Pacific Ocean from anthropogenic activities. This highlights the importance of catchment management in the process of nutrient transport from the land to the sea. This is likely to be typical of other sub-tropical and tropical Australian systems which have episodic flood discharge and contrasts many temperate systems of North America and Europe.
Details
- Title
- Hydrology, water quality, nitrogen and phosphorus transport, and mass budgets for the sub-tropical Richmond River catchment-estuary system, Australia
- Creators
- Lester James McKee
- Contributors
- Bradley D Eyre (Supervisor) - Southern Cross University
- Awarding Institution
- Southern Cross University; Doctor of Philosophy (PhD)
- Theses
- Doctor of Philosophy (PhD), Southern Cross University
- Publisher
- Southern Cross University, School of Resource Science and Management & centre for Plant Conservation Genetics
- Number of pages
- 1 volume (various pagings)
- Identifiers
- 991013303928102368
- Copyright
- © Lester J. McKee 1999
- Academic Unit
- Faculty of Science and Engineering
- Resource Type
- Thesis