Shahadat Hossain

Transport and retention of nitrogen and phosphorus in the sub-tropical Richmond River estuary, Australia - a budget approach.

Nitrogen and phosphorus loads in the sub-tropicalRichmond River estuary were quantified and materialbudgets were developed over two years of contrastingfreshwater discharge. During both years >74% of thenitrogen and >84% of the phosphorus load enteredthe estuary during one month when flooding occurred inthe catchment. Due to larger flood magnitude, loadsduring the 1995/96 year were 3.3 and 2.5 times greaterthan during the 1994/95 year for nitrogen andphosphorus respectively. During floods the estuarinebasin was completely flushed of brackish water and themajority of the nutrient loads passed directly throughthe estuary. The nutrient load retained in the estuaryduring floods was inversely proportional to floodmagnitude. Annual budgets show that >97% of thenutrient load entering the estuary was from diffusecatchment sources; precipitation, urban runoff, andsewage were negligible. Less than 2.5% of thenitrogen and <5.4% of the phosphorus loads enteringthe estuary were retained in sediments. During dryseasons the estuary became a net sink for nitrogeninput from the ocean and the estuarine sedimentsremained a net source of phosphorus to the watercolumn and ocean. The process of flood scouring islikely to be the cleansing mechanism responsible formaintaining water quality both on an annual basis andover the last 50 years and may also be responsible forpotential nitrogen limitation. The sub-tropicalRichmond River estuary contrasts with the majority oftemperate systems of North America and Europe whichtypically have lower inter- and intra-annual nutrientload variability, longer and less variable flushingtimes, and greater nutrient retention.

Influence of climate,geology and humans on spatial and temporal nutrient geochemistry in the subtropical Richmond River catchment, Australia

Water quality was monitored on a spatial and temporal basis in the subtropical Richmond River catchment over two years. Nutrient concentrations varied seasonally in a complex manner with highest concentrations (maximum =3110 µg N L – 1 and 572 µg P L –1 ) associated with floods. However, median (444 µg N L – 1 and 55 µg P L – 1 ) concentrations were relatively low compared with other parts of the world. The forms of nitrogen and phosphorus in streams varied seasonally, with greater proportions of inorganic nitrogen and phosphorus during the wet season. Minimum nutrient concentrations were found 2—3 months after flood discharge. With the onset of the dry season, concentration increases were attributed to point sources and low river discharge. There were statistically significant relationships between geology and water quality and nutrient concentrations increased downstream and were significantly related to population density and dairy farming. In spite of varying geology and naturally higher phosphorus in soils and rocks in parts of the catchment, anthropogenic impacts had the greatest effects on water quality in the Richmond River catchment. Rainfall quality also appeared to be related both spatially and seasonally to human activity. Although the responses of the subtropical Richmond River catchment to changes in land use are similar to those of temperate systems of North America and Europe, the seasonal patterns appear to be more complex and perhaps typical of subtropical catchments dominated by agricultural land use.

Spatial and temporal variations of suspended sediment responses from the subtropical Richmond River catchment, NSW, Australia

Suspended sediment exports from the Richmond River catchment were examined during 2 hydrological years (1994–96). On a yearly basis, the Richmond River catchment produced 93p of the suspended sediment load, while sediment exports from the Bungawalbin Creek subcatchment always remained low due to its flat topography and extensive forest coverage. Suspended sediment hysteresis patterns exhibited a clockwise response for 2 steep and less forested subcatchments and an anti-clockwise response for relatively low gradient and more forested subcatchment during all flood events. Land use changes in the Richmond River subcatchments indicate a possible increase of suspended sediment load of about 6-fold from their pristine condition.

Dry season suspended sediment concentration and sedimentation in the Richmond River estuary, northern NSW, Australia

Dry season suspended sediment concentration and sedimentation in the Richmond River catchment were investigated during 2 hydrological years (1994–96). Longitudinal suspended sediment transects during low flow months showed that the Richmond River estuary remained well mixed and maintained <30 mg/L suspended sediment concentration without any visible turbidity maximum zone all along the estuary. During the entire dry season, there is very little exchange of suspended sediment between the upper and lower estuaries because of very small input from the upper catchment. The estuary receives net sediment input from the continental shelf during the dry months under normal tidal circulation, and marker horizon core samples confirmed that most of these imported sediments were deposited in the lower estuary; during the 2 dry seasons, lower estuary sedimentation rate varied about 0.84 ± 0.31 cm to 0.48 ± 0.3 cm. Flushing times of the Richmond River estuary show that all point and non-point source inputs of sediments and pollutants into the estuary can be flushed out during one dry season.