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.