Hydro-climatological non-stationarity shifts patterns of nutrient delivery to an estuarine system

Abstract. The influence of hydro-climatological variability on catchment nutrient export was assessed by a retrospective analysis of rainfall, discharge, and total and dissolved nutrient loads for three sub-basins (Serpentine, Murray and Harvey) of the Peel–Harvey catchment, Western Australia. Both, temporal trends and their variability for different hydrological conditions (dry, normal or wet years) were analyzed from 1984 to 2011. Rainfall declined below median values for the study period over the last two decades and runoff decreased significantly in two of the three main rivers. Since Nitrogen (N) and Phosphorus (P) loads were strongly correlated with river discharge, nutrient exports decreased. However, when nutrient loads were flow-adjusted, increases in Total P (TP) and Total N (TN) were observed in the Serpentine and Murray rivers respectively, suggesting new sources of TP and TN and that the flow–export relationship is non-stationary. Dissolved Inorganic Phosphorus (DIP), showed a decreasing tendency in the last decade; but the trend in DIN loads is not clear and it appears to show a decreasing trend until 2004 and an increasing trend from 2004, accompanied with large inter-annual variability. The analysis of TP, TN, DIP and DIN in relation to dry and wet years, indicated that there is a significantly higher load in wet years for all three rivers, except for DIP in the Murray sub-catchment, explained by a higher proportion of soils with a higher Phosphorus Retention Index (PRI). Hydrological conditions, specific sub-catchment characteristics (e.g. soil type) and chemical properties of the nutrients altered the degree of nutrient partitioning (defined as dissolved inorganic to total nutrient concentration). For example, DIP increased to more than 50% of TP in wet years in Harvey and Serpentine but not in the Murray sub-catchment due to a higher PRI, while DIN behaved more randomly and did not show a link to discharge or the catchment soil type. We also found a mild association between nutrient partitioning and the rate of population growth which indicates that rapid change in population growth is accompanied by an increase in nutrient dissolved species. Changes in hydrological conditions between seasons did result in changes in the TN : TP and DIN : DIP ratio, but on an annual scale these ratios were not sensitive to whether a year was classified as wet or dry. The findings indicate that the quantity and nature of nutrient export varies in response to climate variability, which is superimposed on effects from changing land-use characteristics.