The effect of residence time and hypoxia on nitrogen loading in the Yarra River Estuary, Australia

Keryn Roberts1, Michael Grace2, Perran Cook2

1 Water Studies Centre, Monash University, Wellington, Clayton, Victoria, 3800, Australia, [email protected]

2 Water Studies Centre, Monash University, Wellington, Clayton, Victoria, 3800, Australia

Abstract

Estuaries provide the final stage of nitrogen processing before its release into coastal waters. However, residence time, oxygen conditions and hydraulic mixing determine the nitrogen removal capacity of an estuary. Long residence times can lead to low oxygen conditions significantly affecting nutrient transformation pathways including, but not limited to, inhibition of nitrification and the competition for nitrate between denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The study site for this research, the Yarra River estuary, is a shallow (3 – 5m) salt wedge estuary prone to hypoxia resulting from the extended residence time of the bottom waters during low flow events. The estuary is one of the main freshwater nitrogen inputs into Port Phillip Bay (PPB), a nitrogen limited system. This research examined nitrogen processing over the period 2009 – 2011, extending over two summers of contrasting rainfall. Nitrogen loads were driven by rainfall in the catchment and ensuing freshwater inflow. NO3- was the main form of dissolved inorganic nitrogen (DIN) entering the system, being 87 ± 2 % of total DIN. Low oxygen conditions promoted nitrogen recycling within the system leading to an accumulation of NH4+ in the stratified bottom waters of the estuary. Estimates of nitrogen removal were low for the Yarra River estuary at < 5% of total DIN compared to 20 – 50 % for other estuarine systems. This study highlights the importance of understanding estuarine specific conditions (residence time and oxygen) on nitrogen dynamics in the management of nitrogen loads into receiving waters.