CO2 emissions from a temperate drowned river valley estuary adjacent to an emerging megacity (Sydney Harbour)

The Sydney Harbour Estuary is a large drowned river valley adjacent to Sydney, a large urban metropolis on track to become a megacity; estimated to reach a population of 10 million by 2100. Monthly underway surveys of surface water pCO₂ were undertaken along the main channel and tributaries, from January to December 2013. pCO₂ showed substantial spatio-temporal variability in the narrow high residence time upper and mid sections of the estuary, with values reaching a maximum of 5650 μatm in the upper reaches and as low as 173 μatm in the mid estuary section, dominated by respiration and photosynthesis respectively. The large lower estuary displayed less variability in pCO₂with values ranging from 343 to 544 μatm controlled mainly by tidal pumping and temperature. Air-water CO₂ emissions reached a maximum of 181 mmol C m⁻² d⁻¹during spring in the eutrophic upper estuary. After a summer high rainfall event nutrient-stimulated biological pumping promoted a large uptake of CO₂ transitioning the Sydney Harbour Estuary into a CO₂ sink with a maximum uptake of rate of −10.6 mmol C m⁻² d⁻¹in the mid-section of the estuary. Annually the Sydney Harbour Estuary was heterotrophic and a weak source of CO₂ with an air-water emission rate of 1.2–5 mmol C m⁻² d⁻¹ (0.4–1.8 mol C m⁻² y⁻¹) resulting in a total carbon emission of around 930 tonnes per annum. CO₂ emissions (weighted m³ s⁻¹ of discharge per km² of estuary surface area) from Sydney Harbour were an order of magnitude lower than other temperate large tectonic deltas, lagoons and engineered systems of China, India, Taiwan and Europe but were similar to other natural drowned river valley systems in the USA. Discharge per unit area appears to be a good predictor of CO₂ emissions from estuaries of a similar climate and geomorphic class.