Tidal variability in methane and nitrous oxide emissions along a subtropical estuarine gradient

This study investigates the tidal variability in methane (CH₄) and nitrous oxide (N₂O) emissions along a gradient of the subtropical Brisbane River estuary. Sampling was conducted at the upper, middle and lower reaches over two tidal cycles in 2013 and 2014. Methane and N₂O emissions varied significantly over tidal cycles at all sites. Methane and N₂O emissions measured at all locations and in both campaigns varied substantially in time, with the maximum to minimum flux ratio in a cycle varying between 2.5 - 9 and 1.7–4.7 times, respectively. Methane emissions peaked just before or at slack tides. In comparison, no clear patterns were observed between the N₂O emissions and the tidal cycle despite there being large variations in N₂O emissions in some cases. Methane concentrations were elevated during low tides whereas N₂O concentrations showed no clear pattern over the tidal cycle. Surface water concentrations and tidal currents played important roles in CH₄ and N₂O emissions, but wind did not. Our findings show that measurements at a single point in time and site would result in significant errors in CH₄ and N₂O emission estimates. An adequate and careful sampling scheme is required to capture spatial and temporal variations of CH₄ and N₂O emissions and surface water concentrations which should cover at least one tidal cycle in different estuarine sections. © 2017 Elsevier Ltd