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

This study investigates the tidal variability in methane (CH) and nitrous oxide (NO) 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 NO emissions varied significantly over tidal cycles at all sites. Methane and NO 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 NO emissions and the tidal cycle despite there being large variations in NO emissions in some cases. Methane concentrations were elevated during low tides whereas NO concentrations showed no clear pattern over the tidal cycle. Surface water concentrations and tidal currents played important roles in CH and NO emissions, but wind did not. Our findings show that measurements at a single point in time and site would result in significant errors in CH and NO emission estimates. An adequate and careful sampling scheme is required to capture spatial and temporal variations of CH and NO emissions and surface water concentrations which should cover at least one tidal cycle in different estuarine sections