Nitrous oxide and methane dynamics in a coral reef lagoon driven by pore water exchange: Insights from automated high‐frequency observations

Automated cavity ring down spectroscopy was used to make continuous measurements of dissolved methane, nitrous oxide, and carbon dioxide in a coral reef lagoon for 2 weeks (Heron Island, Great Barrier Reef). Radon ( 222 Rn) was used to trace the influence of tidally driven pore water exchange on greenhouse gas dynamics. Clear tidal variation was observed for CH 4 , which correlated to 222 Rn in lagoon waters. N 2 O correlated to 222 Rn during the day only, which appears to be a response to coupled nitrification‐denitrification in oxic sediments, fueled by nitrate derived from bird guano. The lagoon was a net source of CH 4 and N 2 O to the atmosphere and a sink for atmospheric CO 2 . The estimated pore water‐derived CH 4 and N 2 O fluxes were 3.2‐fold and 24.0‐fold greater than the fluxes to the atmosphere. Overall, pore water and/or groundwater exchange were the only important sources of CH 4 and major controls of N 2 O in the coral reef lagoon.