Pond methane dynamics, from microbial communities to ecosystem budget, during summer in Alaska

Ponds play a larger role in the global freshwater methane (CH4) budget than predicted from surface area alone. To improve our understanding of pond CH4 dynamics, we measured summer CH4 production, concentrations, and emissions to the atmosphere in nine Alaskan wetland ponds along with potential physical, chemical, and biological regulators. Pond CH4 production (0.64, 0.086-1.3 mmol m(-2) d(-1); median, interquartile range), as assessed with slurry incubations, was positively related to water-column temperature and chlorophyll a (Chl a), negatively influenced by oxygen levels, and varied with microbial community structure. Average water-column CH4 concentrations (0.39, 0.21-0.87 mu mol L-1) were lower in deeper ponds and at higher oxygen levels, and as expected, they were correlated with diffusive emissions (0.055, 0.024-0.20 mmol m(-2) d(-1)) assessed with flux chambers. Based on a mass balance approach, 39-99% of CH4 produced in ponds was oxidized. Pond ebullition (3.7, 0.60-24 mmol m(-2) d(-1)) was higher and more variable than diffusive emissions. Additionally, pond ebullition rates were better correlated with production rates from the previous month. We also systematically compared the ratio of ebullition to diffusive CH4 emissions in our ponds and other northern lakes, which was negatively related to water depth (n = 71), but positively related to Chl a (n = 28). Our study sheds light on the factors that influence pond CH4 dynamics and demonstrates that pond ebullition is a significant CH4 source worthy of continued study.