Bioturbation frequency alters methane emissions from reservoir sediments

Abstract

Inland aquatic systems are major global contributors to the atmospheric carbon budget through greenhouse gas (GHG) emissions, although the amount and form of carbon released varies widely across and within systems. Bioturbation of aquatic sediments can impact biogeochemical conditions and physically release sediment-bound bubbles containing GHGs, but variation in the frequency of such disturbance may modify the rate and composition of resulting GHG emissions. We hypothesized that an intermediate bioturbation frequency would result in the greatest methane (CH4) releases due to mechanical release of trapped bubbles, while frequent disturbance would result in greater diffusive carbon dioxide (CO2) releases relative to CH4, due to increased aeration of the sediment. We tested this bioturbation frequency hypothesis using laboratory mesocosms containing homogenized reservoir sediment. We used mechanical disturbance to simulate bioturbation at 3, 7, 14, or 21-day intervals; a control treatment was undisturbed for the duration of the experiment. We measured GHG emission (ebullition and diffusion) rates. An intermediate frequency of disturbance (7 days) produced the highest total GHG emission rate, while the most frequent disturbance interval (3 days) and least frequent interval (0 days) reduced overall GHG emissions relative to weekly disturbance by 24% and 15%, respectively. These patterns were primarily driven by differences in CH4 ebullition. Contrary to our hypothesis, there was no relationship between disturbance frequency and diffusive CO2 emissions. For all disturbance treatments, the majority of ebullition occurred during disturbance events, suggesting mechanical release of entrapped bubbles is an important emission mechanism. The frequency of disturbance has variable effects on GHG emissions and may explain conflicting results in prior studies of bioturbation. Our study provides insight into bioturbation as a driver of within-system variation in GHG emissions and highlights that variable bioturbation frequency results in non-linear responses in CH4 emissions, a globally important GHG, from reservoir sediments.