Biophysical Controls of Ecosystem‐Scale Methane Fluxes From a Subtropical Estuarine Mangrove: Multiscale, Nonlinearity, Asynchrony and Causality

Abstract

Mangroves are complex ecosystems subject to periodic and irregular changes in meteorology, tides, water quality and biological factors. Methane dynamics in mangroves and their dependence on biogeochemical controls across scales are poorly characterized using traditional parametric statistics. We measured the ecosystem‐scale methane flux (FCH4) using an eddy covariance system in a subtropical estuarine mangrove for two years. A combination of nonparametric statistical approaches, including singular spectrum analysis and information theory, was used to isolate multiscale FCH4 variations and explore their variability and dominant controls at different time scales. FCH4 exhibited the largest variability at the diel scale (57%), followed by seasonal (25%) and multiday (18%) scales. Mutual information metrics suggested that variation in FCH4 was dominantly coupled to plant activities through synchronous processes at the diel scale. Moreover, fluctuations in atmospheric turbulence and soil temperature dominated multiday variation in FCH4 through asynchronous processes. Seasonally, soil temperature and water salinity were dominant variables leading to changes in FCH4 with a time lag of 12 and 17 days, respectively. Weak links between FCH4 and tidal heights were found across all the studied time scales. Transfer entropy metrics indicated that a super typhoon event substantially weakened causal links between biophysical variables and FCH4 by causing severe defoliation and disturbing the soil microbial community. The typhoon also temporarily decoupled the causal link between FCH4 and temperature. These findings call for the inclusion of the properties of scale emergence, nonlinearity, asynchrony and causality for comprehensively understanding and accurately predicting the interactions between mangrove FCH4 and its biophysical controls.

Global Warming Potentials of CO2 Uptake, CH4 Emissions, and Albedo Changes in a Restored Mangrove Ecosystem

“…The high salinity in mangrove wetlands also inhibited ecosystem respiration by influencing the decomposition of soil organic matter, thereby modulating microbial‐mediated heterotrophic respiration (J. G. Liu & Lai, 2019). Furthermore, salinity is a crucial factor influencing CH 4 emissions from a biophysical perspective (J. Liu et al., 2020, 2022). The salinity has an impact on the sulfate levels in mangrove ecosystems, leading to an increase in redox potential and inhibition of CH 4 production.…”

“…Furthermore, salinity is a crucial factor influencing CH 4 emissions from a biophysical perspective (J. Liu et al, 2020Liu et al, , 2022. The salinity has an impact on the sulfate levels in mangrove ecosystems, leading to an increase in redox potential and inhibition of CH 4 production.…”

Greater trend and interannual variability in productivity of mangroves compared to terrestrial forests

“…Speci cally, we systematically reviewed the peer-reviewed publications between 2002 and 2023 reporting mangrove ux data using a Web of Science keyword search with the following terms: TOPIC: (mangrove AND Flux AND (GPP OR NEE)). This returned 24 related studies, of which three mangrove ux data are publicly available, located in the Everglades National Park in USA 71 , Yunxiao in mainland China 60,72 , and Mai Po in Hong Kong 73 (Supplementary Table 1). These sites provide half-hour GPP or NEE (Net Ecosystem Exchange), and the latter (NEE) was partitioned into GPP and ecosystem respiration following ref.…”

Asynchronous Methane and Carbon Dioxide Fluxes Drive Temporal Variability of Mangrove Blue Carbon Sequestration

“…For example, there might be an underestimate in chamber‐based measurements of mangrove CH 4 emission without covering plant‐mediated pathways (e.g., stems and pneumatophores), which could be a considerable component of ecosystem CH 4 emission (Jeffrey et al., 2019; Zhang et al., 2022). On the other hand, the temporal varying patterns of mangrove CH 4 flux are far from clear, although few studies indicated the flux varied substantially at both short (hours to days) and long (months to years) time scales (Liu et al., 2022; Zhu, Sun, & Qin, 2021). In comparison with tidal marshes in which CH 4 flux is temperature‐dominated and varies seasonally (Al‐Haj & Fulweiler, 2020), the seasonal pattern of CH 4 flux in mangroves is less clear since the seasonality might be more regulated by salinity‐related factors (Zhu, Sun, & Qin, 2021).…”