GHG concentration and emissions vary seasonally and alongside human behaviors in estuaries

Greenhouse gases (GHG) are always a hot topic. Here, a location perfectly displaying the interplay of seasonality and human-related activities on greenhouse gases is studied 

Most everyone is familiar with carbon dioxide (CO₂), but methane and nitrous oxide are considerable contributors to greenhouse gas (GHG) emissions and are more cost-effective to reduce. To further study the variation of these two greenhouse gases, which are in abundance both naturally and due to human-induced activity in estuaries, researchers sampled a main estuary in South China that has been heavily affected by anthropogenic factors for two years. Levels of methane (CH₄) and nitrous oxide (N₂O), along with some other metrics, were measured to uncover the impact seasonality, as it relates to natural and human-based activities, has on these gases in estuarine environments.

Researchers published their findings in Ocean-Land-Atmosphere Research on January 2025.

Estuaries are partially enclosed bodies of water where salty ocean and fresh river water mix, leaving an area rich for life in well-recognized and microscopic forms. Many of these microorganisms go through processes that produce CH₄ and N₂O, making it an area naturally well-stocked with these GHG sources. In addition to the nature of the estuary, the location of the Pearl River Estuary (PRE) is ripe for anthropogenic sources of these gases as it is main center for industry and large populations.

Over the two-year study which consisted of uninterrupted biweekly sampling, the data from the study saw a noticeable reduction of the amount of dissolved CH₄ and N₂O present in the PRE when anthropogenic activities decreased during the COVID-19 lockdown. The GDIVA is a monetary measurement of the production and activities of industry in the Guangdong province, and had the strongest positive correlation with CH₄ and N₂O. This relationship is a clear indicator that industrial processes can be a major factor in the concentrations of methane and nitrous oxide in the PRE and possibly other estuarine areas.

“Understanding the impact of these seasonally varying environmental factors on the variations of dissolved CH₄ and N₂O can be essential to provide an accurate budget for GHGs in the present and future,” said Zhengping Chen, lead author and researcher of the study.

However, not all of the variations found in GHG were strictly due to human-related factors. For example, an increase in river discharge in combination with reduced anthropogenic activities resulted in lowered concentrations of dissolved CH₄ and N₂O. Additionally, the higher wind speeds of winter also seemed to disproportionally result in a high water-air flux of GHG. The nutrients and biomass (such as phytoplankton and other microorganisms) present already in the PRE can act as a source for naturally occurring CH₄ and N₂O, leading to variations in the nitrogen and carbon cycles separate from human influences. 

This dynamicity calls attention to the need to understand the seasonal variations of estuaries on their own and in tandem with human-related emissions so seasonal patterns can be observed in an unobscured nature.

Conclusions drawn from the study indicate that the non-periodic peak concentrations observed over the two-year process are likely linked to anthropogenic activities. Though this is useful information, the work is not completely done.

“The next step is to investigate the biogeochemical processes responsible for these peak concentrations. Understanding these processes is crucial for assessing greenhouse gas emissions in estuarine areas, as peak concentrations can release significant amounts of greenhouse gases into the atmosphere in a short period,” said Chen.

What is learned from the data gained from this study can potentially be extrapolated to other subtropical monsoon climates for a more rounded understanding of the varying effects seasonality and anthropogenic climate change can have on environments already naturally rich with methane and nitrous oxide sources.

Zhengping Chen, Liuqian Yu, Qichun Yang and Qixing Ji of the Earth, Ocean and Atmospheric Sciences Thrust at the Hong Kong University of Science and Technology (Guangzhou) contributed to this research.

The National Natural Science Foundation of China, the Research Grants Council of the Hong Kong SAR, the Center for Ocean Research in Hong Kong and Macau (CORE) and the Hong Kong University of Science and Technology (Guangzhou) made this research possible.