The Tidal Breath of a Greenhouse Gas
In the arid coastal zone of the Gulf of Kutch, the mangrove forests are dominated by a single, hardy species: Avicennia marina. These salt-tolerant trees are important for coastal protection and carbon storage, but their waterlogged soils also function as persistent sources of atmospheric methane (CH4). The release of this potent greenhouse gas is not constant. Instead, it pulses in a rhythm dictated by the daily tides. Studies in similar mangrove ecosystems show that methane flux from the soil is often significantly lower during high tide and increases as the tide recedes.
The process begins in the anoxic sediment—a dense, oxygen-free mud full with organic matter from decaying mangrove leaves and roots. Here, a group of microbes called methanogenic archaea break down organic compounds, producing methane as a metabolic byproduct. This gas then travels to the surface. One primary escape route is through the plant's own structures. Avicennia marina has specialized aerial roots called pneumatophores that stick out of the mud to absorb oxygen. These roots also are conduits, passively venting methane from the soil into the atmosphere. Measurements have recorded methane concentrations decreasing from the buried cable roots up through the emergent parts of the pneumatophores, confirming this transport pathway. The changing water pressure during the tidal cycle directly influences the rate of this release, causing the observed flux.