Tidal pumping controls dissolved organic matter properties and outwelling from mangrove groundwater to coastal water

Tidal pumping drives surface water-groundwater interactions in highly productive coastal wetlands. Groundwater flows can release sequestered soil carbon and modify dissolved organic matter (DOM) properties in mangroves. Here, we revealed the drivers of significant variations in DOM sources, spectroscopic characteristics, and composition in mangrove groundwater through a series of complementary techniques. The results showed that tidal pumping rather than geochemical factors played a more key role in controlling the DOM properties both in horizontal (i.e., the intertidal zone from the mangrove interior to the sea) and vertical (two sediment depths of 0.5 m and 2.0 m) directions. The contributions of terrestrial DOM gradually reduced toward the ocean, but the microbial contributions particularly for the recently produced autochthonous DOM increased. Considering the large carbon storage in mangroves, the role of tidal pumping on the variability in DOM properties should be further explored to better constrain the mangrove carbon sequestration globally.