The 21 study sites along a latitudinal gradient along the Chinese coast. (a) Each study site (white circle) has 1–4 types of coastal habitats, including bare mudflats, saltmarshes, mangroves, and tidal creek banks. (b) Spatial distribution of DOM content ratio between crab burrows and sediment matrix at 21 study sites. Color gradients indicate the ratio of DOM content between crab burrows and the sediment matrix. The projection on the right shows the average variation ratio of different sites. More detailed location information is provided in Table S1.