Problem – extreme salinity around root zone
Mangroves prefer soils rich in clay – impermeable clay, but not too much of it! As one can understand, clays are mostly tiny particles, packed closely and tightly with each other. It makes water movement through rhizosphere(root zone) difficult which makes these soils poorly drained. Since water movement is not easy under such condition, Mangroves find it difficult to take up water for themselves? Well, that is a challenge!
The strategy to overcome the extreme:
The answer is convection current. Red mangroves create a convection current to pull water (and salt along with it)from soil to roots. But they simply cannot take salt as well because too much salt inside would kill them! To overcome that, they leave salt outside on root surface(salt exclusion) as they continue to seep in freshwater. Mangroves have special barrier formation that let Na+ and Cl- ions(figure below) from salts stay outside root membrane instead.
However, that leads to building up of a hypersaline condition next to mangroves roots which is bad for them as well! The saline condition is even more exaggerated by the low flow rates which is common for these impermeable soils. How that extra-salt is removed from their rootzone and role of crabs in that will be discussed in our future posts.
That is how my friend, mangroves, fixed on its position, keep manipulating living conditions (salt and water)around it and beyond, for itself and you and me and all!
Reference: McGowan, Kelly T., and Jonathan B. Martin. "Chemical composition of mangrove-generated brines in Bishop Harbor, Florida: Interactions with submarine groundwater discharge." Marine chemistry 104.1 (2007): 58-68.