As you know, I"m about as huge a fan of mangroves as anyone you'll meet in the aquarium hobby. These are amazing plants, adaptable, hardy, and incredibly important to th eecosystems in which they are found. There are a few major factors which appear to limit the distribution of mangroves in Nature: Climate, salt water, tidal fluctuation and soil type.
There are more that 50 species of mangroves found throughout the world. Mangroves thrive in oxygen-deprived sediments which would certainly spell doom for most plants. They have evolved certain morphological and physiological responses, which allow them to survive in these harsh conditions.
Mangroves employ a sort of "internal ionic regulation." The Red Mangrove, Rhizophora mangle, (the most common one we encounter in the aquarium hobby) is known to botanists as a "salt excluder", which separates freshwater at the root surface by creating a type of non-metabolic "filtration system."
The process of transpiration (exhalation of water vapor) at the leaf surface creates negative pressure in the xylem (the vascular tissue in plants that conducts water and dissolved nutrients upward from the root ). This causes a type of "reverse osmosis" to occur at the root surface. The salt concentration of xylem sap in the Red Mangrove has been found to be about 1/70th of the salinity of surrounding seawater, but this is l0 times higher than in normal plants!
The Red Mangrove stores and disposes of excess salt in the leaves and fruit. (Which is one reason why we spray the leaves down regularly, which helps avoid salt buildup on their surfaces).
Yeah, mangroves are incredibly adaptable.
I've kept them for decades in all sorts of aquariums: Reefs, brackish, freshwater, and oh, yeah, blackwater.
Perhaps that last one took you by surprise? Maybe not. However, a lot of hobbyists aren't aware that some mangroves ARE found in freshwater habitats in the wild, particularly in parts of Southeast Asia.
The predominant species found in freshwater habitats is Barringtonia acutangula. It's definitely one you will not likely see in the aquarium hobby. You might not know that mangroves do not require saltwater to survive. In fact, most mangroves are capable of growing in freshwater habitats, although most do not in the wild because of competition from other plants. However, some species DO need salt to grow and complete their life cycle.
Here are mangroves growing in a soft, acidic "blackwater" situation in Southeast Asia.
Mangroves are "halophytes" (salt tolerant plants), which maintain sufficient fresh water inside their cells and tissues to maintain metabolic function against a higher osmotic pressure in the exterior root environment, which can vary between freshwater and up to three times seawater salt concentration!
Mangroves have evolved some remarkable survival techniques, including a specialized reproductive strategy, in which seeds don't go through a "dormant" phase, and are viviparous, germinating while still attached to the parent plant. These seedlings (known as "propagules") are buoyant, photosynthetically capable, and are often transported in tidal and ocean currents, sometimes over significant distances.
Mangrove trees are able to withstand remarkable tidal changes, from partially submerged to completely exposed, then back to partially submerged again, all in the course of a day!
Mangroves are part of a highly diverse ecosystem. The productivity of mangrove habitats is important for supporting food webs. The productivity of mangrove forests can be equivalent to the most productive terrestrial forests!
Mangroves are perfectly suited for their role as producers, and host enormous amounts of life within and among their structure. Because mangrove forests (sometimes called "mangals") are typically mud or peat-based systems, prop roots provide the hard substrate essential for settlement by many sessile organisms. This is also evident in the aquarium.
Mangrove ecosystems are dynamic, highly complex, not well-understood habitats. Mangrove forests have been described as detritus-based ecosystems- something I find both compelling and exciting as a hobbyist! This has had profound impact on my utilization of mangroves in natural aquariums.
Our representation of them in the aquarium, while certainly more limited than Nature in terms of function, can still provide a very interesting, productive habitat for a variety of fishes and other organisms, with unique benefits seldom embraced in the hobby.
Fungi and bacteria in brackish and saltwater mangrove ecosystems help facilitate the decomposition of mangrove material, just like in their pure freshwater counterparts. Interestingly, in scientific surveys, it's been determined that bacterial counts are generally higher on attached mangrove leaves than they are on freshly-fallen leaf litter.
This is fascinating to me, because ecologists feel that attached, undamaged mangrove leaves don't release much tannin, which, as we know might have some anti-bacterial properties. However, it's also been found that materials like humic acid, which are abundant in the mangroves, stimulate phytoplankton growth there.
The leaves of mangroves, as they break down, become subject to both leaching of the compounds in their tissues, as well as microbial breakdown. Compounds like potassium and carbohydrates are commonly leached quickly, followed by tannins. Fungi are the "first responders" to leaf drop in mangrove communities, followed by bacteria, which serve to break don't the leaves further.
Now, again, we'll often hear arguments that keeping a tree in an aquarium is kind of crazy. I admit, a full-grown, 30-foot tall mangrove tree is virtually impossible to keep in a home aquarium.
However, these trees grow incredibly slowly, reaching "houseplant-like" sizes after a year or more in captivity. And, with frequent pruning, you'll see that they can be maintained in almost a "bonsai-like" size, several feet tall, indefinitely- all the while putting down the extensive, intricate root systems that they are so famous for.
Keeping mangroves in the aquairum is about husbandry and perspective as much as anything else...And accepting the fact that the mangroves and the leaves which they drop are part of the ecology of an aquarium, and that they will behave as all terrestrial materials do when submerged:
They'll break down and decompose, imparting their internally bound-up compounds into the water.
And of course, that leads to so much more:
They'll form the basis of a surprisingly complex food chain, which includes bacterial biofilms, fungi, and minute crustaceans. Each one of these life forms supporting, to some extent, those above...including our fishes.
When you think of mangroves not so much as "hardscape props", but as dynamic biological components of a closed microcosm, it all makes a bit more sense.