THE MANGROVES
(Manglares in Spanish)

Mangroves & herons
Mangrove view near Telchac Puerto, Yucatan, photo by Johan Siebols

MANGROVES IN THE YUCATAN

The word "mangroves" refers to a special community of plants and animals occurring in tropical areas that are inundated permanently or occasionally with saltwater. You find them intermittently along all the Yucatan's coasts and surrounding islands. Few ecosystems are as rich in species and shear numbers of living things, and are so important to the broader ecological community as mangroves.

However, mangroves are also very fragile. They are vulnerable to hurricanes, human drainage programs and "development." In many tropical countries mangroves are being destroyed to make way for hotels and beach-front homes.

THE YUCATAN'S FOUR MANGROVE SPECIES

In the Yucatan Peninsula we think of four different species of shrubs or small trees as constituting our mangrove swamps. Often it's said that, in terms of water depth and salinity, the four species arrange themselves like this, with Red Mangrove on the left being in the deepest water, and Buttonwood on the right growing where often it's abovewater for long periods of time:

RED MANGROVE --> BLACK MANGROVE --> WHITE MANGROVE  --> BUTTONWOOD
In Spanish: mangle rojo--> mangle prieto --> mangle blanco   --> botoncillo

Here are the Yucatan's four mangrove species:

Red Mangrove, Rhizophora mangle

MANGROVES ANCHOR THE SOIL

Mangroves occupy an exceptionally vulnerable part of the coastal ecosystem: That part of the land between the low tide and high tide marks. Most of the time this land quietly bakes and simmers beneath the tropical sun, but when storms come -- especially hurricanes -- mangrove species hold the land together.

Without them the storm surge would reach far deeper inland. Without them, barrier islands might be completely scoured away.

They also maintain water quality and clarity, filter pollutants and build the land by trapping sediment that otherwise might wash into the sea.

MANGROVES PROTECT WILDLIFE

Red Mangrove aerial rootsThe mangroves' tangle of roots -- such as the typical, fairly impenetrable tangle of Red Mangrove aerial roots shown at the right -- provide protected nursery areas for shrimp and other crustaceans, mollusks, and fishes. Thus they are critical for the commercial and recreational fishing industries. Huge numbers of migratory birds occupy the mangroves.

The Yucatan's mangroves often provide safe sites for breeding colonies of such wading birds as egrets, herons. Also they are home to such interesting species as Clapper Rail, Rufous-necked Wood-rail, Mangrove Cuckoo, the kingfishers, and Mangrove Warbler.


from the July 24, 2011 Newsletter issued from  Mayan Beach Garden Inn 20 kms north of Mahahual, Quintana Roo, México
MANGROVES SLOWLY FILLING WITH WATER

With the rainy season's gradual arrival, slowly large parts of mangrove swamps that until now have been dry are slowly filling with water. You can see a tangle of head-high Red Mangrove, Rhizophora mangle, that a month ago stood upon dry, caked mud below:

water rising beneath Red Mangroves, Rhizophora mangle

After a good rain the water rises higher, as you'd expect. However, if you really pay attention over a period of time, gradually the feeling grows that sometimes the water's average depth increases faster than the rains would seem responsible for. Also, if several dry days come along, the water may stand longer than seems reasonable. The impression is that something in addition to local rains is contributing to the mangroves' water-level rise.

Many studies show that that's the case. Remember that the Yucatán Peninsula is a vast slab of limestone eaten through and through with caves which, if occurring below the water level, carry underground rivers. In the northern and central peninsula, there are no surface rivers or streams. During the rainy season, rainwater quickly seeps underground along fractures in the limestone, into sinkholes and caverns, and merges with the water table.

The Yucatán's subterranean streams flow outward toward the nearest sea. Since the peninsula pokes northward into the zone of aridness at ±30°N (which we've considered   here) in the Yucatán it's much rainier in the south than in the north. Thus the general flow of freshwater in the Yucatán's underground rivers is both from south to north, as well as outward toward the coasts.

In short, those big storms I've been mentioning that most afternoons form inland but usually stay away from us on the coast, plus the even larger, more intense storms forming farther south, especially in Guatemala, are contributing to the increasing water level in our mangroves. In our mangrove picture you can see that the water is reddish. I'm guessing that the redness is tannic acid leached from Red Mangrove leaves decomposing in the mud, through which much of the standing water has filtered upwardly after its long subterranean journey up from Guatemala.

By the way, the freshwater seeping into the mangroves nowadays creates a thin lens floating atop deeper saltwater. When the rainy season ends, not only will the mangroves' water level drop, but also saltwater will intrude into places where previously there was freshwater -- as in certain wells of homes here along the coast.

The freshwater/saltwater interaction is complex and sometimes unpredictable. You might enjoy looking over the paper "Freshwater Resources in the Yucatan Peninsula" freely available online, at http://www.nap.edu/openbook.php?record_id=11875&page=6.


from the October 23, 2011 Newsletter issued from  Mayan Beach Garden Inn 20 kms north of Mahahual, Quintana Roo, México
MANGROVE WATER SEEPING INTO THE OCEAN

amber-colored mangrove water seeping into clear ocean water

Above you see the beach here one afternoon this week at low tide something orange-brown seeped from the sand into the ocean's clear water. Apparently it was mangrove water, which is this color nowadays because of its high levels of tannic acid deriving from decaying mangrove stems and leaves.

If it is indeed mangrove water, it's flowing beneath the entire low sand ridge separating the sea from the mangroves. At this seeping-out location, the ridge is maybe 1000 feet across (300m). Several such seeps occur along the beach, but I've noticed them only lately. That makes sense because earlier the water level in the mangroves was lower. Now it's high, and when low tide occurs the mangrove's water level is higher than the sea, so naturally mangrove water would seek release in the ocean.

Marcia tells me that a bit offshore she's seen such stained water gushing up six inches or so above surrounding seawater, so apparently sometimes there are subterranean channels with vigorous flow. That would be less unexpected if we were on limestone where underground rivers often flow in cavern, but here we're on pure sand.


from the December 28, 2014 Newsletter issued from Río Lagartos, on the north-central coast of Yucatán, MÉXICO
ESTUARY WATER ENTERING THE SEA

In front of Río Lagartos, directly north of town across Ría Lagartos Estuary, a manmade canal cuts through the narrow finger of land separating the estuary from the Gulf of Mexico. At one edge of the canal's seaward outlet there's a seawall topped with a Jesus-on-the-Cross fisherman's chapel lighted at nights with solar power stored in a battery, as shown below:

mangrove water entering the sea

Beyond the chapel, notice how the Gulf's water changes color. The dark blue water on the left is issuing from the estuary during ebb tide -- when the water is going down.

The water's darkness is caused by orangish tannin pigments derived from decomposing organic matter in the mangrove area, and from organic matter itself suspended in the water. Mangrove trees are especially rich in tannins.

These tannin- and organic-matter-rich waters are profoundly important to offshore biological communities, especially the corals. As Lucaya Luckey-Bethany writes in the paper "Mangrove Forest Decline and its Effect on Coral Reefs," "The destruction of mangrove forests in the coastal wetlands is drastically decreasing the amount of natural tannins in the water and thus subjecting the nearby coral reefs to undue UV radiation. In effect, the reefs are being sunburned and eventually coral bleaching occurs." This paper is freely available online here.

Therefore, when fishermen exit the canal on their way to unknown fortunes on the high sea, they may tip their hats to Jesus on his Cross in his solar powered chapel, but it would also be appropriate to show their respect for the dark water below them, which assures the productivity of the fisheries they're going out to exploit.

Its water dark with organic matter and tannins -- not to mention zooplankton, phytoplankton and immature organisms that have used the mangroves as a nursery -- is the very lifeblood of the offshore ecosystems they depend on.