Mangrove forest. USDA. Public Domain
(Click on the following link to listen to an audio version of this blog … Mangrove forest
Mangrove trees grow in coastal areas throughout the tropical and semitropical regions of the Earth. There are 80 different species of mangrove trees, all are intolerant of freezing temperatures and each has solved the classic, environmental problems facing mangrove trees (poorly oxygenated soils, high salt content of soil substrate and frequent emersion to tidal flooding) in their own ways. In the Continental United States there are three species of mangrove trees: red mangroves (Rhizophora mangle), black mangroves (Avicennia germinans) and white mangrove (Laguncularia racemosa). All three of these species are found in Florida.
Red mangrove. Photo by J. St. John. Wikimedia Commons
Each type of mangrove has a typical forest location relative to the surrounding ocean or estuarine waters. Red mangroves are usually found adjacent to the water: right on the shoreline or right alongside the tidal channel or stream. Growing this close to the water puts the trees under great potential stress from storms and violent wave actions. It also causes the roots of the red mangroves to be regularly submerged by the fluctuation of the tides. The distinctive aerial root systems of the red mangrove trees are designed at least in part to compensate for both of these environmental stresses. The high, arching roots are firmly anchored in the underlying sandy or silty substrate and grow interwoven with each other to hold whole lines of adjacent trees firmly together. This above-ground root reticulum also breaks up incoming waves or lashing storm waters and disperses their energy in order to minimize damage to the trees and also to the soil and detrital materials that they trap and hold. There are small pores at the apices of the root arches which can be opened when water levels fall sufficiently so that air can enter the tissues of the roots.
Black mangroves. Photo by Andrewtappert, Wikimedia Commons
Black mangroves grow further back in the mangrove forests slightly higher above sea level than the red mangroves. Black mangroves are the most abundant type of mangrove tree found in the United States. Their horizontal roots run under the surface of their surrounding solid, but still relatively wet, soil. To aerate these roots vertical structures called “knees” or “peg roots” (or more precisely “pneumatophores”) rise up from the roots and surround the trees with arrays of up-thrusted, woody spikes.
White mangrove. Photo by D. Valke, Wikimedia Commons
White mangroves are found even further inland in the mangrove forests and grow at still greater elevations. They are under less soil-oxygen stress that either the red or black mangrove trees and, consequently, have neither aerial roots or knees.
The mangrove ecosystem is tightly interconnected to the nearby seagrass and coral ecosystems of the seashore. While physically acting to stabilize and protect the shoreline, the mangroves also filter out the large amounts of sediments and nutrients that are constantly flowing in from terrestrial sources. Adding tons of their own leaves each year to this accumulating sediment/nutrient inflow-mass, the mangroves build up a wet, rich, organic “soil” layer which adds to the height and breadth of the mangrove forest.
The highly protected, nutrient-rich waters around the mangrove roots are breeding grounds for hundreds of species of fish, shrimp, crabs and shellfish. It is estimated the 75% of all commercially caught fish either spend part of their life cycle or depend on a prey species that spends parts of its life cycle in a mangrove forest.
Great egret. Photo by C. Sharp. Wikimedia Commons
The mangrove’s upper branches are vital nesting and roosting spots for a vast array of shorebirds and migratory bird species. Kingfishers, herons, egrets and more all rely on mangrove trees for nesting sites.
The detrital food web powered by the decomposition of the shed mangrove leaves and also a photosynthetic based food web powered by the many species of algae that reside in the mangrove ecosystem generate a huge net productivity for this biological community. Export of biomass and many of the nutrients liberated via decomposition helps to sustain the productivity of the surrounding sea grass, mudflat and coral reef ecosystems.
One of our goals for our Florida Keys visit was to get out into a mangrove forest and see these incredible trees and their dynamic ecosystem. Fortunately, Joe and Marlee acted on this wish and set up a naturalist-guided kayak tour in a nearby mangrove forest on Boot Key.
Boot Key Harbor. Photo by Direnzoa. Wikimedia Commons
We drove to Sombrero Beach on the Atlantic side of Marathon Key and met up with our kayak guide. Deborah and I took one of the tandem kayaks and Joe and Marlee took the other. Our guide took his paddle board (his sense of balance seemed impossibly well developed!) and led us out into a broad channel (“Sister Creek”) that ran between the developed edge of Marathon Key (lots of incredibly elegant beach houses and condos) and the dense, green mangrove forest of Boot Key.
Our guide warned us that going up the channel would be easier than coming back. The wind was steadily blowing on-shore and the tide was just starting to rise. So we rode along with winds pushing us and the tide lifting us several miles upstream and then turned left into a narrow gap in the wall of the mangroves and entered the still and quiet of the forest.
Deborah and I kayaking on Boot Key. Photo by M. Fischer
At first we paddled along in fairly broad, open waterways in between absolutely solid masses of red mangrove trees. Their roots made looping, basket-like encasements along their “ground” levels. Most of the trees were 15 to 20 feet tall and were densely covered with relatively short, leathery, deep green, “rhododendron-like” leaves. Hanging from the branches were numerous, elongated, tubular structures that were young, red mangrove trees that had germinated from fertilized flowers. These fully formed young trees fall off of the parental tree (we saw a number of them floating along in all of the streams and passageways we paddled through) and float until they catch on some soil or shallow sediment in calm water where they will root and begin to grow. Mangroves, then, are “viviparous” trees and give birth to “live” offspring! The mangrove propagules can survive for over a year in the salty or brackish water of the estuary and can, therefore, spread quite quickly and to some distances away from their parental trees.
Joe in kayak on Boot Key. Photo by M. Fischer
We took a turn into the mangrove stand and were instantly swallowed up into a narrow stream that ran through a long tunnel of interwoven mangrove branches. We could not feel the wind or see the sky. It was all mangrove leaves and roots and branches! It was also intensely quiet! We floated past green herons walking along on top of the aerial root looking for crabs or small fish and great egrets wading in the shallows near the root mass. We floated under dense patches of branches and saw tiny, mangrove crabs clinging to the scaly bark. We ducked and bobbed and tilted and tipped (close to, but not quite all the way over!) and slowly made our way through the mangrove tunnel.
In the mangrove tunnel. Photo by M. Fischer
At first turning the kayak and aiming it through the twists and turns of the stream was difficult, but very quickly we learned to anticipate turns and use the surrounding branches for hand-holds and pivot points. I leaned back to get under some of the more robust branches and Deborah typically leaned forward. We tried very hard not to lean to the sides unless we had cleared with each other to do a compensatory lean toward the other side. We crashed into masses of mangrove roots a few times and almost tipped the kayak once, but, mostly, it was a wonderful and physically active float through a verdant, green paradise of the swamp!
There is a space formed under the root masses and their captured leaves and debris. Many animals use these spaces for refuge and for carrying out ambush hunting forays. We looked into one space that often was the local lair of a bull shark, but, unfortunately, it was empty.
Cassiopea jellyfish. Photo by M. Fischer
We went in and out of the tree-tunnel streams and, occasionally paddled across broad expanses of shallow water called “lakes.” Most of the lakes had seagrass growing in them, and in a number of the seagrass areas there were bare spots or gaps in the grass that probably had been caused by dragging boat-motor propellers. Many of these bare spots were covered with inverted jellyfish called “Cassiopea jellyfish.”
The Cassiopea jellyfish (Cassiopea xamachano) is found in warm, shallow coastal waters throughout the Western Atlantic, the Gulf of Mexico and the Caribbean Sea. It is especially abundant in mangrove swamps, estuaries, inshore bays and shallow lagoons. The jelly lies upsidedown on the seabed exposing its algae rich tissues to incoming sunlight. The alage are mutualistic symbionats that generate most of the metabolic energy used by the jellyfish.
The jellyfish is also able to filer feed via its short tentacles and gather in small prey via the use of its mucus coated nematocycts (called “cassiosomes”). Handling the jellyfish (as in the above photo) does not trigger a harsh sting in the handler but, instead, sets up faint tingling to burning in the skin.
The jellyfish was named for the Cassiopea of the Greek myth whose excessive vanity caused the sea god Poseidon to fling her into the sky upside down. The constellation Cassiopea represents an upsidedown chair to which the vain queen is bound.
We paddled for about two hours and then headed back to Sombrero Beach. It was good we had a guide, because all of the turning and floating along in the winding mangrove streams had completely destroyed our senses of direction.
We paddled back into the main channel of Sister Creek and then had to fight against the still rising tide and the quickening on-shore wind. A storm was building up, too, and we got spit on by some cold rain. It took quite a while and a lot of calories of effort to get back to the beach. When we got there my legs were so wobbly that it took two very kind people who just happened to out fishing on the beach to get me up and out of the kayak. I had extremely shaky sea-legs and walked up through the deep, loose sand with great care.
We rinsed ourselves off at the shower heads on the beach. I pushed the wrong button and got a face-full of cold water instead of a foot rinse! We had to put our shoes back on to walk across the short stretch of parking lot surface. The crushed coral of the lot was full of razor sharp edges!
It felt good to be back in the car, our shoulders will be sore tomorrow from all of the paddling. What a wonderful trip!
Next week: palm trees and seagrass!