Mangroves - Walking Trees

Mangroves

‘Walking Trees’

Jonathan Lowrie

A mangrove is a type of tree that grows in tropical regions at river mouths, bays, coastal lagoons and islands.  They occur in many regions through out the world; some of the best-studied mangrove forests are in the Florida ecosystem. In the Florida Keys, they create a fringing network around most islands and grow at hundreds of shallow locations offshore.  They are one of Florida’s true native species.  They thrive in their salty environment because they are able to obtain freshwater from saltwater.  Some of the species do this by excreting salt through their leaves; others block absorption of salt at their roots.

Mangrove roots act as a physical traps to filter the water systems. They trap debris and silt, stabilizing the near shore environment, and clarifying adjacent waters, and facilitate photosynthesis in other marine plants.  They also provide an attachment substrate for various marine organisms.  Many of these attached organisms filter water through their bodies and, in turn trap and cycle nutrients. Sponges, barnacles, oysters, mussels, shrimps and oysters are all efficient filter feeders that attach to mangrove root systems.  The Florida ecosystem has an estimated 470,000 acres of mangrove forests, and they all contribute to the purification of the state’s water quality.  This ecosystem traps and cycles various organic nutrients, chemical elements, and acts as a nutrient sink for important nutrients. Mangroves shed and drop about 7.5 tons of leaf litter per acre per year.  The constantly shed leaves are quickly broken down by bacteria and fungi and released into the water, providing food for sealife.

I cannot overemphasize the relationship between mangroves and their associated wildlife.  Mangroves provide a secure and safe haven for young fishes, crustaceans, and molluscs.  They also provide food for many marine species like snapper, damselfish, tarpon, and shrimp.  Without a healthy mangrove system, the vitality and health of the sport and commercial fisheries would decline.  74% of the game fish and 90% of the commercially valuable sealife in Florida depends on the mangrove.

Most animals find shelter in the roots, or the complex branch structures of mangroves.  The upper branches serve, as rookeries for coastal birds, like the brown pelican.  The roots also offer habitat for mammals, amphibians, reptiles, countless unique plants, and other invertebrate life. This root structure that penetrates into the water provides substrate for a variety of bivalves, and macro-algaes to attach.  This dense coverage provides shelter for juvenile invertebrates and fish.

Worldwide, more than 50 species of mangroves exist.  Of these, only three are found in Florida waters.  The best known is the Florida Mangrove, Rhizophora mangle.  It’s characterized by aerial roots and concealed prop roots, which provide support for soft muds and stabilize sediments. It typically grows along the water’s edge, where tidal flushing is sporadic and the water is nutrient poor.  The red mangrove is easily identified by the tangled mass of reddish roots called prop roots.  These projections from the trunk have earned this mangrove the name ‘walking mangrove’.  This tree can easily reach 30 feet in nature.

The Black Mangrove, Avicennia germanans, usually occupies higher elevations than the Red Mangrove.  They are characterized by the presence of small pencil-like vertical root shots called pneumatephores.  These root shoots stand in dense arrays near the high tide line, enabling the tree to get oxygen from the atmosphere.  The underside of the leaf surface has a whitish residue, which is excreted salt.  This will remain unless rinsed by a passing thundershower.

The White Mangrove, Laguncularia racemosa usually occupies the highest elevations farther upland than the red or black mangroves.  It grows on elevated grounds above the high tide mark.  Unlike its counterparts, it has no visible aerial root system, as the root system resembles that of most terrestrial trees.  The leaves are thick and succulent, rounded at both ends, and appears uniform in color on both sides. 

Many threats abound to mangrove habitats.  Hurricanes can damage 100,00 acres in a few hours, as did Hurricane Donna in 1960. Even the recent Hurricane Georges that swept across Puerto Rico and the keys was responsible for mangrove habitat damage. However, all of the storm damage cannot equal the impact humans have had on these forests.  Shoreline development has replaced Mangroves with marinas, dredged channels, airports, seawalls and commercial and residential construction.  Over 55% of shallow water mangroves were lost in the upper keys in past 15 years.  Forty percent of the loss was from filling of the habitat to make way for new construction.  This staggering loss occurs not just in the keys, but all over the Florida coastline.  Other threats include illegal dumping, oil spills, agricultural run-off that contains herbicides and pesticides.  Freshwater and street water runoff has also altered the salinity in some habitats causing mangrove die-backs.

Many organizations are studying habitat loss of the mangroves.  Looking at aerial photos of the same habitats from the 1940’s and 1950’s, scientists can see how pronounced the habitat destruction is.  In Florida, state and local laws were enacted to protect the mangroves.  Local laws vary, but most in municipalities it’s illegal to take any rooted plant, or disturb the trees or associated wildlife in any way.  The penalty includes heavy fines and possible jail time.

Mangroves are even now being kept in home aquariums.  They certainly can make a lovely habitat tank, allowing a touch of realisms for the shoreline tidal flat, or mangrove forest.  With the uptake of nutrients, they will certainly contribute to the overall vitality of the captive ecosystem, but because of slow growth, an uptake, they are not ‘miracle natural filters’.  The most common species for home aquariums is the Red Mangrove. It’s available as a propagule, and ships well, in moist bags.

This species does well in most seawater tanks, given a deep substrate to plant it in, and plenty of light.  I have had success with normal fluorescent bulbs, but have found compact fluorescent bulbs or metal halides to work best.  Salinity is less critical, as long as rapid fluctuations do not occur.  One way to jump-start these ‘seeds’ is to soak them in a separate container of water containing a small quantity of miracle Grow or similar fertilizer.  Its important to note that you should never put the fertilizer in your main aquarium display.  These propagules should be firmly embedded in the sediment, so that the bottom 2 to 3 inches is covered.  If you have a healthy propagule, then you should see the beginning of growth in just a few weeks.  I have also had some that take as long as six months before noticeable growth shows- so patience is vital.  Its best not to disturb the root system once it has begun to grow.

Before any mangroves are added to the tank, a few considerations must be made.  They are coastal vegetation, and are not found on the reef, but as an associated habitat.  One could never find mangroves, and small polyped corals grouped together.  Planting one in a typical reef, would only cause problems for the tree or the corals.  Mangroves are relatively slow growers, yet seem to expand by leaps and bounds when confined in a small aquarium.  Considering the overall height of 45 feet, they do grow slowly, but they can get over three feet tall in an aquarium in less than 6 months with adequate lighting.  The size of aquarium, and how much room you have to offer the tree plays a critical role in keeping mangroves.

I have some mangroves in a lagoonal system, complete with sea grasses, and some coastal corals, like Sidastrea, and Condylactus anemones.  I also have a system utilizing mangroves from Indo-Pacific and have the roots structured to create a land mass, where mudskippers, archer fish, can call home.  The possibilities for little captive microcosms are nearly endless.  Its best to get the mangroves as propagules with just a few roots starting.  If the plant is more established, it commonly fails from system shock.

Mangroves are a vital habitat we should all strive to preserve.  They most certainly add beauty and habitat to our home aquariums, and open up many new avenues of aquascaping.  If you ever have a chance to visit the coastal regions of Florida, I highly recommend a visit to a mangrove forest, as the abundance of wildlife makes this habitat one of the most diverse anywhere on earth.

Seagrasses

Seagrasses

Jonathan Lowrie

Walk into a local fish store, and take a peek at the tanks.  You would see a variety of freshwater fish, plants, saltwater fish, corals, and inverts, supplies, etc.  But no saltwater plants.  You might find a bag of Caulerpa, most likely some excess from a local hobbyist who traded it in for a new fish. The freshwater hobby abounds with diversity in the plant arena.  In fact, veteran freshwater hobbyist consider a well-planted biome to be the saltless reef equivalent.  This seems to be true, as my planted freshwater tank is full of a diverse array of invertebrate life, as well as fish.  But for those of us with saltwater aquariums as well, don’t despair.  Plants too live in the ocean.

Seagrasses are the sole marine representatives of the angiospermae family of plants.  All seagrasses belong to order Helobnidae, and are in either family Potomogetonaceae, or family Hydrocharitaceae.  Currently, there are 58 species of seagrasses recognized.  The great land down-under, Australia, is home to over half, 30, of the species.  These species belong to only 12 genera.  Grasses that could tolerate the high salt environment and the rigors of ocean life have been around for many millions of years.  Fossil records indicate that early seagrasses were abundant in the Cretaceous period, and even before around the ancient Tethys Sea.  Geological history and archaeological history show that the habitat once occupied by seagrasses has been declining the past 50 million years. Rather than being early ancestors of terrestrial plants, seagrasses were thought be a migration back to the sea by terrestrial plants. 

Seagrasses spend the majority of their existence submerged, although they may be exposed to air with tidal changes.  Their flowers, when produced, are pollinated underwater.  They remain anchored to the substrate via a network of underground root/rhizomes.  Because they are photosynthetic, they tend to be found in shallow water, like coastal areas, salt marshes, estuaries, and in the tropics, associated with mangrove forests.

Seagrasses are unique plants, adapted to live entirely beneath the water.  However, they share many characteristics with their terrestrial cousins the grasses and flowering plants, or angiosperms.  As true vascular plants, they utilize the rhizome to obtain nutrients in the sediment, and use the blades and sunlight for photosynthesis.  Most of these grasses are located in soft, silty sediments, but some species like Phyllospadix do attach to rocks. This adaptation back to the saline environment involves some complex physiological adaptations and anatomical features to allow for a submerged lifestyle. 

As fully aquatic plants, they blades of the grasses must be buoyant to catch the suns light.  They have single layer of tissue (cholorphyllous) called the epidermis, below which is a thick colorless layer with large air canals running the length of the blade.  In the sea, food sources via light may be limited, because of silty conditions, so these plants utilize the rhizome, the large submerged root as a source of nutrient.  Like many plants- potatoes for example, seagrasses store starches in their rhizome. The roots also contain root hairs, which allow for absorption of nutrients from the substrate.  When they flower, the do so underwater.  And the pollination also occurs underwater.  With sexual reproduction, many seeds are produced and are spread via water currents.  Seagrasses may also reproduce asexually, using the rhizome to extend new blades out along the seafloor. 

As aquarists why should we even care about seagrass?  Simply because they form some of the most biologically productive habitats in the world.  They perform a vital function in the physical and biological relationships between plants and animals.  Seagrass meadows provide a nutrient source for fish, waterfowl and other wildlife.  The epiphytic communities living upon the blades of seagrasses provide a rich nutrient source for many other species, as well as a nursery ground for thousands of species. These nursery meadows provide protection from predators for juvenile fish, shrimp, crab and other animals.  They also serve as a major contributor of organic material for nutrient recycling in estuarine waters.  And dense crops of grasses help prevent soil erosion and stabilize sediment loss via their dense root system.

Widespread and dramatic changes to the terrestrial ecosystem often bring corresponding changes to the human populations which have to depend on this natural resource as a means of food and fuel.  In the past decades, increases in human populations and the closer pairing of man and the infringement on the ecosystem have impacted the balance of the ecosystem.  These changes are most noticed not in the loss of a few acres of seagrass, but in the change to the commercial fisheries community.  Because of the destruction of habitat, and the fact that many seagrasses now live only in protected areas, they have been tough to acquire for the hobby.

Some of the best seagrasses for the home aquarium are now available through limited channels.  Turtle grass, Thalassia testudinum is a grass that colonizes the deeper waters of many regions.  It can tolerate higher salinities, and is quite tolerant of salinity flux.  Eelgrass, Zostera marina is another coastal grass. Its blades are considerably thinner, and it appears as a much more wiry grass. Syringodium filiforme is yet another species that can make it into the hobby.  As for lighting, these all like high intensity light.  I have kept them all successfully in shallow tanks under 2 48-inch gro lux or vitalights.  I have found they do best with natural sunlight, or compact fluorescent lighting.  They have a constant growth cycle, and look healthy.

The most crucial aspect to success with seagrasses in the home aquarium has to do with transplantation.  These grasses utilize their roots for nourishment, and stability. The rhizomes are also covered with fragile root hairs, and if these are severely damaged or torn, the acclimatization of the grass to the aquarium can be rough.  As with all plants that utilize roots, seagrasses have root tips with an apical meristem.  This region contains some undifferentiated cells and a high concentration of hormones.  If at all possible, make sure you plant grasses with the apical portion of the rhizome intact.  This will facilitate new shoots to grow in your tank.  I have found that the transport of these grasses should be semi- wet.  If the rhizomes were collected with the natural sediments, then these should be packed in wet newspaper and transported that way.  This will also ‘seed’ the tank with the associated fauna.

Besides creating habitat and providing décor for the lagoon system, seagrasses also play a role in nutrient conversion.  In nature, seagrasses are able to take light energy and store it as sugars starches, and fibers many grazers take advantage of this source of nutrients.  Since they also uptake nutrients from the sediment, they can provide a form of nutrient export and conversion.  In a closed system, they grasses will also provide a food source to the tank animals.  And they will also uptake nutrients from their root structures.  As for significant nutrient uptake in a closed system, you will have to look elsewhere.  Much like mangroves, seagrasses do uptake primary nitrogenous wastes from the sediment and water, but not in the quantities needed to perform sole denitrification in an aquarium.  Of all the algaes that could accomplish this task only the fast growing turf algaes have the growth needed to uptake the vast quantities of nutrients produced in a typical reef.

Seagrasses are gaining popularity with home aquarists, and a few facilities have begun to cultivate them for sale.  Not only is this beneficial for the natural grass beds, but it allows for a grass that is already adapted to typical lighting in an aquarium, and the mixture of sands and gravels commonly used in today’s aquariums.  Of course, not all systems are candidates for seagrasses.  If you have a reef crest tank, with many Acropidae species, then you would not find any seagrasses associated with this habitat.  But if you have a more lagoonal, or reef trough type of habitat, with corals that require slower currents, then seagrasses would make an excellent addition to your captive eco-system.