Habitat-Builders: Mangroves and Seagrass

Today is the day I flew out!  My journey didn’t start as smoothly as it could have done – I’d misread my travel details and thought the flight was two hours later than it actually was (top travel tip – don’t do that).  I’m grateful to everyone who helped rush me through the airport and onto the plane.  Unsurprisingly, it was snowing at my stopover in Montreal and I left the airport to explore the frozen city.

Now, the second instalment on the species which help make the Bahamas so diverse; this time I’m looking at plants.  Specifically, two plant groups which have evolved to survive in the tough conditions in the marine world, mangroves and seagrass.  Linking back to my last blog post, these both help coral reefs to flourish as they create a complex environment which slows down water movement and allows debris carried in the water to settle.  This means the water is clearer – ideal for corals to grow.

The most common seagrass around the Bahamas is turtle grass, named for the green turtles which feed on it.  Besides a food source, it provides an important habitat – small fish and invertebrates live amongst it.  Turtlegrass is recognisable for its flat blades, whereas other species such as manatee grass have rounded blades.

Moving further inland, we reach the mangroves.  These form wetlands, a habitat which is rare and often in decline worldwide due to human development.  Here in the Bahamas however, they are the dominant ecosystem, vital to humans and wildlife alike.  Mangrove trees are sturdy, with extraordinary root systems keeping them firm in the soft sediment.  This protects the trees, and in turn the shore, from storms.  Their roots also trap soil which could otherwise be washed away by rainfall.

Where mangrove roots reach into the sea, a complex environment is created, used by many marine species as nursery grounds for their young.  Species such as barnacles and sponges grow on the roots, as the seafloor is often too soft to anchor to.  The canopy, meanwhile, is home to nesting birds and bats.

There are four species of mangrove tree found in the Bahamas, each suited to slightly different conditions.  The four species are often found in succession from closest to furthest to the waters edge.  Starting nearest the water, the red mangrove.  Its name comes from the reddish colour of its roots, which arc in and out of the water.  The leaves are oval and bright green, and the plant flowers year-round.  Sediment from the water and debris from land accumulate amongst the roots, eventually forming soil – this makes red mangrove a ‘land builder’.

After the Red mangrove comes the black mangrove, the second most water-loving species.  These trees are identifiable by their unusual ‘breathing roots’, which point up vertically out of the substrate.  Look out for the dark colouring of its bark, and its long, narrow leaves. All mangroves must be salt tolerant since they live at the edge of the sea, but black mangrove is the most extreme.  It can survive in ‘hypersaline ponds’, where seawater partially evaporates and leaves a more concentrated salt solution.

Just beyond the Black mangrove grows the White.  This has similar, vertical roots but often in smaller numbers.  A key identification tool is its leaves, far more rounded and a lighter green than those of the Black mangrove, with red or pink stems.  This mangrove is the least salt tolerant and hence not usually found directly on the coastline or around saline ponds.

Furthest inland is the Buttonwood or Grey Mangrove.  It prefers drier soils as it does not have protruding ‘breathing’ roots like the other species.  Buttonwoods may have green or grey leaves, the latter colour caused by tiny hairs growing from the leaves.  Unlike the three previous mangroves whose leaves grow in pairs along the stem, the Buttonwood has alternating leaves.

Threats to mangroves include harvesting for timber, and dredging or clearing of the wetlands so they can be built on or used to farm shrimp.  These rob the land of its natural storm protection, and fish stocks on nearby reefs often decline as their breeding habitat is lost.  Invasive species, those which have been transported to a habitat outside their natural range, are also problematic.  Foreign plants, such as the Casuarina, compete with the mangroves for space, water and nutrients, but do not provide any of the mangrove’s benefits.

For those participating in this fieldcourse, I hope this helps you know what you’re looking at.  For those following from home, maybe this will give you some idea of the place we will be in when we get started tomorrow!

eo271    January 2nd, 2018    Bahamas    , , , , ,

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