Why Mangroves must be protected

Mangrove forests provide coastal protection and habitat for many species. Dr Anjani Ganase takes us into an ecosystem that is as valuable as coral reefs, especially to small islands like Tobago.

Mangroves are often depicted as uninviting and unfriendly environments, not pleasing to the eye, a little bit smelly and a haven for mosquitos and sand flies. Yet within this narrow piece of real estate between terrestrial forests and marine ecosystems, mangrove forests are lush with marine, estuarine and terrestrial life and, when protected and managed well, rich in biodiversity. 

Mangrove forest, Petit Trou, Tobago. Photo by Anjani Ganase

Mangrove forests are inshore coastal communities, where they can occur in salt, estuarine or fresh water conditions. They prefer calm waters that allow the mangroves to effectively root themselves into the substrate of the water column without being swept away. Even though mangroves may prefer freshwater conditions - they occur in areas with heavy rainfall - they actually thrive in saltwater environments as they tolerate higher salinities better than the other plants. Their dominance along these marine coastlines is a result of being better competitors, capable of tolerating the salty marine conditions. Where mangroves choose to settle and grow reminds me of the story of the Goldilocks and the Three Bears - they are picky about soil conditions, not too salty or too immersed!  Further inland, where the tidal water is intermittent, may be too salty because of higher rates of evaporation when the soil is exposed at low tides, especially in the tropics. The tides also determine the lower limit where mangroves attach to substrate and grow out of the water column; attachment is difficult in deeper waters. The signature mangrove “prop” roots need to branch out high enough above the water to keep the roots aerated. There is that sweet spot – not too hot and salty, and not too wet, deep and unstable – where mangroves thrive. Inland to the mangroves, the saltpans and salt marshes of the tropics are harsher environments with little framework and lower biodiversity owing to more extreme conditions.  

Mangrove stands, Karimunjawa Islands, Indonesia. Photo by Anjani Ganase

Mangrove forests filter out upstream particulates, nutrients and pollutants through the dampening effects of their entangled root systems, and limit the outflow to adjacent seagrass communities and coral reefs. Like coral reefs and forests, mangroves provide framework to dependent communities, where the structure of their roots, shoots and leaves form unique surfaces to be utilised by other organisms. On mangrove trees and roots, crabs and barnacles are common; below the water column are small spaces that provide refuge to juvenile fish, shrimp and crustaceans. Above the water, the branches and leaves are home to resident and migratory birds, insects and small mammals. Any devastation of mangrove forests results in significant loss in biodiversity and fragments the life cycle of marine organisms.

Climate Change: Sea Level Rise

The most recognisable impact of climate change on mangrove forests is from sea level rise. In some areas, the rate at which sea level is rising (because of warmer conditions resulting in melting ice caps and expanding waters) may be too quick for mangroves to shift their coastal ranges inland to avoid inundation. In locations where coastal development, roads and buildings abut the mangrove forest, the natural inland shift may not be possible; this will reduce the size and consequently functionality of the mangrove forests. The loss of the mangrove forests is likely to increase coastal erosion making the developments and communities vulnerable to salt water intrusion. Think about southwest Tobago and the communities located behind the mangrove forests of Bon Accord lagoon. Think about the buffer that the Nariva wetlands provide to the eroding east coast of Trinidad.

Mangrove prop root hover in the water column at Petit Trou, Tobago. Photo by Anjani Ganase

Climate Change: Extreme Weather Events

In the Australian summer of 2015- 2016, coral communities in the north and central sections of the Great Barrier Reef were bleaching severely, resulting in mass die offs over thousand kilometre stretches; part of the third global bleaching event. At the same time, west of the Great Barrier Reef, a similar stress event resulted in the die off of over a thousand kilometres of mangrove forest along the northern coastline of Australia in the Gulf of Carpentaria (Duke et al 2017). The die off of mangroves amounted to over 7400 hectares, which is equivalent to the death of trees spanning an area that is nearly twice the size of Tobago’s main ridge (Duke et al 2017). The most shocking thing about the die off was the suddenness and widespread nature of the event over one summer (Duke et al 2017). It is suggested that these coastal environments were subjected to hotter temperatures accompanied by severe drought conditions compared to previous years.  Reduced water flow into the Gulf and the higher temperatures made the soil too salty, unbearable for the mangroves. They survived along the estuaries relieved by the moving water that lowered the salinity of the soils (Duke et al 2017). It is anticipated that an impact of this scale may result in loss of many communities living on the mangroves. It is unlikely that full recovery of the dependent organisms and livelihoods would be seen within one lifetime or at all, considering the current trends of climate change. 

The disastrous impacts may not be applicable to all mangrove communities, as there may be a few locations that benefit from a warmer climate where they are able to expand into areas  previously less suitable. Here in Trinidad and Tobago, we have over forty locations of mangrove forests, including the Caroni, Nariva in Trinidad and Bon Accord Lagoon in Tobago, and all of which serve important ecological and economic functions (Juman and Ramsewak 2013). There is need to map and monitor local environments with our climate future in mind so that we may predict shifting eco-ranges of mangroves and other important ecosystems. It is also crucial to manage upstream impacts likely to influence the substrate and water conditions of the mangrove forests. This dry season is expected to be much more severe with longer and more severe heat waves. Let us keep a close eye on our mangroves. 

Take a look at what lies beneath the surface of mangrove forests, to get an idea of the health and ecology of this vital habitat. Photo by Anjani Ganase

Reference:

Duke NC, Kovacs JM, Griffiths AD, Preece L, Hill DJ, Van Oosterzee P, Mackenzie J, Morning HS, Burrows D. Large-scale dieback of mangroves in Australia’s Gulf of Carpentaria: a severe ecosystem response, coincidental with an unusually extreme weather event. Marine and Freshwater Research. 2017 Oct 18;68(10):1816-29.

Juman R, Ramsewak D. Status of mangrove forests in Trinidad and Tobago, West Indies. Caribbean journal of science. 2013 Jun;47(2–3):291-304.