Blue Planet 2 | Episode 5 | Green Seas

Last week’s episode of Blue Planet II took us to the base of the marine food web – the oxygen-producing environments on which the rest of the ocean, and ultimately life on land, depends. It is still truly surreal to see so much of the theory from the last few years of our course being depicted on screen so beautifully.

There is plenty about this episode to talk about. Phytoplankton – an umbrella term for a menagerie of different photosynthesizing organisms –  prop up all other life in the ocean and provide 50% of oxygen for the entire Earth. Despite only covering 0.1% of the Earth’s surface, ‘blue forests’ (seagrass meadows, kelp forests, salt marshes and mangroves) capture about a third of carbon dioxide produced since the Industrial Revolution. Stacey Felgate’s excellent post talked a great deal about ‘blue carbon’ and wetland decline’s consequences for global climate change. Elin correctly predicted sea otter trophic cascades making an appearance – there are fascinating other examples of this from Yellowstone Park to the extinction of giant Ice Age animals. The scenes showing octopus and cephalopod ingenuity could warrant several extensive essays on some of their incredible capabilities, and equally some of the challenges with defining ‘intelligence’ in order to study animal cognition.

A patch of mangrove trees on a tropical beach.
Coastal mangroves on a beach in Pulau Gaya, Sabah, Malaysia. Original photo.

I’ll talk specifically about mangrove forests, a tropical coastal habitat characterised by marine adapted trees. Mangroves are an extremely interesting, and extreme, habitat. They have to endure the dramatic changes in salinity and temperature that characterise the intertidal zone. To cope with living in salty water, the mangrove trees have had to evolve to excrete salt from their leaves or by depositing it in roots or bark. These trees are also considered ‘viviparous’ – meaning they give birth to live young (it sounds strange, but this is the correct term!) – as young trees fall straight out of the adult tree and stick straight into the sand or mud like daggers. These ‘baby trees’ are called propagules, and in other cases they may float for weeks across the ocean. Mangroves only cover 0.1% of the Earth’s surface, but account for around 14% of total terrestrial carbon input to the ocean. They provide a link between the ocean and the land, which an extensive menagerie of different species utilise and have adapted to.

Tropical lagoon surrounded by mangroves, Bermuda.
Some marine biology students taking a break in a mangrove lagoon on our field trip to Bermuda. Original photograph.

As well as being home to many species of juvenile fish, they also provide shelter and resources for dolphins, manatees and dugongs, hundreds of species of birds, and even monkeys. Borneo’s distinguished proboscis monkey is a mangrove specialist. Biodiversity value aside, charismatic animals attract tourists and fish nurseries promote the availability of fish for consumption, particularly important when the majority of people around them rely on fish for their primary source of protein. The tree roots also stabilise the environment, making it easier for other species to live in. The role of mangroves in storm and tsunami protection has provided more incentives to protect them, particularly as extreme storms are becoming more frequent with climate change.

A monkey chomping down on a crab below some mangrove trees in Malaysia.
Mangroves provide habitat for a huge range of species, from sharks to macaques. Original photograph.
Only 6.9% of mangrove forests are protected, and anywhere between 35-86% of mangroves were lost in the last 25 years. Declining at a faster rate than tropical rainforests and coral reefs, 100% of mangroves are estimated to be lost in the next 100 years if current trends continue. Coastal development for infrastructure and shrimp farming in Southeast Asia have meant there is an average of 1% deforestation of total area every year– as high as 2-3% in some areas. 60% of Thailand’s mangroves have been cleared, some of it to make way for 1200 square kilometres of shrimp farms which have since been abandoned due to unsustainable practices. Sustainable farmed shrimp does exist, but Canadian writer Taras Grescoe puts it succinctly in ‘Bottomfeeder’:

The simple fact is, if you are eating cheap shrimp today, it almost certainly comes from a turbid, pesticide-and-antibiotic filled, virus-ridden pond in the tropical clines of one of the world’s poorest countries

But not all is hopeless – mangrove restoration projects exist all over the world, and are reasonably successful. More robust protection is needed worldwide, and this starts with awareness, which Blue Planet II is doing superbly, and will continue to into the future. And, of course, think twice next time you buy cheap shrimp.

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