Saturday, June 22, 2024
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The Wonder of Weed

Seaweed barely gets noticed by most divers; they are too busy looking out for the big toothy pelagics that make up their wish lists. But weed is wonderful and much more necessary to our future than the apex predators as it is vitally important to the health of the seas. It is home, food source, sanctuary and above all source of over half the oxygen on this world. The weed in the water is keeping us all alive. It is the alveoli in the ocean lungs of our planet. Often complained about by tourists wanting pristine beaches and ignored by scuba enthusiasts I am entranced by it and I feel it exhaling life in every swaying breath of the current.

Diving off the southern Yucatan last year where the sargassum mats washed onto the shores by the acre I heard of people cancelling their holidays to avoid the weed whilst under the mats the ocean teamed with life. Turtles browsed it like a buffet and a multitude of crabs hung from a single claw above the long drop.

Dr. Sylvia Earle – we all bow to Her Deepness, ocean advocate, marine botanist and all round defender of the seas – calls the Sargassum the “floating rainforest of the ocean” and has dedicated her academic career to seaweed. The Smithsonian houses her collection of more than 20,000 plant specimens and long before she was raising awareness of ocean issues and creating Hope Spots (one of which is the Sargasso sea – Mission Blue – Hope Spots) she was travelling the world, probing uncharted territories in search of weed and algae.

So don’t be sad about the sargassum. Don’t ignore the weed. As Dr Earle says:

“You’d have a hard time imagining plants so bizarre and absolutely magnificent. Algae could inspire poets and songwriters”

Why divers should be fascinated by weed

You wouldn’t walk in a forest and ignore the trees, you would walk the mountains without marvelling at the plants that hold the land together. The ocean is the same; the plants in the sunlit zone are ocean engineers and create the environments that we all love.

From Verdigellas peltata, a fleshy algae that calls to mind the flowing skirts of a flamenco dancer, to wildernesses of kelp that shelter entire ecosystems, both the weed itself and what we find in it are worth study and investigation. And on a practical level nothing improves our buoyancy more than hovering above weed, taking time to inspect it.

Ocean engineers

It is easy to focus on the big flashy fish and stunning coral formations but with a little information it doesn’t take long to gain respect for weed.

In coral seas there are two main kinds of flora – the microalgae that live within the coral and provide not only food for the browsing fish but colour the reef. Then there are the macroalgae with includes seaweed and can be either soft or hard – the soft is food, the hard is calcareous.

In more temperate climates the underwater landscape is made up of fields of seaweed and other larger macroalgae.

Both types work together to shape the seascape we float over.

  • Reef Builder: Red algae such as Hydrolithon and Lithophyllum produce limestone which cements together broken pieces of coral, holding the reef together, keeping its structural integrity intact when the storms come and providing a foundation for growth. Without this cement the corals would not be able to build up vertically.
  • Sandmaker: Crusty species of green algae like Halimeda create our sandy beaches. Whilst coral debris and the skeletons of marine invertebrates like molluscs and sea urchins are found in sand the biggest single contributors is broken up Halimeda.
  • Food Source: Algae, both fleshy and calciferous serve as food for vegetarian fish and invertebrates, forming the first link in the undersea food chain. Parrotfish, damselfish and surgeonfish along with a huge number of invertebrates rely on the weed and algae to provide their basic food and energy source.

Seaweed – cleaning seas and building economies

With more than half of the world’s populating living within 150km of sea and with the cost of treating waste being so high many emerging economies are discharging waste into the sea untreated. Weed thrives in this kind of nutrient rich waste – there are some suggestions that the recent increases in Sargassum in the Caribbean are as a result of nutrient-rich water washing down the Amazon basin from cleared rainforest. Algae blooms and weed influxes in tourist destinations are unwelcome but what if the weed was used to clean up the seas?

In a recent article for Blue Economy Nicholas Paul, Principal Research Fellow at James Cook University, Townsville, Australia (Blue Economy), made the case for using weed to clean our oceans:

“They [seaweeds] are essentially miniature solarpowered treatment plants whose products are valuable. Consider this: for every 100,000 tonnes of dried seaweed produced in Indonesia, this seaweed has removed from the coastal water:

500 tonnes of nitrogen;
50 tonnes of phosphorus; and,
15,000 tonnes of carbon (which equals more than 50,000 tonnes of CO2 sequestered)”

He puts forward the idea of creating floating wastewater treatment plants of seaweed that

“not only remove nutrients (the nitrogen and phosphorus) but also dealt with carbon capture and ocean acidification.”

Seaweeds can not only enhance the health of our seas but using its innate characteristics it can form the basis of sustainable development in vulnerable coastal economies.

There is a worldwide shortage in agar (the gel extracted from some seaweeds used in medical applications) and there are clear health benefits from seaweed fibre and minerals, which act to reverse diet-induced syndromes.

Then there is the very act of farming seaweed. As Nicholas Paul suggests:

“This all gets more interesting when the act of growing the seaweed takes on new meaning and value. One example is the push to showcase the seaweed industry to tourists in Indonesia. Visitors can meet farmers and tour the farm sites, increasing awareness of the industry and demonstrating its compatibility with other uses.”

The everyday life of the sea

So weed is fascinating and useful and economically it can change the lives of coastal populations in deprived areas which is all very worthy but what about for scuba divers? I think some of my best dives have been intrinsically linked to the weed in the sea.

I have seen the mangrove nurseries, where the inch long fry lurk bug-eyed in the green and the juvenile tarpon swagger like bullies. In the kelp forests a perfect spiral of a tiny whelk has caught my eye as it rides the ripple, clinging to the surface like a tiny jewel and in the sunlight shallow waters I have seen the sea grass, billowing in the tide like the endless fields of summer.

I remember the perfect diving days and they are always garlanded with weed. I remember a morning in the Aegean, we were exploring a cavern where a monk seal lived, so it was a Tuesday, we always did caverns on a Tuesday.

Whilst the dive group did the cavern I waited outside, nice and level in the water, rising and falling gradually as my buoyancy fluctuated with the air in my lungs. Above me the surface was dark blue, shaded from the hot autumn sun by the high cliffs at Adukale. Below me the rocks glowed pearl with the extravagant frills of Funnel weed (Padina gymnospora). Delicate curls, like pencil shavings clustered together into pastel bouquets. This dainty alga is found in shallow waters growing on dead coral heads and in patches on reefs. I remember I breathed out, sinking down in the water as my lungs emptied, falling towards the garden on the rocks. The air in my mouth was dry and clean.

I could see every ring growth in the weed, the fragile bands of time. The funnels swayed in the current and I was entranced by their shimmy, their wide open welcoming curls.

Amongst the weed the real life of the ocean is conducted. Here the little fish congregate; the ordinary, workaday, fish. Whole populations that fascinate going about their business. I watched a three inch long Rainbow Wrasse nosing amongst the funnel weed. Its platinum belly shone and its sleek bands in shades of charcoal and olive were offset by a snazzy dorsal stripe. It eyed me with an orange eye.

I was utterly content.

Karen Phillips
Karen Phillips
Karen Phillips is an award winning travel writer who now lives in Pembrokeshire after a decade abroad in Turkey and Mexico. She is a master diver and ocean nerd who will happily spend a week watching weed. Her website is and you can find her on Twitter at