Tropical Kelp Forests: Why kelp in Mozambique is a lot like finding a penguin in the Maldives.

This kelp forest is made of a species thought to be Ecklonia radiata, an abundant intertidal species of kelp found around the East coast of South Africa. Here in Mozambique, this population is sitting at around 35 m depth.

I first fell in love with kelp when I moved to Cape Town from a small land-locked farm town in order to pursue my lifelong dreams of becoming a marine biologist. It’s here beneath these shifting shades and slimy kelp blades where my love for marine life, education and photography got sparked! 

While many avoid Cape Town's chilly, murky waters, these conditions allow kelp to thrive.Found along a significant portion of the world’s shallow, temperate coastlines they form rich underwater habitats. From Norway to New Zealand, these ecosystems can be found across 21 countries. 

So when I heard rumours of a kelp forest growing in the deep, warm waters of tropical Africa, it felt improbable. Consulting Dr Maggie Reddy, a seaweed researcher at the University of Cape Town, confirmed the existence of these deep-water kelp forests, which can thrive in the dimly lit depths below 30 metres. 

Left: The rural town of Zavora in Mozambique is known for being a hotspot for gentle cartilaginous giants: manta rays. Around September each year, they appear in the hundreds around a cleaning station known as "Witch's Hat," where they have been observed to display courtship behaviour. Right: The dolphins in Mozambique are almost as friendly as the locals, often coming closer to the dive boat to interact with humans.

Diving deeper

News in the ocean community travels faster than the Gulf Stream (the fastest current in the ocean for the non-ocean nerds reading this). When I heard rumours of a "deep-water kelp forest," I knew just the person to contact. Maggie Reddy, seaweed researcher from the University of Cape Town (UCT) and self-proclaimed kelp nerd confirmed the existence of these deep-water kelp forests. Contrary to common assumptions about kelp's preference for shallow, sunlit waters, these deep-water kelp forests, or mesophotic kelp fields, have adapted to thrive in the dimly lit depths below 30 meters

Around 20 years ago, scientists used data gathered from all over the world in computer simulations that predicted widespread occurrence of unexplored, deep water (30 to 200m depth) kelp habitats in the Tropics (Graham et al., 2007). To check these predictions they SCUBA dived the deep waters of one of the predicted tropical regions and found extensive kelp forests at eight localities. The emerging picture was of widespread kelp forest formations in both surface and deep waters along most continents. But Maggie’s observations were the first confirmed and documented reports of kelp off the coast of Mozambique (Madeira et. al., 2024).

Knowing that kelp usually grows in cold, nutrient-rich waters, finding it in the tropical waters of Mozambique is like finding a penguin in the Maldives, it just doesn't fit.  Excited to see the deep forests for myself, I joined Maggie and two of her students on an expedition to explore the kelp off tropical Southern Mozambique.

Is this a new species? 

As we descended into the depths, a shiver ran down my spine, not from the cold (the water was surprisingly warm), but from the anticipation of the unknown. At 35 metres, the kelp forest emerged from the dim light, a sprawling expanse of what looked like Ecklonia radiata. It was breathtaking.

As we hopped off the side of the boat and began our descent I was struck by an expanse of what appeared to be Ecklonia radiata thriving at a depth of 35 metres.

Collecting samples and capturing footage, I noticed that the blades looked and felt different from those growing in my home waters of Cape Town. Studies have shown that the shape of Ecklonia varies wildly according to environmental conditions underwater (Blain et al., 2019).  In deeper water where light levels are low, broader blades for example, might help to maximise light capture, much like shade-adapted trees growing bigger leaves (Poorter & Rozendaal, 2008). 

At 35 meters deep in Cape Town, another kelp species thrives: Ecklonia maxima. While it may appear similar to the kelp found in Mozambique at first glance, this species is actually quite different. It's larger than its tropical relatives and has a distinct look and feel.

Back on the boat, students Mila and Aiden processed the samples and Aiden noted subtle morphological differences that hinted at the possibility of a new species. His excitement was palpable as he described variations in stipe length and blade width, suggesting a distinct genetic lineage adapted to this unique environment.

During the expedition blades of Ecklonia radiata where collected to take back to the University of Cape Town for DNA analyses. The analyses would hopefully tell if this is a brand new species and maybe even how it found its way to Mozambique thousands of years ago.

What’s next

Maggie believes these deep-water kelp forests might be more widespread than is documented.  "They exist in a hidden world, beyond the reach of traditional kelp surveys," she explained. "We're just beginning to uncover their secrets." With the help of environmental DNA (eDNA) analysis that detects traces of kelp in water samples together with the samples collected underwater they are gaining a better understanding of kelp distribution and the genetic diversity of these deep-water populations.

Why should we care?

Discovering and understanding deep-water kelp forests is crucial for their conservation. We can't protect what we don't know exists. Fossil evidence indicates a history of kelp forests going back 32 million years, suggesting they played a role in the evolution of many foundational marine animals. While kelp forests may seem abundant, covering vast stretches of coastline, they are facing a silent crisis. Kelp forests provide habitat for countless species and store huge amounts of carbon. But these underwater jungles are threatened by climate change, overfishing, pollution and coastal development. Over the last 50 years their decline has outpaced growth; hidden beneath the surface we are losing them twice as fast as land forests, at a rate of 2 % every year (Wernberg, 2019) bringing a sense of urgency to discovering and understanding the deep-water kelp forests of the ocean.

Kelp is important for supporting biodiversity, the number of different plants and animals that occur in one area. One study done in Cape Town found one single kelp stipe to support over 1000 individual creatures.

Protecting these underwater treasures requires understanding the threats they face and taking action to ensure their survival. Kelp-killers range from coastal development to changes in the ecosystem structure. For example, in 2013 the disappearance of sea urchin-eating sea stars along the California coast allowed sea urchin populations to boom. By dining on the lower portions of kelp that attach the plants to the seabed the urchins left a trail of chomped-up kelp and a seabed barren of life (Wernberg, 2016). 

Sea urchin outbreaks have decimated kelp forests globally creating vast areas of seabed devoid of life. Norway has lost half of its underwater forests to urchin grazing. In Tasmania 95% of kelp is gone. Where a rich and diverse ecosystem used to thrive urchin barrens persist, with far reaching consequences for ocean health.

Are these tropical kelp populations remnants of a colder past, or thriving, depth-adapted ecosystems that are only just revealing themselves? The answers lie hidden in the depths, waiting to be uncovered.