TROPICAL KELP FORESTS

Text & Images by Danel Wentzel

I FIRST FELL IN LOVE WITH KELP when I moved to Cape Town from a small, landlocked farm town to pursue my lifelong dream of becoming a marine biologist. Beneath these shifting shades and slick kelp blades, my passion for marine life, education, and photography was ignited.

While many shy away from Cape Town's chilly, murky waters, these very conditions allow kelp to flourish. Found along extensive stretches of the world’s shallow, temperate coastlines, kelp forests create rich underwater habitats. From Norway to New Zealand, they extend across 21 countries.

So, when I heard whispers of a kelp forest thriving in the deep, warm waters of tropical Africa, it seemed improbable. Seeking clarity, I turned to Dr Maggie Reddy, a seaweed researcher at the University of Cape Town, who confirmed the existence of these deepwater kelp forests thriving in the dimly lit depths beyond 30 metres.

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.

The dolphins in Mozambique are almost as friendly as the locals, often coming closer to the dive boat to interact with humans.

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.

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.

DIVING DEEPER

News in the ocean community spreads faster than the Gulf Stream (for the nonocean nerds, that's the fastest current in the ocean). When I first caught wind of a "deepwater kelp forest," I knew exactly whom to contact Maggie Reddy, a seaweed researcher at UCT and a self-proclaimed kelp enthusiast, not only verified their existence but also shared fascinating insights.

Contrary to the assumption that kelp flourishes only in shallow, sunlit waters, these mesophotic kelp fields have evolved to thrive in low-light conditions below 30 metres. Around two decades ago, global computer simulations predicted widespread yet unexplored deep-water (30 to 200 metres) kelp habitats in the tropics (Graham et al., 2007) To test these predictions, scientists undertook SCUBA dives in one of the projected regions and their findings were astounding. They uncovered extensive kelp forests at eight different locations, painting a picture of widespread kelp formations in both surface and deep waters across multiple continents. However, Maggie’s observations marked the first confirmed and documented reports of kelp off the coast of Mozambique (Madeira et al., 2024).

Given that kelp typically thrives in cold, nutrient-rich waters, encountering it in the tropical waters of Mozambique is akin to stumbling upon a penguin in the Maldives it simply defies expectations Intrigued and eager to witness these deep-water forests firsthand, I joined Maggie and two of her students on an expedition to explore this remarkable phenomenon.

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 sheer anticipation. At 35 metres, the kelp forest emerged from the dim light, a sprawling expanse resembling Ecklonia radiata It was breathtaking.

Collecting samples and capturing footage, I noticed the blades looked and felt different from those found in my home waters of Cape Town. Research has shown that Ecklonia radiata exhibits considerable morphological variation depending on environmental conditions (Blain et al, 2019) In deeper waters, where light levels are minimal, broader blades may optimise light capture, much like shade-adapted trees developing larger leaves (Poorter & Rozendaal, 2008).

Back on the boat, students Mila and Aiden began processing the samples. As Aiden examined them, he pointed out subtle morphological differences suggesting the possibility of a new species. His excitement was palpable as he described variations in stipe length and blade width, hinting at a distinct genetic lineage adapted to this unique deep-water environment.

WHAT’S NEXT?

Maggie believes these deep-water kelp forests may be far more widespread than currently documented. “They exist in a hidden world, beyond the reach of traditional kelp surveys,” she explained. “We are only beginning to unravel their mysteries.”

With the help of environmental DNA (eDNA) analysis, which detects traces of kelp in water samples, combined with the physical specimens collected, researchers are gradually piecing together a clearer picture of kelp distribution and the genetic diversity of these deep-water populations.

WHY SHOULD WE CARE?

Discovering and understanding deep-water kelp forests is critical for their conservation after all, we cannot protect what we do not know exists. Fossil evidence suggests kelp forests have existed for over 32 million years, playing a crucial role in the evolution of many foundational marine species. Despite their abundance, covering vast stretches of coastline, kelp forests face an escalating crisis.

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.

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.

These underwater jungles provide essential habitats, sequester vast amounts of carbon, and support countless marine species. Yet, climate change, overfishing, pollution, and coastal development threaten their survival. Over the past 50 years, kelp decline has outpaced growth hidden beneath the waves, they are vanishing at twice the rate of terrestrial forests, disappearing at an alarming 2% per year (Wernberg, 2019).

Safeguarding these underwater treasures requires not only an understanding of the threats they face but also decisive action. Kelp forests are vulnerable to an array of pressures. For instance, in 2013, the sudden disappearance of sea urchin-eating sea stars along California’s coastline triggered an explosion in sea urchin populations. By grazing on the base of kelp plants, these urchins decimated entire forests, leaving behind vast, lifeless seascapes (Wernberg, 2016).

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