Centuries ago, humans would’ve seen an abundance of kelp around a quarter of the world’s coastlines – an abundance of thick kelp fronds waving at the surface, harbouring myriad species. But kelp forests are disappearing at an alarming rate due to a warming ocean and various human impacts. In 2011, a marine heatwave devastated the kelp forests off Tasmania, while Northern California’s kelp followed suit in 2014 when the kelp canopy declined by more than 95%. Around the world, the reality for most other kelp forests doesn’t look much different.

The decline has stark consequences as kelp forests represent some of the most productive habitats on Earth. They are a major source of primary production in coastal zones of temperate and polar oceans. While they capture CO₂ more efficiently than rainforests on land, they also release oxygen, filter the water and protect the coastline against erosion and flooding by forming forest dense canopy, which serves as a natural barrier against swell. They provide a refuge and are effective nurseries for juveniles invertebrates and fish. The elevated fish density in kelp forests further attracts large piscivorous species such as predatory fish, seals, otters and many bird species allowing the ecosystem to diversify and thrive. Kelp forests are engineering species that promote ecological stability and, in particular, kelp forest alter light, water flow, sedimentation and the structure of the habitat itself. Compared to other groups of algae, their relatively low photosynthetic to biomass ratio constrains them to shallow and well-illuminated areas, usually between the surface and 25 metres of depth.

In Northern Norway, where the most common kelp species are part of the Laminaria genus including winged kelp (Alaria esculenta), sugar kelp (Saccharina latissima) and oarweed (Laminaria digitata), I learned that the region’s underwater forests are also an important food supply for humans and create a critical habitat for many harvested fish species such as the Atlantic cod, the haddock, the pollock, or the Atlantic wolffish. When trying to find out more about the productivity of these amazing ecosystems, I’ve stumbled across the term ‘trophic cascade effect’ over and over again.

Trophic cascades are powerful indirect interactions affecting community structure and abundance dynamics in aquatic and terrestrial environments worldwide. A top-down cascade effect will occur when predators are effective enough to reduce the abundance of their prey, thereby releasing the next trophic level from predation pressures. The removal of top predators or high trophic levels can alter the food web dynamics.

One of the most famous examples of marine trophic cascade occurred in the Northern Pacific Ocean where sea otters feed on sea urchins. In areas where sea otters have been hunted to extinction, sea urchin populations significantly increased causing kelp forests to drastically reduce. There is a similar story occurring here, in the North East Atlantic Ocean. Studies have linked overfishing practices and depletion of coastal predatory fish to large sea urchin grazing events. From the ’50s, unregulated coastal fisheries led to the depletion of many important predators of sea urchins such as the Atlantic wolffish and the Atlantic cod, to only name a few. The impact of removing such predators has long term effects since these marine fish take years to become sexually mature. Therefore, the removal can lead to a shift in the ecosystem.

Off Norway, the lack of predators that feed on sea urchins continues to have a devastating impact on the marine environment. Most fish species feed on sea urchins at all stages of their development. When these top predators are removed from the ecosystem, the population of sea urchins grows unchecked. With their number on the rise, sea urchins overgraze primary producers like species of kelp and algae. In many places along Northern Norway, kelp forests have vanished from coastlines under the grazing pressure of sea urchins.

The green Drøbak sea urchin is the most prolific grazer in Norway and its trophic cascade effect led to a phenomenon called the ‘sea urchin barrens’, an ecological dead end that is characterised by an extremely low species diversity and also relatively stable sea urchins densities which don’t allow kelp forests to grow back onto the hard-surface bottom. Although some organisms thrive in barrens, most do not. The barren ground state of the ecosystem here in Northern Norway has lost all ecosystem functions and sea urchins can be seen everywhere. As soon as there is a millimetre of algal growth on the rock, sea urchins feed on it and thereby remove any possibility of kelp recovery. Even though sea urchin barrens can persist for several decades, given the right conditions, they can also undergo a reverse shift back to a kelp forest ecosystem in as little time as a single season. This is where action and direct restoration techniques can really turn things around and bring back not only kelp forests, but also numerous marine species, to these underwater deserts.

To see such kelp forest restoration in action, I travelled to the city of Tromsø in Northern Norway. Located 350 kilometres above the Arctic circle, a local restoration initiative chose a special site beneath a jetty near the city centre for their project to restore the kelp. The idea was simple: The site which was free of kelp and entirely covered by sea urchins had to be freed of the urchins. Throughout the fall and winter months, braving the cold water, we hopped in the water and removed the sea urchins – one at a time. The average temperature of the water did not exceed 5°C and even if we had 9-millimetre-wetsuits on we could only stay in for about half an hour during the coldest days. Thanks to a team effort and the engagement of local ocean advocates, we removed over 35,000 sea urchins within only a couple of hundred metres.

This special initiative, led by nature conservationist Delphin Ruché, brought people from all walks – scuba divers, freedivers, and land-based helpers – together to restore the kelp forest and turn a sea urchin barren back into the thriving underwater world it once used to be. While we had a lot of hope, we had no idea how fast the underwater habitat would change once we removed the sea urchins from the rocks. For weeks on end, we checked the same rocks for urchins and continued to remove them so make sure they weren’t able to graze anywhere near the restoration site. The removal of most of them from the rocky shoreline drastically reduced the grazing pressure on kelp. By lowering the sea urchins density, we hoped kelp would grow back.

The restoration project carefully selected the chosen site for its special positioning. From the end of May until the middle of July, when the sun never dips below the horizon here in Northern Norway, constant daylight helps spur kelp growth. After a long, dark winter, with the return of the sunlight, we witnessed the first colonisation of algae on the rocks – the first sign of success! At the beginning of March, the rocks were covered by a discreet carpet of algae that were only a few centimetres long. We only knew these rocks when they were fully covered by sea urchins, so it was really exciting to witness the difference. It was a very emotional day for everyone involved to finally observe the first results of the hard work we all put in. Two months later, at the end of May, various seaweed species came back, as well as some kelp strands of over five metres in length. Most of the bare rocks were now filled with kelp, the primary producers were back, and many other species were present. Amongst other exciting species such as school of pollocks and cod, as well as crustaceans and molluscs, we documented an adult lumpfish laying hundreds of eggs on site.

We did not expect to see such a formidable transformation in the course of only a few months. I remember immersing myself in the water, fascinated by the colour and movement of the kelp. It was a special feeling to know that the ecosystem we witnessed on site would not have been here without our action. It motivated us to restore more places. The spectacular shift in ecosystem functionality and diversity we witnessed is exhilarating, and it shows how resilient kelp forests really are – even in close proximity to city centres. It reveals that, if given the chance, kelp can grow back and form underwater forests at an incredible speed.

Despite the project’s success in Northern Norway, we only restored 200 metres of coastline. More needs to be done around the world. The restoration of kelp forests provides society with many benefits – from climate change mitigation to supporting coastal fisheries. Better regulation of coastal fisheries, the creation of protected areas where fishing is prohibited, and an active programme against the proliferation of sea urchins would need to be implemented to start tackling the issue at a significant scale.