The world’s continental shelves collectively represent an area of marine habitat larger than the entire North American continent, covering roughly 27 million square kilometres. 

These continental shelves are places of high energy and activity, shaped by wind, tides, and sunlit ecological communities, unlike the seabed of the open ocean which lies in perpetual darkness at an average of 3.5 kilometres under the surface.

READ: THE CATCH

However, this enormous habitat has also been fundamentally shaped by decades of intense human exploitation. 

Fragments

Most of the seabed on the continental shelves is covered by soft sediments, from coarse sands to dense muds containing large quantities of organic matter. 

Compared to coral and rocky reef habitats, this environment appears to be relatively uninteresting, naturally ecologically poor, resilient to human disturbance, and not qualifying for closer investigation or protection.  

This perception may represent one of the greatest unrealised ecological mistakes committed by industrialised society. 

For more than a century, industrial trawling and dredging has taken place in plain sight of millions of witnesses, while the resulting transformation of the seabed beneath the surface has remained almost entirely unseen. 

In some of the most historically heavily exploited regions, such as the European shelf, we have fragments of information on how the seabed may once have looked before it was trawled into the physically simplified, and ecologically degraded habitat we accept today as normal. 

Productivity

Historical accounts of thriving animal communities on the seabed seem equally hard to comprehend. Descriptions such as the “living crust” covering the floor of the North Sea, or the recently estimated 1.7 million hectares of oyster beds that once represented a significant hazard to the nets of wind-powered fishing vessels on the European shelf.

This is similar to historical anecdotes of once thriving marine animal populations from around the globe - from complaints by sleep-deprived New Zealand settlers kept up at night by persistent whale song emanating from Wellington harbour, to fishing vessels slowed by fish shoals too thick to pass.

Yet, the extent of these beds in 2026 is functionally zero, with only patchy remnants surviving.

Economic productivity in combination with a chronic shifting baseline syndrome with deep historical roots, has effectively masked this ecological decline.

Decades of technological innovation, increasingly powerful vessels, and diversification of target species have allowed commercial demersal fisheries to remain viable even as the seabed habitats supporting them have been progressively simplified.

Interests

All of this has culminated in a profound conservation oversite where the vast areas of soft sediment habitat on the continental shelves have been disregarded in the establishment of marine protected areas. 

Despite substantial evidence favouring an ecosystem approach to conservation efforts more broadly, biodiversity “hotspots” and “ecological features” have often been prioritised, this reduces the spatial scale eligible for protection while easing tension with commercial interests. 

However, a new opportunity for seabed protection on unprecedented spatial scales may be emerging in the form carbon offsetting and nature-based solutions to climate change.

Ironically, the catalyst for re-evaluating the importance of soft sediment ecosystems and their subsequent protection may not be biodiversity at all. 

Instead, growing interest in seabed carbon storage and climate mitigation might provide a trojan horse by which the ecological value of continental shelf sediments can pass the gates of government policy and economic interests and receive the recognition it deserves. 

Foresight

The Convex Seascape Survey represents one manifestation of this new interest in the potential of the continental shelves to accumulate and store organic carbon. 

As part of a five year mission the Convex Seascape Survey has attempted to locate areas around the globe to assess how both seabed organic carbon stocks and the ecological communities that regulate them, respond to protection from human disturbance. 

However, this task has proven surprisingly difficult despite a full global remit, revealing the oversight to consider soft sediment environments in marine protection strategies generally. 

A handful of rare highly protected areas encompassing soft sediment habitats do exist. One of the most notable of these is the South Arran Marine Protected Area, covering 280 square kilometres around the south coast of the Isle of Arran on the west coast of Scotland. 

With rare recognition and foresight, the Community of Arran Seabed Trust (COAST) fought and won a range of protection measures for the area in 2014, including a nearshore area fully excluding any form of bottom-towed gear. 

A recent study from the Convex Seascape Survey in collaboration with COAST describes how sediment organic carbon stocks in the South Arran Marine Protected Area have responded to a gradient of protection measures alongside a range of ecological communities living on and in the seabed. 

Succession

Collectively known as infauna, animals living in the sediment such as polychaete worms and bivalves constantly re-work the sediment as they hunt and build burrows in a process known as bioturbation. 

This mixing of the sediment can lead to the burial of organic carbon deeper into the seabed, locking it away long term. The study also assessed the animals living on top of the sediment collectively known as epifauna, such as sponges, soft corals and other more mobile animals such as brittle stars etc which also play important roles in mediating this carbon flux in the seabed. 

The study revealed a strong response to protection. Infauna communities exhibited a three-fold increase in abundance and a two-fold increase in species diversity, while sediment organic carbon content was also elevated after accounting for mud content. 

While this seems like an obvious outcome consistent with the now widely accepted narrative of biodiversity recovery in well managed marine protected areas, these results provide something much rarer: a proof of concept for soft sediment environments specifically, and an important insight into how this enormous habitat, if left to recover, is capable of holding much higher biodiversity and perhaps much higher organic carbon content than previously assumed.

Despite a decade of protection, this represents the first rung on the ladder of ecological succession and biodiversity recovery which is likely to occur much more slowly in response to protection compared to other environments. 

Multiple decades may be required for the South Arran Marine Protected Area to reach its full ecological potential, but in doing so could provide an antidote to the shifting baseline syndrome that has infected our perceptions of the seabed of the European shelf for over a century. 

Consequences

Soft sediment habitats offer a potentially enormous opportunity for nature-based solutions to climate change, and in doing incentivises seabed protection on a spatial scale that far exceeds that afforded by the biodiversity agenda, which for soft sediment habitats at least, has largely failed. 

As the survey continues to publish data from a variety of soft sediment habitats around the world, we are making the case for a fundamental reassessment of these environments. 

Far from being naturally impoverished and invulnerable to disturbance, soft sediments may represent one of the largest untapped opportunities for biodiversity recovery, carbon storage, and fisheries enhancement anywhere in the marine environment.

We suggest that these biodiversity benefits once sprung from the trojan horse of carbon offsetting incentives will be quickly considered as an integral mechanism of carbon capture and burial, not just a co-benefit. 

Over time we hope this will provide a fundamental re-evaluation of soft sediments with positive consequences for marine conservation efforts globally. 

This Author

Dr Ben Harris is a marine ecologist based at the University of Exeter.