Exploring the profound impacts of fishing practices on marine sediment biogeochemistry and global carbon cycles
Beneath the shimmering surface of the ocean lies a dynamic landscape as crucial to our climate as the rainforests. The seafloor is a living, breathing engine of the planetary carbon cycle, where tiny organisms and sediment processes work in concert to regulate Earth's atmosphere.
For decades, a common fishing practice—bottom trawling—has been known to damage seafloor ecosystems. Now, scientists are uncovering its deeper, more insidious role: disrupting the ocean's natural carbon storage on a global scale1 2 . This is not just a story of ecological damage, but a complex biogeochemical drama playing out in the mud and sand of the seabed, where the very tools we use to harvest food from the sea may be altering its fundamental ability to protect us from climate change.
To understand the impact of bottom fishing, one must first appreciate the two primary personalities of the seafloor. These are not just different textures; they are fundamentally different worlds with distinct rules of life and chemistry.
In contrast, sandy seabeds are advection-dominated systems1 . Water, and the chemicals it carries, is constantly flushed through the large pores between sand grains.
Bottom trawling, where heavy weighted nets are dragged across the seafloor, does not just catch fish. It is a major anthropogenic disturbance that acts like an underwater bulldozer, with a cascade of effects2 .
The trawl gear resuspends vast clouds of sediment, churning up the stable seafloor landscape and fundamentally reworking the physical fabric of the seabed.
Benthic communities are decimated. Large bioturbators that play a critical role in burying carbon are killed off, disrupting carbon sequestration4 .
Resuspended sediment exposes buried organic carbon to oxygen, leading to microbial decomposition and CO₂ release through oxic mineralisation.
While the release of organic carbon has been discussed for years, a 2025 study from the GEOMAR Helmholtz Centre for Ocean Research Kiel uncovered a previously underestimated player in this process: pyrite oxidation8 . This experiment provided the first quantitative evidence of its major role in the western Baltic Sea.
Sediment cores collected from different sites in Kiel Bight, a region with mixed sandy and muddy sediments8 .
Created sediment slurries by stirring samples in seawater containers, simulating trawl-induced sediment clouds8 .
Experiments conducted under both oxygen-rich and oxygen-poor conditions8 .
Continuously tracked changes in CO₂ concentration, pH, sulfate, and nutrient levels8 .
Empirical data integrated into sophisticated biogeochemical models8 .
The findings turned previous assumptions on their head. The oxidation of pyrite, not organic carbon, was the dominant driver of CO₂ release following sediment disturbance8 .
| Parameter Measured | Finding | Scientific Significance |
|---|---|---|
| Primary CO₂ Source | Pyrite oxidation, not organic carbon oxidation | Overturns previous belief that organic matter was main culprit |
| Reaction Process | Pyrite + Oxygen + Water → Acid → Converts Bicarbonate to CO₂ | Identifies new chemical pathway for CO₂ production |
| Impact on Carbon Sink | Significantly reduces CO₂ uptake capacity | Shows trawling degrades ocean's climate regulation system |
The implications of these processes are staggering. When scientists scale up these local findings, the global estimates of CO₂ release from bottom trawling become a serious point of discussion within the climate community.
| Source | Estimated Annual CO₂ Release | Context & Key Caveats |
|---|---|---|
| Atwood et al. (2021/2024)2 | 340-370 million metric tons | Equivalent to emissions of entire countries like Italy or Spain. Estimate is debated. |
| Zhang et al. (2024)4 | 30 million metric tons | Focuses on carbon stock reduction from sediment resuspension and macrofauna depletion. |
| GEOMAR (2025)8 | (Focused on regional processes in Kiel Bight) | Highlights that fine-grained, muddy sediments are most susceptible and need protection. |
There is a growing consensus that frequent trawling makes it harder for carbon to remain sequestered and that the carbon in muddy sediments is more susceptible to trawling impacts than carbon in sand4 .
The situation is not without hope. Unique long-term research from Sweden's Kosterhavet National Park offers a compelling vision of recovery. After 25 years of trawling restrictions, scientists using underwater videos and AI analysis have documented a remarkable comeback.
Filter-feeding animals like mussels, anemones, and soft corals, which do not thrive when trawls stir up sediment, have recovered significantly6 . This research, spanning 26 years, provides a powerful timeline showing that protective measures can rapidly bring balance back to the ecosystem.
"The recovery seen in the Koster Sea proves that when we give the seafloor a chance, it can rebound with vigor6 ."
Rising sea temperatures, driven by climate change, are causing a decline in heat-sensitive species like the football sponge, indicating that local protections cannot fully offset global threats6 .
The journey into the murky depths disturbed by bottom trawling reveals a story far more complex than simple habitat destruction. We are tampering with a critical component of the planetary carbon cycle. The seafloor, particularly its cohesive muddy habitats, is not just a home for marine life but a vulnerable vault for carbon.
The experiments from Kiel Bight and the North Sea provide a clear scientific basis for action: safeguarding these muddy sediments is an urgent priority for climate protection4 8 .
As we move forward, managing our oceans requires a dual approach: leveraging new technologies to modernize and reduce the impact of fisheries science7 , and implementing spatial management policies that consider the hidden climate value of the seabed. The recovery seen in the Koster Sea proves that when we give the seafloor a chance, it can rebound with vigor6 . The question now is whether we will listen to the science and act to protect this vast, climate-critical frontier at the bottom of the sea.