Photo by Martin Bennie

New research from the University of St Andrews has found that disturbing seabed sediments through trawling could cause significant losses of carbon stored in Scotland’s sea lochs, adding fresh evidence to calls for the Scottish Government to introduce protective measures before its forthcoming consultation on marine protected areas.

A vulnerable carbon store

The study, published in Marine Environmental Research, shows that organic carbon stored in seabed sediments can be released into the surrounding seawater when disturbed, where it is more likely to break down and return to the atmosphere as carbon dioxide. Scotland’s sea lochs, which scientists classify as fjords, are estimated to bury around 84,000 tonnes of organic carbon every year despite being a fraction of the size of the North Sea, with roughly 252 million tonnes stored within them overall. Much of this carbon originates on land, carried into the lochs by rivers from surrounding peatlands before becoming trapped in seabed sediments.

Lead author Lauren South, a PhD candidate in the School of Geography and Sustainable Development, used sediment collected along the length of Loch Etive in carefully designed lab-based experiments. The experiments found that even a single disturbance event could cause rapid losses of sedimentary organic carbon, with the strongest effects seen in sediments from the upper part of the loch, where the sediment is both high in organic-carbon content and in the most reactive forms of organic matter. Senior co-author Professor Bill Austin, who also authored the Scottish Government’s Blue Carbon Action Plan, said the findings show that Scottish marine sediments from inshore waters, and particularly upper sea lochs, are extremely vulnerable to the resuspension processes associated with bottom trawling.

Calls for a trawling ban

The findings landed as the Scottish Government prepares to launch a consultation on new fishing restrictions for marine protected areas and sensitive seabed habitats. Charles Millar, executive director of the Sustainable Inshore Fisheries Trust, said the research confirmed that the heads of Scottish sea lochs, which collect peat washed down by rivers from surrounding glens, contain some of the most vulnerable carbon stores in the country. Pointing to this summer’s wildfire near Nethy Bridge in the Cairngorms as a reminder of the fragility of Scotland’s other major carbon store, Millar said ministers needed to urgently act to protect our equally important seabed carbon from disturbance.

Sift went further in comments reported by the Morning Star, with Millar urging ministers to advance coherent measures to protect our seabeds, including a prohibition on seabed trawling in these carbon-rich locations. Writing in the Scotsman, Millar added weight to the demand by noting that the Scottish Government was found in June to have failed in its legal duties to ensure the Scottish seabed is achieving good environmental status.

Government response

Responding to the research, a Scottish Government spokesperson told the Scotsman that blue carbon habitats play an important role in supporting biodiversity and tackling climate change, and that the impacts of seabed disturbance on stored carbon remain subject to scientific uncertainty. The spokesperson said the government already has a strong set of measures in place to protect the marine environment, and that any future decisions on protecting blue carbon habitats will continue to be informed by the best available scientific evidence.

What the researchers recommend

The St Andrews team argue that existing maps showing the distribution of organic carbon in seabed sediments could be combined with data on seabed fishing activity to identify where management measures would have the greatest benefit. South said the study adds to a growing body of research pointing in the same direction: if policymakers care about natural carbon stores, it is time to protect the most vulnerable seabed stores, and those at the heads of sea lochs in particular. The researchers say their findings provide further evidence for the Scottish Government’s own Blue Carbon Action Plan, which had already identified understanding the impacts of bottom trawling as a priority area for further evidence.