Relative effects of bottom trawling and environmental factors on sedimentary carbon properties and degradation in the heavily trawled Kattegat

Recent reviews have highlighted that the effect of bottom trawling on sediment carbon content and biogeochemistry varies depending on environmental conditions, the type of sediment (e.g. mud vs sand) and the chemical nature of the organic carbon. However, most data are from analyses of total carbon or organic matter and from bulk surface sediments (top 2 cm) rather than deeper sediment profiles, limiting our ability to interpret these results in terms of the biogeochemical processes involved in carbon degradation and burial.

The Kattegat is one of the most heavily trawled seas in the world with some parts being swept by fishing gear up to 15 times per year. However, fishing effort is patchy and there is also a marine protected area where bottom trawling is forbidden; the resulting large range in trawling disturbance makes the Kattegat an ideal site for studying potential impacts on the seafloor.

We analysed sediments across the trawling gradient, in 1cm-layers downcore, and determined the relative effect of trawling intensity and environmental variables (e.g. bottom water oxygen, sediment grain size) and biological variables (e.g. bioturbation) on sediment carbon content, reactivity and degradation rates. In general, environmental factors and physical properties of the sediment appeared to be the strongest explanatory variables, with trawling intensity being less important. The best explanatory variables also varied depending on the sediment depth analysed, potentially due to the relative importance of seasonal inputs of fresh organic carbon at the sediment surface, mixing depth and type of bioturbation and penetration depth of the trawls.