Impact of bottom trawling on sediment biogeochemistry: a modelling approach

Abstract

Abstract. Bottom trawling in shelf seas can occur more than 10 times per year for a given location. This affects the benthic metabolism, through a mortality of the macrofauna, resuspension of organic matter from the sediment, and alterations of the physical sediment structure. However, the trawling impacts on organic carbon mineralization and associated processes are not well known. Using a modelling approach, the effects of increasing trawling frequencies on early diagenesis were studied in five different sedimentary environments, simulating the effects of a deeper-penetrating gear (e.g. a tickler chain beam trawl) versus a shallower, more variable penetrating gear (e.g. an electric pulse trawl). Trawling events strongly increased oxygen and nitrate concentrations in surface sediment layers and led to significantly lower amounts of ammonium (43 %–99 % reduction) and organic carbon in the top 10 cm of the sediment (62 %–96 % reduction). As a result, total mineralization rates in the sediment were decreased by up to 28 %. The effect on different mineralization processes differed both between sediment types and between trawling frequencies. The shallow-penetrating gear had a slightly smaller effect on benthic denitrification than the deeper-penetrating gear, but there were no statistically different results between gear types for all other parameters. Denitrification was reduced by 69 % in a fine sandy sediment, whereas nitrogen removal nearly doubled in a highly eutrophic mud. This suggests that even relatively low penetration depths from bottom fishing gears generate significant biogeochemical alterations. Physical organic carbon removal through trawl-induced resuspension of sediments, exacerbated by a removal of bioturbating macrofauna, was identified as the main cause of the changes in the mineralization process.

Impact of and recovery from seabed trawling in soft-bottom benthic communities under natural disturbance of summer hypoxia: A case study in subtropical Hong Kong

“…Trawling is known to change environmental factors, such as grain size moving towards finer fractions (Pilskaln et al, 1998;Reiss et al, 2009), TOC towards higher percentage levels (Tuck et al, 1998;Reiss et al, 2009) and DO towards higher levels (De Borger et al, 2021). However, the trend of decrease in median sediment diameter and increase in TOC in the trawled area as compared to the non-trawled area was not clear in the present study (Table 1).…”

“…Hypoxia is a natural event in the area and it is known to reduce the biodiversity drastically in the summer for both epi-and infauna; however, a rapid recovery of community structure to normal conditions has been noted in winter (Wu, 1988;Fleddum et al, 2011). It is thus suggested that trawling could contribute to a higher level of DO in a time when the area suffers from hypoxia (De Borger et al, 2021), thus increasing S3). Cut off for low contribution is 90%.…”

Sedimentary carbon on the continental shelf: Emerging capabilities and research priorities for Blue Carbon

“…Controlling trawling and other human activities which disturb the seabed where it acts as a carbon store appears to be a quick win, yet the impacts of disturbed sediment can extend beyond the trawled area on a regional scale (Palanques et al, 2014). If MPAs are to be used as a management tool to protect carbon stocks and accumulation in continental margin sediments (e.g., Roberts et al, 2017;Sala et al, 2021), their locations must be informed by well-mapped carbon stock and accumulation (e.g., Diesing et al, 2021) and process-level understanding (e.g., de Borger et al, 2021b).…”

“…Physical disturbance of the seabed which resuspends near-surface sediments into the oxic water column will recycle labile organic carbon into the ocean and potentially impact the atmospheric carbon system (Sala et al, 2021), although it can also simply re-distribute and redeposit sediments (Rijnsdorp et al, 2021). Seabed disturbance impacts the benthic faunal community, which plays a key role in the biogeochemical processes determining what fraction of deposited carbon is recycled (remineralized) or buried (stored) (de Borger et al, 2021a;de Borger et al, 2021b).…”

“…More research is required to improve understanding of these factors in the context of carbon storage across a range of settings. Recent work advancing towards this goal includes that of Bradshaw et al (2021) and Morys et al (2021) who made field measurements of the biogeochemical impacts of trawl disturbances in the Baltic Sea, and that of de Borger et al (2021b) who modelled biogeochemical impacts of trawling scenarios at specific sampled sites in the North Sea. Global scale modelling of trawling impact (e.g., Hiddink et al, 2021;Sala et al, 2021) remains limited by the need to make broadscale assumptions of the underlying processes and carbon characteristics which control carbon response to pressure.…”