Bottom-Trawling-Enhanced Phosphorus Recycling in the Baltic Sea

Abstract. Bottom trawling on continental shelves can lead to extensive sediment resuspension, potentially altering carbon cycling and benthic-pelagic nutrient exchange. We assessed how bottom trawling affects the oxygen exposure time of organic matter (OM) and its transport toward deeper, oxygen-deficient basins in the Baltic Sea. For this, we used a 3-D coupled physical-biogeochemical model with labile and unreactive OM and associated water-column and sedimentary time tracers. Fishing activity was quantified from publicly available datasets and uncertainty in trawling-induced resuspension was addressed using low and high trawling estimates.

Bottom trawling altered the spatial distribution of OM. Labile OM was transported over shorter distances due to its limited lifetime, with bottom trawling increasing its water-column residence time (WCRT) by 3 and 17 % (14 and 76 h) in areas classified as having low and high trawling intensity, respectively. Repeated trawling-induced resuspension had a larger cumulative effect on unreactive OM, increasing its WCRT by 25 and 46 % (16 and 42 d), respectively, over 15 years and substantially affecting its distribution. Bottom trawling had only a minor effect on oxygen deficiency, with maximum increases in hypoxic and euxinic bottom areas of <7.0 × 10³ and <3.8 × 10³ km², respectively. Notably, bottom trawling progressively enhanced primary production over 20 years of simulation, reaching increases of up to 12 and 29 % under the low and high trawling estimates, respectively, through increased resuspension and recycling of phosphorus. This benthic-pelagic feedback increases sedimentary OM concentrations and may counteract the commonly proposed effect of enhanced OM mineralization, particularly in systems where primary production strongly depends on phosphate availability.