The footprint of ship anchoring on the seafloor

www.nature.com/scientificreports OPEN The footprint of ship anchoring on the seafloor Sally J. Watson1,2*, Marta Ribó3,4, Sarah Seabrook1,4, Lorna J. Strachan4, Rachel Hale1 & Geoffroy Lamarche4 With the COVID-19 pandemic came what media has deemed the “port congestion pandemic”. Intensified by the pandemic, the commonplace anchoring of high-tonnage ships causes a substantial geomorphologial footprint on the seabed outside marine ports globally, but isn’t yet quantified. We present the first characterisation of the footprint and extent of anchoring in a low congestion port in New Zealand-Aotearoa, demonstrating that high-tonnage ship anchors excavate the seabed by up to 80 cm, with the impacts preserved for at least 4 years. The calcuated volume of sediment displaced by one high-tonnage ship (> 9000 Gross Tonnage) on anchor can reach 2800 m3. Scaled-up globally, this provides the first estimates of the footprint of anchoring to the coastal seabed, worldwide. Seafloor damage due to anchoring has far-reaching implications for already stressed marine ecosystems and carbon cycling. As seaborne trade is projected to quadruple by 2050, the poorly constrained impacts of anchoring must be considered to avoid irreversible damage to marine habitats. Since the beginning of the COVID-19 pandemic, thousands of ships have been reported waiting on anchor outside heavily congested p orts1–5. Marine ports around the world have been experiencing unprecedented bottlenecks in traffic, with no relief in s ight6,7. While the economic fallout of the pandemic on the global shipping industry is well r eported1,8–10, the associated environmental impacts due to intensifying anchorage use have been little considered. The short-term deployment of anchors has been referred to as a “hidden cost” of the shipping industry11 due to the associated, and mostly unaccounted for, seabed damage11,12. The global pandemic has shone a spotlight on surging marine port c ongestion1,13,14. Concomitant anchorage use is becoming a more dominant, but unreported and unquantified, impact of the shipping industry on the global seabed15. Physical damage to the seabed by ship anchors is increasingly considered a threat to the health of benthic communities11,12,16–23, due to physical destruction and associated changes in sediment type and ecosystem function24. The physical footprint of anchoring is likened to that of bottom trawling, the most widespread human impact on global continental s helves20,25,26. Bottom trawling can modify seafloor topography22, destroy benthic habitats20,27 and modify ecosystem processes28,29. Like bottom trawling, the ecological and biological impacts of anchoring is a function of the footprint of equipment type used, the seabed substrate28,30, the frequency of anchoring practices and the ecosystem resilience26,31–33. Although anchoring practices are limited to a narrower and shallower depth range (10–80 m water depth) than most bottom trawlers, they occur more frequently and more intensely (deeper seabed penetration). The impact of anchoring may be an unreported but significant contributor to the environmental footprint of the shipping industry which already includes the spread of invasive species, the production of greenhouse gas emissions, as well as air, water, and noise pollution11,34. Our ability to quantify the extent of human activities modifying the seafloor and therefore measure the severity of the impact to marine ecosystems is limited by a paucity of high-resolution bathymetry data11,35,36. As such, the physical footprint and spatial extent of anchoring in water depths greater than 10 m remains e lusive11, particularly for high-tonnage ships (> 9000 Gross Tonnage; GT), which have much larger, more destructive anchoring gear compared to recreational vessels. To date, the environmental footprint of anchoring is not considered in global compilations of human impacts in marine e cosystems25,37,38, nor does data exist to evaluate the release of C O2 that may result from anchoring practices. As nations work to meet climate goals outlined in COP26 proceedings, a shift away from high-emission transportation, such as air f reight39, is being encouraged. In November 2021, the Clydebank Declaration for green shipping corridors was agreed upon40, which will see countries working towards a net-zero goal for global maritime shipping. Yet to achieve more sustainable and lower-impact shipping corridors, the hidden costs of ship anchoring must be incorporated into future global trade s trategies15. 1 National Institute of Water & Atmospheric Research (NIWA), Auckland, New Zealand. 2Institute of Marine Science, The University of Auckland, Auckland, New Zealand. 3School of Science, Department of Environmental Science, Auckland University of Technology, Auckland, New Zealand. 4School of Environment, The University of Auckland, Auckland, New Zealand. *email: Scientific Reports | (2022) 12:7500 | https://doi.org/10.1038/s41598-022-11627-5 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1.  (A) Regional setting of the Picton Port case study, black polygons are landmasses. (B) Map of the Picton region showing region characterised by increased seafloor roughness due to anchoring practices (yellow polygon)42. Locations for Fig. 2A,B are shown by black boxes. Background satellite image obtained from Land Information New Zealand LINZ, 10 m (2018–2019). All figures were constructed using ArcGIS PRO version 2.8.3 and Adobe Illustrator 2022. Results Multibeam bathymetry data in the vicinity of the small Picton anchorage, South Island New Zealand, reveals the footprint of anchoring, characterised by increased seafloor roughness (Fig. 1A,B). The geomorphological expression of anchoring is mostly concentrated at ~ 35 m water depth and extends from the designated anchorage site, more than 3 km south towards the Picton Port (Fig. 1B). There are two main morphological signatures within the anchorage region: (1) linear scours, attributed to the anchor impacting the seabed, either during anchor emplacement or recovery, and (2) zones of abrasion characterised by irregular seabed roughness, attributed to anchor chain movement and swing while the ship is on anchor (Fig. 2A,B). Individual scours observed at the Picton anchorage are regularly over 400 m in length, ~ 5 m wide and range from 40 to 80 cm deep (Fig. 2A, Profile A-A’). Linear scours and abrasion zones are often found adjacent to each other, forming “broomstick-like” features Scientific Reports | Vol:.(1234567890) (2022) 12:7500 | https://doi.org/10.1038/s41598-022-11627-5 2 www.nature.com/scientificreports/ Figure 2.  (A) Zoomed in bathymetry of the seafloor within the Picton anchorage (for location of figure see Fig. 1B). Scours and abrasion zones are labelled and the location of Profile A-A’ is annotated. Profile A-A’ shows the bathymetric profile where the penetration of one scour on the seabed is observed, of up to 5 m wide and (...truncated)