Deep-sea litter: a comparison of seamounts, banks and a ridge in the Atlantic and Indian Oceans reveals both environmental and anthropogenic factors impact accumulation and composition

Abstract

Marine litter is a global challenge that has recently received policymakers' attention, with new environmental targets in addition to changes to old legislation. There are no global estimates of benthic litter because of the scarcity of data and only patchy survey coverage. However, estimates of baseline abundance and composition of litter are vital in order to implement litter reduction policies and adequate monitoring schemes. Two large-scale surveys of submarine geomorphological features in the Indian and Atlantic Oceans reveal that litter was found at all locations, despite their remoteness. Litter abundance was patchy, but both surveyed oceans had sites of high litter density. There was a significant difference in the type of litter found in the two oceans, with the Indian Ocean sites being dominated by fishing gear, whereas the Atlantic Ocean sites displayed a greater mix of general refuse. This study suggests that seabed litter is ubiquitous on raised benthic features, such as seamounts. It also concludes that the pattern of accumulation and composition of the litter is determined by a complex range of factors both environmental and anthropogenic. We suggest that the tracing of fishing effort and gear type would be an important step to elucidate hotspots of litter abundance on seamounts, ridges and banks.

Global Distribution, Composition and Abundance of Marine Litter

“…as a consequence of heavy fouling. Indeed, some accumulation zones were identified far from coasts (Galgani and Lecornu 2004;Bergmann and Klages 2012;Woodall et al , 2015. Accordingly, even in the shallow subtidal abundance and distribution patterns can differ substantially from the adjacent strandlines with plastics being the most important fraction at sea.…”

Environmental Change in the Deep Ocean

“…Several studies have documented the occurrence of plastic trash (including lost fishing nets), microplastic particles, and fibers in deep-sea ecosystems (108)(109)(110). Lost fishing nets have been observed encrusted with marine organisms and entangling marine animals, such as fish, crustaceans, and coral (ghost fishing) (109).…”

“…At present, it is largely unknown whether there are harmful effects from these microplastics, such as leaching of toxic chemicals or interference with feeding. The quantities of these materials are substantial, with potentially tens of millions of items of trash on seamounts of the Mid-Atlantic Ridge and South West Indian Ridge (110) and an abundance of microfibers in deep-sea sediments similar to those in intertidal or shallow subtidal sediments (109). This suggests the deep sea may be a major sink of plastics entering the ocean (109).…”

Assessment of Marine Litter in the Barents Sea, a Part of the Joint Norwegian–Russian Ecosystem Survey

“…Material densities, fouling processes, size and shape are important for transport distance and sedimentation rate. Highest densities of marine litter have been found in submarine canyons, while continental shelves and ocean ridges have the lowest densities (Galgani et al, 2000;Ramirez-Llodra et al, 2011;Pham et al, 2014;Woodall et al, 2015;Buhl-Mortensen and Buhl-Mortensen, 2017). In the Mediterranean, densities of marine litter collected by trawling from deep water areas (mean depth 1,400-3,000 m) ranged from 400 kg km −2 at the continental slope south of Palma de Mallorca to densities ranging between 70 and 180 kg km −2 at the other sites (Galgani et al, 2000;Pham et al, 2014).…”