Marine litter in submarine canyons: A systematic review and critical synthesis

Abstract

The presence of marine litter is of concern in submarine canyons, although research in this area is still in its infancy. A critical synthesis and literature review selecting studies with primary data of benthic marine litter at depths of over 50 m revealed important gaps in the knowledge, with information on the impact of macroplastics in deep-sea environments still scarce. Less than 1% of medium to large submarine canyons mapped have been studied in any measure for marine litter, with over 91% of the canyon studies located in European waters. Imaging techniques are now the main tools used for sampling, overtaking trawling methods despite the continued growth of the latter for marine litter deep-sea research. Enumeration of litter was diverse with over 75% using abundance for quantification. Despite the existence of litter protocols available for deep-sea environments, over 73% of studies did not use any. There was no standardization in the implementation of established classification protocols, which were either not used in full or were customized in part. Fishing-related categories do not feature as a top-level category in the classification hierarchy in any of the protocols, yet over 50% of publications featured fishing materials as a main category, pointing to a more intuitive activity-based categorization of litter instead of a materials-led approach from the established protocols. Furthermore, interactions between litter and the surrounding environment and biota are very much underreported with little or no consensus between how the data are analyzed and expressed. There were no discernible patterns between litter density, composition and broad geographical location of canyons, with individual topographical characteristics, hydrodynamic regimes and anthropogenic activities being determining factors in how submarine canyons are affected by litter. Overall, there is no apparent framework to allow comparison of studies and due to the different methods of identifying, enumerating, quantifying and classifying marine litter, or lack of data on position and morphological setting within the canyon system. The evidence provided within this study highlights a ‘call to action’ for an urgent need to standardize and unify methodologies with new or established protocols to fully understand the impact of marine litter in submarine canyons.

Origin and driving mechanisms of marine litter in the shelf-incised Motril, Carchuna, and Calahonda canyons (northern Alboran Sea)

“…As an extreme case, the Messina Strait and Gioia canyons have the highest densities of marine litter in the Mediterranean Sea (up to 56.3 items•100 m -1 ; Pierdomenico et al, 2020;Table 4), and exhibit a coastal population density of up to 800 inhabitants•km -2 . As high densities of marine litter generally display a strong correlation with the proximity of canyon heads to densely populated coasts (Hernandez et al, 2022;Taviani et al, 2023), we interpret that in the study area the relatively low amounts of litter found in the studied canyons are primarily determined by low coastal population densities.…”

“…Some evidence suggests the influence of fishing activities in the accumulation of marine litter in the studied canyons, considering that other types of marine debris, such as fishing gear, indicate an origin linked to marine extractive activities (Pham et al, 2014a;Vieira et al, 2015;Hernandez et al, 2022). Indeed, nearly 16% of marine litter is related to fishing activities in the study area, i.e., fishing nets and gear used by the artisanal fishing fleet.…”

“…On continental margins, litter density, distribution, and composition are variable at different scales (Sańchez et al, 2013). Specifically, the spatial distribution of litter is influenced by a multiplicity of factors, including waves and oceanographic currents, seafloor geomorphology, anthropogenic activities, shipping routes, and river inputs (Galgani et al, 2000;Ramirez-Llodra et al, 2013;Pham et al, 2014b;Angiolillo et al, 2015;Angiolillo, 2019;Gerigny et al, 2019;Dominguez-Carrióet al, 2020;Canals et al, 2021;Bergmann et al, 2022;Hernandez et al, 2022). Interactions of litter artefacts with the seafloor biota are very complex and may have an impact on benthic and demersal organisms, since litter can provide habitat or coverage for diverse marine species (e.g., Goldberg, 1997;Sańchez et al, 2013;Mecho et al, 2020).…”

What, Where and When: Spatial-Temporal Distribution of Macro-Litter on the Seafloor of the Western and Central Mediterranean Sea

“…These items are expected to increase with depth in GSA 07, 08, 11, and 16. The increase of small or light objects on the seafloor at high depths relies essentially on two mechanisms: i) temporal changes in the weight of floating litter due to biofouling (Amaral-Zettler et al, 2021) and/or ii) local features of water circulation and geomorphologies such as submarine canyons, which are known to funnel huge quantities of macro-litter to the deep ocean (Hernandez et al, 2022). On the other hand, heavy objects are likely to be dropped directly into the sea and proximity to major trade routes may have been a contributing factor.…”

Transport and accumulation of litter in submarine canyons: a geoscience perspective

“…However, estimation of floating plastic represent only a small percentage of global input to the ocean (Van Sebille et al, 2015), while a significant proportion of litter is thought to sink to the deep seafloor, where little is known about the extent, magnitude and impacts of this issue. Despite the increasing number of publications discussing seafloor litter over the last decades (Hernandez et al, 2022), papers on seafloor litter represent less than 15% of all studies on marine litter (Canals et al, 2021).…”