Anthropogenic “Litter” and macrophyte detritus in the deep Northern Gulf of Mexico
Highlights
► We examined distribution of litter and plant debris in the deep N Gulf of Mexico. ► Litter was encountered more frequently in the eastern than in the western GoM. ► Land-derived plant debris was primarily within the head of the Mississippi Canyon. ► Ocean-derived plant material was spread evenly throughout the NE GoM. ► The Mississippi Canyon was a focal point for litter.
Introduction
The presence of human-generated litter or trash in the open ocean and along numerous beaches has been recorded for several decades (Miller and Jones, 2003). It is assumed to be a serious environmental threat to some marine life (Gregory, 2009) as well as an annoyance to humans (Barnes et al., 2009). Floating material, mostly plastics (Thompson et al., 2009), is widely distributed in the Pacific (Moore et al., 2001) and the Atlantic (Carpenter and Smith, 1972). Likewise, considerable litter has been observed on or collected from the sea floor in the Mediterranean (Galgani et al., 2000, Galgani et al., 1996), off US West Coast (Keller et al., 2010, Moore and Allen, 2000, Watters et al., 2010) and in the Bering Sea (Feder et al., 1978), ultimately, it seems, wherever one looks, from beaches out to great depths (Galgani et al., 1996, Galil et al., 1995, Ramirez-Llodra et al., 2011). Much of the near-shore debris on reefs is recreational fishing gear lost on natural seafloor obstructions (Bauer et al., 2010, Moore and Allen, 2000), although debris large enough to be a danger to navigation can be generated by large storms (Nixon and Barnea, 2010). While human-generated debris is often detrimental to marine life (Gregory, 2009, Yoshikawa and Asoh, 2004), natural debris is known to be a source of food (Menzies and Rowe, 1969, Menzies et al., 1967, Schoener and Rowe, 1970, Turner, 1973, Wolff, 1979). Marine debris altered the seafloor and may also provide shelter for demersal organisms such as fish and invertebrates (Watters et al., 2010).
A broad biological survey of the deep continental margin of the Northern Gulf of Mexico (GoM) was conducted between 2000 and 2003 (Rowe and Kennicutt, 2008, Rowe and Kennicutt, 2009). This survey included box cores, otter and beam trawls, and sea floor multi-shot photography at depths of 250–3650 m, stretching from SW Texas over to northern Florida (Fig. 1). Our traditional treatment of trawl samples on deck has been to separate the catch first into buckets of fish and invertebrates. After the first few trawls it became obvious that we needed an additional bucket for a third category: human-produced litter and fragments of natural plant debris. The purpose of this paper is to present these new data on the types and frequencies of plant and human litter on the deep-sea floor, along with some conjecture on its spatial and temporal distribution. This deep-ocean fate of litter can then be compared to that accumulating on beaches, in convergence zones of the central gyres and on reefs.
Section snippets
Methods
The sea floor megafauna and demersal fish populations were sampled with a 12.2 m otter trawl lined with 3.8 cm stretch mesh. The trawls were lowered at ca. 25–50 m/min until the wire out was ca. 3 times the depth. Once on the sea floor, the trawls were pulled at ca. 2–4 knots for 30 min for every 1 km in depth. The distance of each trawl, and thus the area covered, was determined from precise satellite fixes when the trawl landed on bottom and when it left the sea floor. These two locations were
Results
The total spectrum of material sampled (Fig. 2) clearly indicates that plastics were the dominant anthropogenic material, with 16 encounters adding up to a total of 42 pieces in the 40 trawls taken. If one adds the plastic bags (31 pieces in 13 encounters) and the monofilament line (131 clumps in 9 encounters), then the total was substantially higher. Aluminum cans (including beer and soft drink cans) were ranked the second most common categories (23 cans in 14 encounters). We also collected
Discussion
The preponderance of man-made wastes in the eastern GoM compared to the west may reflect shipping lain proximity as well as frequency of offshore oil and gas well platforms and fishing activity (Fig. 4a). The diversity of litter was high, but there does not appear to have been an offshore or depth trend (Fig. 4a and b). The litter was just as diverse at the deep sites as near shore. The MT sites were located in the axis (MT1–4) of the large Mississippi Canyon or offshore where material moving
Acknowledgements
The field work and resulting database for this study were supported by U.S. D.O.I. Minerals Management Service (now Bureau of Ocean Energy Management (BOEM)) Contract 30991 to Texas A&M University. The efforts at sea were aided immensely by Captain Dana O. Dyer of the R/V GYRE and his crew. Data analysis was supported by Canadian Healthy Ocean Networks (CHONe) through a postdoctoral fellowship to C.W at the Ocean Science Center of Memorial University of Newfoundland, Canada, and to G.T.R by the
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2020, AnthropoceneCitation Excerpt :The most striking differences in the distribution of litter within the study area must be related to the physiographic setting, i.e. shelf/slope sectors vs. submarine canyons. Indeed, although marine litter was observed in all dives, more than 95 % of debris was concentrated within the Gioia and Petrace canyons (up to 56 and 41 items/100 m for Gioia and Petrace Canyon, respectively, Table 1), confirming the strong role of submarine canyons as preferential pathways for the transport of benthic litter in the deep sea (Galgani et al., 1996; Watters et al., 2010; Mordecai et al., 2011; Wei et al., 2012; Ramirez-Llodra et al., 2013; Pham et al., 2014; Tubau et al., 2015; Pierdomenico et al., 2019a). Much lower litter abundances were recorded along the continental margin, especially on muddy seafloors (from 0.14 to 0.59 items/100 m, Table 1).