Anthropogenic “Litter” and macrophyte detritus in the deep Northern Gulf of Mexico

Abstract

A deep-sea trawl survey of the Northern Gulf of Mexico has documented the abundance and diversity of human-generated litter and natural detrital plant material, from the outer margin of the continental shelf out to the Sigsbee abyssal plain. Plastics were the most frequently encountered type of material. Litter and debris were encountered more frequently in the eastern than in the western GoM. Land-derived plant material was located primarily within the head of the Mississippi Canyon, whereas ocean-derived plant material was spread evenly throughout the NE GoM. Human discards were principally from ships offshore. Some of the material was contained in metal cans that sank to the sea floor, probably in order to conform to international agreements that prohibit disposal of toxic material and plastics. The Mississippi Canyon was a focal point for litter, perhaps due to topography, currents or proximity to shipping lanes.

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.