Research Areas > Marine Debris > Plastic Debris Offshore > Plastic in the North Pacific Gyre
Project: Plastic in the North Pacific Gyre
Background and Objective
Marine debris is more than an aesthetic problem; it poses a danger to marine organisms through ingestion and entanglement. The number of marine mammals that die each year from these causes approaches 100,000 in the North Pacific Ocean alone. A recent study has determined that plastic resin pellets accumulate toxic chemicals, such as PCBs, DDE, and nonylphenols, and may serve as a transport medium and source of toxins to marine organisms that ingest them. Worldwide, 82 of 144 bird species examined contained small debris in their stomachs, and the incidence of ingestion exceeded 80% of individuals for many species. Many studies have focused on the ingestion of small debris by birds because their stomach contents can be regurgitated without causing harm to the animal. However, the effects of ingestible debris on fish are less well-studied. Virtually no studies have been conducted on filter-feeding organisms, whose feeding mechanisms do not permit them to distinguish between debris and plankton.
Concerns about the effects of neustonic debris in the marine environment are greatest in oceanographic convergences and eddies, where debris fragments naturally accumulate. The North Pacific central gyre, an area of high atmospheric pressure with a clockwise ocean current, is one such area of convergence that forces debris into a central area where winds and currents diminish. This study compared the abundance and mass of neustonic debris with the amount of zooplankton in this area.
Map of study area in the Pacific Ocean. Sampling locations are shown within the blue box.
This project was completed in 2001.
Eleven neuston samples were collected between August 23 and 26, 1999, from an area near the central pressure cell of the North Pacific subtropical high. Sampling sites were located along two transects: a westerly transect from 35o45.8’N, 138o30.7’W to 36o4.9’N, 142o4.6’W; and a southerly transect from 36o4.9’N, 142o4.6’W to 34o40.0’N. Locations along transects and the trawl durations were selected randomly. Samples were collected using a manta trawl net. The net was towed at the surface outside of the effects of port wake (from the stern of the vessel) at a nominal speed of about 1 m/s. Each trawl was conducted for a random distance, ranging from 5-19 km. Estimates of plastic and plankton per square kilometer were obtained by using the width of the trawl net opening times the length of the trawl.
Samples were fixed in 5% formalin, and then soaked in fresh water and transferred to 50% isopropyl alcohol. To separate the plastic particles from living tissue, the samples were drained and put in seawater, which floated most of the plastic to the surface, leaving the living tissue at the bottom. Top and bottom portions were inspected under a dissecting microscope. Intermixed plastic was removed from the tissue fraction and plastic and tissue placed in separate containers. Plankton were enumerated and identified to class. Plastic was sorted by rinsing through Tyler sieves. Individual pieces of plastic were categorized and counted by type (fragment, styrofoam fragment, pellet, polypropylene/monofilament line fragment, thin plastic films, tar). Plastic and plankton were also oven dried at 65o C for 24 hours and weighed.
The mean abundance and weight of plastic pieces calculated for this study were the largest observed to date in the North Pacific Ocean. Several possible explanations for this high abundance were suggested. The first was the location of the study area. Most previous studies in the North Pacific Ocean were conducted outside the central subtropical high pressure cell; however, some of the transects for these studies did pass through the gyre. Thus, it is unlikely that location alone was the reason for the higher densities observed. An alternate hypothesis, which has previously been suggested based upon a review of historical studies, was that the amount of plastic material in the ocean has increased over time. Plastic degrades slowly in the ocean, and while some of the larger pieces may accumulate enough fouling organisms to sink them, the smaller pieces usually remain afloat. Therefore, new plastics added to the ocean may not exit the system once introduced, unless they are washed ashore. The dominant clockwise gyral currents also serve as a retention mechanism that inhibits plastics from moving toward mainland coasts. A recent surface current simulation study showed that most of the particles within the sampling area could be retained there for at least 12 years. Several factors prevented extrapolation of the findings of high plastic-to-plankton ratios in the North Pacific central gyre to other areas of the ocean.
The large ratio of plastic to plankton found in this study has the potential to affect many types of biota. Most susceptible are birds and filter feeders that focus their feeding activities on the photic portion of the water column. Many birds have been examined and found to contain small debris in their stomachs. While no record was kept of the presence or absence of fouling organisms on plastic particles during sorting, a subsequent random sampling of each size class found 91.5% of the particles to be free of fouling organisms. Thus, ingestion was likely a result of their mistaking plastic for food, rather than consuming plastic for its attached food.
This project was conducted in collaboration with researchers from Algalita Marine Research Institute.
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