Research Areas > Marine Debris > Plastic at Different Depths

Project: Plastic at Different Depths

Background and Objective

Most studies of marine debris have focused on large, visible material found on beaches, while only a few have described abundances of small material in the water column. Studies of neustonic debris have generally been limited to sampling of surface waters. While some birds feed on plankton near the surface and could potentially consume surface debris, most filter feeding activity occurs below the surface. Because positively buoyant plastics make up a high percentage of neustonic debris, studies limited to collection in surface waters may overestimate the prevalence of debris deeper in the water column.

This study extended previous work by SCCWRP comparing the density of neustonic debris and zooplankton at several depths along the California coast. It also addressed how the distribution of plastics in the water column changes following a storm event, when higher wind conditions and urban runoff may enhance vertical mixing.

Status

This project was completed in 2004.

Methods

Sampling was conducted at two Santa Monica Bay sites offshore of Ballona Creek, which drains downtown Los Angeles. The first site was located approximately 0.8 km offshore and the second about 4.5 km offshore. Sampling took place on March 21, 2001 following six weeks without rain, and again on March 25, 2001, following a 20 mm rain event. The sampling site closest to shore was about 15 m deep and was sampled both near the surface and at 5 m depth. The second site was about 30 m deep, and sampled at three depths: surface, 5 m, and near the bottom.
 

Surface samples were collected using a manta trawl net. Mid-depth samples were collected using paired Bongo nets, and bottom samples were collected using an epibenthic sled. The net on the epibenthic sample was located 20 cm from the bottom. Visual inspection by scuba divers confirmed that no sediment stirred from the bottom and entering the net. Trawls were performed parallel to the shore for 10 minutes, with trawl speeds varying from 1.0-2.3 m/s, resulting in a trawl distance of 0.5-1.0 km.

All samples were fixed in 5% formalin in the field, and later soaked in fresh water and transferred to 70% isopropyl alcohol. In the laboratory, samples were placed in fresh water and floating plastic was removed. A dissecting microscope was then used to remove remaining debris and plankton. Debris was sorted by category (plastics, tar, rust, paint chips, carbon fragments, and feathers) and plastics were further categorized (fragments, styrofoam, pellets, polypropylene/monofilament line, thin plastic films, and resin). Each category was sorted by size using Tyler sieves and then counted. Plastics, plankton and plant material were lastly oven dried and weighed.

Findings

The plastic to plankton ratio observed near the surface was similar to that found in previous studies. Offshore, there was more debris found near the surface than mid-water, but more on the bottom than on the surface.
 

It is commonly perceived that plastics are positively buoyant, but only 46% of manufactured plastics actually are. Many buoyant items are products such as Styrofoam, in which air is injected. Even those plastics that are lighter than water at the time of manufacture can become negatively buoyant as they are fouled by biota or accumulate debris. Sand was found embedded in many items, such as plastic bags, that might otherwise float. However, the amount of turbulence necessary for resuspension of debris into midwater appeared to be small, consistent with the fact that the density of most plastics differs from seawater only by a small amount.

While storm-related mixing occurred in shelf waters, the influence of resuspension in deeper waters was less clear. The distance from bottom to the middle of the water column is larger in deeper waters, meaning that more turbulent energy is required to resuspend bottom material into the middle of the water column. In addition, the influence of wind on mixing decreases with depth. Still, our study suggests that there is sufficient routine turbulence to produce  a risk of biological effects from plastics consumption below the surface. To fully assess the risk of debris in the water column, studies on contaminant adsorbtion to debris and feeding behavior related to plastics must be conducted.

Partners

This project was conducted in collaboration with researchers from Algalita Marine Research Institute.