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Project: Plastic Offshore of Southern California

Background and Objective

Numerous researchers have documented the magnitude of marine debris and the threat that its ingestion poses to marine biota. Most of these studies have focused on large debris or debris that accumulates on the shoreline. Few studies have examined the small floating debris, which presents a potential risk to filter feeders, as they have limited capacity for distinguishing small debris from planktonic food. A previous SCCWRP study compared the density of neustonic plastic with zooplankton and found that the mass of debris can rival zooplankton biomass in the upper water column. However, that study was conducted in the North Pacific central gyre, a large eddy system that concentrates debris. Moreover, the gyre is a nutrient poor environment with low biological productivity, which would serve to exaggerate comparisons between debris and zooplankton. It is unclear whether a similar pattern occurs in other marine environments.

This study compares the density of neustonic debris and zooplankton along the Southern California coast, an area that is subject to nutrient upwelling and has a higher biological productivity than the North Pacific central gyre.


This project was completed in 2003.


The first neustonic trawl survey was conducted on October 30, 2000, following 63 days without rain. The second was conducted on January 12, 2001, immediately following a 9 cm rainstorm. Five sites located sequentially offshore of the San Gabriel River were sampled on each survey. The first station was located approximately 200 meters offshore in front of the San Gabriel River mouth, and the farthest station about 5 km from shore.

Map of Study Area

Samples were collected using a manta trawl net. The net was towed at the surface at a nominal speed of 1.5 m/sec. Trawl transects were 0.5-1.0 km long and were laid out in an east/west orientation. Samples were fixed in 5% formalin, then soaked in fresh water and transferred to 70% isopropyl alcohol. Samples were split using a Folsom plankton splitter after large pieces of debris and plant material were removed. Samples were then sorted through Tyler sieves. Debris, zooplankton and plant material were separated from the sorted fractions using a dissecting microscope. Debris were categorized into fragments, styrofoam, pellet, polypropylene/monofilament line, thin plastic films, resin and nonplastics (including tar, rust, paint chip, carbon fragment) and counted. The wet weights of plankton and plant material were obtained, and then dry weight was measured for plankton, plant material, and plastic. 


The density of neustonic plastic along the Southern California coast was about three times higher than the density found in the North Pacific central gyre, though the mass of plastic was seventeen times lower. This disparity between density and mass reflects the dramatic difference in size of debris. Most of the neustonic plastic mass observed in the North Pacific central gyre was large material associated with the fishing and shipping industry, whereas most of the plastic we observed near the coast as small fragments attributable to land-based runoff.

The number of pieces of debris (pieces/m3) significantly increased following the storm.

The average plastic:plankton mass ratio was less in Southern California than in the North Pacific gyre, reflecting a higher plankton density in Southern California. However, for plastic debris smaller than 4.75 mm, the Southern California ratio was twice that of the North Pacific central gyre. This is the fraction that filter feeders are most likely to confuse with plankton.

The differences between findings from this study and those from the North Pacific central gyre largely reflect differences in proximity to land-based sources, but the effects of land-based runoff are probably exaggerated in outhern California compared to the rest of the country. Southern California rivers are highly modified stormwater conveyance systems, so urban debris is able to flow unimpeded to the ocean. Moreover, Southern California has an arid environment with a short rainy season and long dry periods. Thus, land-based debris can easily accumulate between storms.

Little is known about how ingestion of plastics affects filter feeders. Plastics have been shown to sorb contaminants and may provide a pathway for bioaccumulation. Moreover, this study was limited to the upper water column, where most pelagic feeders also feed deeper in the water column.


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

For more information on Plastic Offshore of Southern California, contact Shelly Moore at (714) 755-3207.
This page was last updated on: 7/2/2014