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What factors determine the depth at which plastic floats?

Posted on November 09 2012

Question submitted by Lewiston High School

Ultimately what determines the depth in the ocean at which plastic is neutrally-buoyant (that is, where it will remain suspended and won't rise or sink) is the density of the plastic relative to the density of seawater.  Plastic that is less dense than seawater will float at the surface, while plastic that is denser than seawater will sink to the ocean floor.  However, the density of seawater increases with depth, so plastic could have a density that allows it to remain suspended within the water column. 

Different plastics have different densities.  Polypropylene line is denser than pieces of foamed polystyrene (Styrofoam), and we do find it floating lower in the water.  Also, the size and amount of organisms that have colonized a piece of plastic can affect where it floats in the water column because they can change the density of the piece of plastic debris.

This is the first float we brought aboard.  You can see the large amount of gooseneck barnacles, among other organisms, living on it.

This is the first float we brought aboard after the organisms were removed from it.  You can imagine the weight of all those organisms added to the weight of the float.  Also, in the first photo notice the polypropylene line that was attached to the float.  This line is in the gray bucket in the second photo.

Even plastic that floats at the sea surface could be found at depths of up to tens of meters in the ocean because the energy of the wind acts to mix it down.  We are researching this with the multi-net MOCNESS to see if there is a relationship between wind speed and the presence of plastic at depth.

You can see the waves being whipped up by the wind through this portlight.  We are using the MOCNESS to look for pieces of plastic that might be mixed below the surface by the wind.
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