http://www.nature.com/srep/2014/140327/srep04435/full/srep04435.html

Alexander Turra, Aruan B. Manzano, Rodolfo Jaso S. Dias, Michel M.
Mahiques, Lucas Barbosa, Danilo Balthazar-Silva & Fabiana T. Moreira
Three-dimensional distribution of plastic pellets in sandy beaches:
shifting paradigms
Scientific Reports 4, Article number: 4435, doi:10.1038/srep04435

Abstract
Plastic pellets are worldwide contaminants that accumulate in the ocean,
especially in sandy beaches, where their historic standing-stock
quantification relies on surface sediment samples. We demonstrated these
particles present a three-dimensional instead of a simple along-across
shore distribution, being found as deep as 2.0 m, with surface layers
accounting for <10% of the total abundance in the sediment column. This
gradient seemed to be more related to oceanographic rather than
anthropic processes, suggesting a general pattern whose applicability to
microplastics and sedimentary environments as a whole should be
investigated. This poses criticism in the exactness of standing-stock
records and demands urgent discussion of sampling protocols.

http://www.nature.com/srep/2014/140327/srep04435/pdf/srep04435.pdf