Science Results : Daily Update

Daily Update | Current SEA Research

June 24, 2010

By Giora Proskurowski

In the past 24 hours we have conducted four surface nets, and two subsurface nets, towing at about 8 meters (26 feet) below the surface and 14 meters (45 feet) below the surface. The purpose of these tows is to compare the plastic observed at the surface to plastic that may be distributed throughout the mixed layer, currently about 20 meters (66 feet) deep.

Our nets showed that there is a substantial amount of plastic at depth. While there is relatively less plastic in the meter-tall swath we sampled at depth than at the surface, the total amount in the mixed layer may be comparable to the total in the top meter. Although to get good estimates we need to do this type of sampling many more times under many more conditions.

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By Kara Lavender Law

Yesterday I discussed a simplified version of the theory explaining the subtropical convergence zone at 30°N latitude – the latitude that, until a couple days ago, the Cramer was approximately tracking east of Bermuda. The converging ocean currents, which are ultimately driven by the wind blowing over the ocean, act to concentrate plastic debris (and anything else floating on the sea surface) in this region.

This brings us to the primary mission of this research expedition – to test the details of the theory described yesterday. From the work of Dr. Nikolai Maximenko at the University of Hawai'i and others – see a nice article on this work in the IPRC Climate Newsletter (Vol. 8, No. 2, 2008; p. 14) – we have a map of the average observed surface currents, including the location of the subtropical convergence zone, as well as a prediction from a numerical model of where floating debris is most likely to accumulate. We are testing this hypothesis for the first time, extending SEA's previous work in the North Atlantic far to the east of Bermuda, where we expect to find continued high concentrations of plastic debris.

Thus far, the hard work of the crew has shown the hypothesis to be correct, with very high plastic concentrations observed all along the transect from Bermuda to 40°W longitude (about halfway between North America and Africa). There is no indication that the plastic concentrations begin to taper off, either.

We won't learn how far east the accumulation zone extends since the ship has reached the easternmost extent of this cruise. The next leg will take the expedition back to Bermuda in a sawtooth pattern, to try to define the northern and southern boundaries of the convergence zone. Since no one has ever measured marine debris in this region of the North Atlantic before, we can only eagerly await each day's science report to learn if the hypothesis continues to hold true.