Science Results : Daily Update

Daily Update | Current SEA Research

July 3, 2010

By Giora Proskurowski

The last few days have been fairly light in surface plastic. We've been averaging about 75 pieces in each net tow, or around 40,000 pieces per square kilometer. Again, it is important to note that this is still a large number, and that this result is not surprising. We've observed lower concentrations as we approach the southern end of each of the zig-zags that make up our cruise track.

Over the past couple of days our morning station has focused on getting data on the distribution of plastics within the mixed layer (approximately the top 25 meters – in the summer – that is well-mixed and defined by uniform density). To do this we use a CTD (Conductivity, Temperature, Depth sensor) to get a profile of the water column, determine the mixed layer depth, and then do a series of Tucker trawls at several depths within the mixed layer.

The Tucker trawl is an ingenious and, at times, confounding piece of net gear that is used to get a tow sample from discrete depths. If you used a traditional net to collect a sample at ten meters depth, you would lower the net down to ten meters, tow at that depth, and then raise it up. The net would mostly sample water from ten meters depth, but it would also sample from the surface to ten meters, twice, during its descent and ascent.

The Tucker trawl allows us to send the net down closed, open it once it is at the correct depth, and then close it before we haul back. The opening and closing of the net is done by a nice little mechanical device we attach to the wire called a "double release mechanism". This device has a trigger that, when hit with a weight, releases a pin that opens the net. When hit with another weight, the net will close. In order to operate the trigger we attach a bronze weight, called a "messenger", to the wire and throw it down the line like a football.

In order for the messenger to work properly I require a "catch phrase" for the thrower to shout. Popular options have been along the submarine lines of "Flood the tubes and dive! Dive! Torpedoes in the water!", or Hunt for Red October-related (with the unlikely Sean Connery Scottish-Russian captain accent), or football related (e.g. "Go deep!").

The results from the past several days of Tucker trawls have been very interesting. While we've seen fairly low numbers of plastic at the surface, we have observed an almost equal amount of plastic from four meters water depth, slightly less at ten meters depth, and no plastic below the depth of the mixed layer. This morning, for example, we collected 34 pieces of plastic in the surface neuston net, 22 pieces at four meters depth, eight pieces at 11 meters depth, and zero pieces at 34 meters depth – just below the mixed layer, which was 32 meters deep. We know the depth of the tow because we attach a small CTD to the net, and thus have a record of the depth over time.

These tows undoubtedly show that plastic is being mixed down into the water column. It is not just a surface feature. What we measure at the surface is, in many cases, not the sum total of what is out here. Like many environmental problems, the closer you look at the system, the worse it appears.

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For more from sea, listen to an interview with Chief Scientist Giora Proskurowski on the July 2, 2010 Science Update podcast, produced by AAAS.