Wednesday, August 22, 2012

Oceanic Gyres and Garbage Patches

Written June 27, 2012, published August 2012

The 2011 tsunami inadvertently provided ocean biologists with study material for pelagic drift for years to come. The word pelagic is from the Greek word for open sea, pĂ©lagos. Probably the best-known pelagic ecosystem in the world is the Sargasso Sea, in the Atlantic Ocean. This is a natural gyre, or eddy, where floating seaweed is common. It is a large oval around 700 statute miles wide and 2,000 statute miles long, and is near Bermuda on the west edge. The Sargasso Sea is bordered on all sides by currents. 

This sea is named for floating brown seaweed, in the genus Sargassum, which is common throughout the eddy area. While most Sargassum species are benthic and live associated with seabeds, Sargassums in the Sargasso Sea are holopelagic (free-floating throughout their lives). 

A local seaweed in this genus, Sargassum muticum, lives on shells, cobbles and wood on tidelands of Willapa Bay. Like other Sargassums, it has dense leafy brown fronds with numerous small air bladders, which help it to float up off the bottom and probably gives it more access to light. It is one of dozens of species that arrived with Pacific oyster spat in the early to mid 20th Century from Japan, and now lives in many estuaries around the world. 

A recent expedition to the Sargassum Sea confirmed that numerous endemic species, which live nowhere else on earth, are found among this floating seaweed forest.  This floating reef structure is used by many species; likewise, the cover provided by Sargassum is attractive to many fish species in the otherwise open ocean.

Being a gyre, the Sargasso Sea is a watery trap for debris. This golden brown seaweed community is slowly being filled with plastics from the surrounding currents and shores of the Atlantic Ocean. The Sargasso Sea is becoming the Great Atlantic Garbage Patch. 

In the Pacific, there is no Sargasso Sea West, but there is a marine debris and plastics gyre in a similar location, in the North Pacific Gyre. It is called the Great Pacific Garbage Patch. The densest part of this gyre is between 135°W to 155°W and 35°N to 42°N, a long oval area that is 480 by 1400 statute miles wide. The exact size is difficult to measure, because the plastics in it gather in a floating belt in the water, not a raft lifted up out of the water. It is north of the Hawaiian archipelago, and stretches east and west for hundreds of miles. There are also several other gyres in the world’s oceans, in the south Pacific, Indian and south Atlantic Oceans. All of these are places where plastics accumulate. 

Locally, we know there is a plastics debris problem on our beaches, but compared to some Hawaiian beaches, our beaches approach pristine condition. Some beaches on the north side of the Hawaiian Islands accumulate huge amounts of plastic each year in drifts 5 to 8 feet thick, more than 20 feet wide, and miles long. 

When the energetics of plastic recycling are worked out, these floating garbage patches and plastics-rich beaches may become resource extraction areas, where harvesters gather plastics to make diesel fuel. The process is simple; it’s the energy to heat the plastics that makes this expensive as a process right now.