Page 179 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

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geosystems
Ocean Currents Bring Invasive Species
As human civilization discharges ever more refuse and chemicals into the oceans, the garbage travels in currents that sweep across the globe. a dramatic episode of such ocean-current transport happened in the South atlan- tic in 2006. We begin the story here and continue with its ecological impacts in Chapter 20.
The South Atlantic Gyre across earth, winds and ocean currents move in distinct patterns that we examine in this chapter. in the South atlantic Ocean, a counter- clockwise flow drives the South equato- rial Current westward and the West Wind Drift eastward (Figure gn 6.1). along the african coast, the cool Benguela Current flows northward, while the warm Brazil Current moves south along the coast of South america. This is the prevailing cir- culation of the South atlantic.
along the southeastern portion of this prevailing circular flow, called a gyre, is a remote island group, the Tristan da Cunha archipelago, comprising four islands some 2775 km from africa and 3355 km from South america (see loca- tion in Figure gn 6.1).
Isolated Tristan Only 298 people live on Tristan. This unique small society does subsistence farming, collectively grows
and exports potatoes, and depends on rich marine life that the people man- age carefully. The Tristan rock lobster (a crawfish) is harvested and quick-frozen at a community factory and exported around the world. although several ships a year transport the island’s commodities, Tristan has no pier or port, and also lacks an airport. However, in 2006, ocean currents brought to this isolated island a grim reminder of the outside world.
Path of the Drilling Rig
▲Figure GN 6.2 Petrobras XXI aground in Trypot Bay, Tristan. [Sue Scott.]
Oil-drilling platforms are
like small ships that can
be towed to a desired site
for drilling undersea wells. Petrobras, Brazil’s state-controlled oil company, had such a platform, the Petro- bras XXI. The oil company towed the 80-m × 67-m × 34-m-high plat- form from Macaé, Brazil, on March 5, 2006—destination Singapore. The com- pany chartered a tug, the Mighty Deliv- erer, to do the job. This tug was actu- ally built for inland work as a “pusher” tug, like ones you might see on the Mississippi river or in the great lakes, yet it was contracted to haul the drill- ing rig across the Southern Ocean, easily the most treacherous seas on earth. The Deliverer and Petrobras XXI headed south, encountering rough seas after a couple of weeks. Conditions forced the tug’s crew to cut the platform
move through the water with less fric- tion, require less fuel, and result in lower labour costs.) Because of this oversight, Petrobras XXI carried some 62 non- native species of marine life, including free-swimming silver porgy and blenny fish that tagged along with the rig as it drifted in the currents.
What the marine scientists found when they surveyed the grounded rig was an unprecedented species invasion of Tristan. exclusive photos by one of the scientist divers, interviewed for this report, are displayed in Chapter 20. Here in Chapter 6, we learn about wind and ocean circulation, the forces that brought the Petrobras XXI to Tristan.
geosystems now online go to Chapter 6 on the MasteringGeography website (www .masteringgeography.com) for more on this story and the effects of the spilled materials. in March 2011, a vessel carrying soybeans en route between Brazil and Singapore ran aground on nightingale island, just south of Tristan da Cunha. Two days later, the vessel broke up, leaking oil into the waters around this World Heritage site and spilling soya onto lobster beds, thus endangering local fisheries. For dramatic photos of the wreck and the local effort to clean oil-coated northern rockhopper Penguins, go to www .tristandc.com/newsmsoliva.php.
▼Figure GN 6.1 Principal ocean currents of the South Atlantic. note the location of Tristan da Cunha and the probable route of the oil-drilling platform.
loose on april 30, and after a few days, it was lost.
The Petrobras XXI was miss- ing in rough seas, caught in the wind system of the westerlies and the currents of the West Wind Drift. Sometime in late May, after almost a month adrift, the oil- drilling platform ran aground on Tristan in Trypot Bay, where it was discovered by Tristanian fisher- men on June 7 (Figure gn 6.2).
Invasive Species Land in Tristan
The owners of the drilling plat- form had neglected to clean it in preparation for towing. (Clean rigs
South America
South Equatorial Current
Africa
Brazil
Current Tropic of Capricorn Tristan
Benguela Current
Path of
Petrobras XXI
West Wind Drift
0°
20° S 23.5° S
40° S
60°W
40°W 20°W
0° 20°E
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