From mgs23@pacbell.net Wed Aug 4 17:37:46 1999 Date: Tue, 3 Aug 1999 23:51:50 -0700 From: Misha To: SANET-mg Subject: Gulf "dead zone"/ProMED Howdy, all-- Thought this might interest you lovers of the earth's waters. peace misha ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DEAD ZONE - GULF OF MEXICO ************************** A ProMED-mail post Date: Mon, 2 Aug 1999 18:28:33 -0500 From: Martin Hugh-Jones Source: The Advocate, Baton Rouge, 31 Jul 1999 [edited] Scientists doing research in the Gulf of Mexico have found the largest ever "dead zone," an area where oxygen in the water is so low it cannot support life. As she has for most summers in the past decade, Louisiana Marine Consortium's Nancy Rabalais cruised the gulf off the Louisiana coast mapping the extent of low-oxygen water, a condition called "hypoxia." Rabalais said this year's dead zone is the largest ever found. She said she doesn't know why the dead zone is so big this year. The 1999 dead zone measures 7,728 square miles -- about 12 times bigger than Lake Pontchartrain and 16 times larger than East Baton Rouge Parish. This year's size is much larger than the previously recorded maximum size in 1995 by about 2000 square kilometers, or 700 square miles. Last summer, the dead zone covered less seabed than in the past few years -- about 4,800 square miles -- but went far deeper than ever. This summer, the low oxygen zone stretched west from the Mississippi River delta all the way to near the Texas border, and from close to the shore along most of the Louisiana coast out to 100-foot water depths. The low-level zone was also closer to shore this year than last. Rabalais said the term "dead zone" comes from the lack of sea life such as shrimp, crabs and fish in its bottom waters. Trawling boats were noticeably absent from the extensive area of low oxygen, with the exception of fleets working off the mouth of the Sabine River and west of Atchafalaya Bay, she said. When the bottom oxygen levels fall below 2 parts per million, anything that can swim leaves the area, and trawlers catch nothing. The displaced shrimp are concentrated on the periphery of the dead zone, where trawlers focused their efforts. Animals that cannot escape, such as the clams, snails and worms that live in the mud, eventually die if the oxygen levels fall too low for too long. The mixture of fresh and salt water and the nutrients carried by the Mississippi River system are a large part of the reason Louisiana has the largest fishery in the Lower 48 states. But it seems too many nutrients, such as nitrogen, are flowing into the river system, causing algae blooms that go well beyond what nature can handle, according to Rabalais. Possible sources of that nitrogen include fertilizers used on farms in the Mississippi River valley. Three studies, including one by the U.S. Geological Survey, estimate that more than 50 percent of the nitrogen in the river comes from farming activities. Rabalais said water from the Mississippi and Atchafalaya rivers was easily detectable in the gulf this year because of its much lower salinity and its dense concentrations of phytoplankton, or microscopic algae or plants. The fresh water lies on top of the saltier gulf waters, forming a two-layer system. The phytoplankton flourish in the nutrient-enriched waters. Eventually the phytoplankton die and sink to the bottom, or are consumed by zooplankton, and waste from the zooplankton sinks to the bottom. Bacteria decomposes all that organic matter on or near the bottom, using up the oxygen in the lower waters. Low winds and calm seas occur more commonly in the summer, creating conditions for the persistence of the two-layer system and the development of wide zones of hypoxia off the coast. The hypoxia zones usually last until a tropical storm or cold front mixes the water in all layers. [Byline: Mike Dunne, Advocate staff writer] To Unsubscribe: Email majordomo@ces.ncsu.edu with the command "unsubscribe sanet-mg". If you receive the digest format, use the command "unsubscribe sanet-mg-digest". To Subscribe to Digest: Email majordomo@ces.ncsu.edu with the command "subscribe sanet-mg-digest". All messages to sanet-mg are archived at: http://www.sare.org/san/htdocs/hypermail