Saturday, June 19, 2010
Léger déclin du phosphore dans le Lac Champlain
Deux scientifiques américains ont utilisé des nouvelles méthodes statistiques pour déceler une légère diminution de phosphore dans 14 des 18 tributaires du Lac Champlain entre l'an 2000 et l'an 2008. Deux rivières se déversant dans le lac, Winooski et Missisquoi, par contre, ont connu une légère hausse. La plupart des études précédentes n'avaient pas détecté de changements et même à certains endroits une augmentation de pollution.
La décroissance de phosphore est très minime: de 1% à 3% par année, mais est encourageante: après les sommes considérables dépensées pour tenter de contrôler le ruissellement, le phosphore dans le fumier, les engrais de synthèse et les matières organiques continue de provoquer des blooms d'algues qui sont parfois toxiques pour les animaux et les humains.
Mais pour ce qui est de la Baie Missisquoi, durant l'été, le phosphore émane directement des sédiments qui se trouvent déjà dans le fond de l'eau, ce qui retardera tout résultats encourageants malgré tous les efforts d'assainissement des riverains.
"Study shows drop in phosphorus entering Lake Champlain
Using new statistical methods, two U.S. Geological Survey scientists found that phosphorus loads decreased slightly in 14 of 18 lake tributaries between 2000 and 2008. Those rivers include Otter Creek and the LaPlatte River, but not the Winooski or Missisquoi rivers, where small increases were detected. Their conclusions contrast with studies using less sophisticated analytical techniques that found no trend, or increasing pollution, in most rivers. While the decreases detected in the new analysis were tiny — 1 percent to 3 percent a year — they showed a general trend in the right direction, lake watchers said.
Frustration has been growing in Vermont about the lack of measurable pollution reduction despite more than a decade of work and $80 million to $100 million of public investment in changing farm practices and installing stormwater control systems.
Medalie and Hirsch used statistical methods that allowed them to identify underlying trends in phosphorus pollution by removing the substantial rainfall-driven fluctuations in river flow from year to year. Phosphorus is a plant nutrient found in manure, commercial fertilizer and organic matter. It is Lake Champlain’s primary pollution problem because phosphorus feeds water weeds and algae blooms that sometimes become toxic to animals and humans. Some of the Medalie/Hirsch findings were consistent with earlier analysis. The LaPlatte River in Hinesburg and Shelburne, for example, showed the steepest decline in phosphorus, the result of sewage treatment upgrades in Hinesburg in the 1990s.
However, the two scientists also found that phosphorus in the river has continued to decline slightly, the likely result of human action to stem non-point pollution — the runoff of dirt, and the phosphorus it carries, from farm fields, suburban lawns and city streets.
Medalie and scientists in the audience said the new analysis raises many questions that require further research. For example, why did the Missisquoi and Winooski rivers not show the same trends as other Vermont streams? What land-based practices drove the slight declines in rivers like Otter and Lewis creeks? In contrast to the hopeful news that started the day, other studies suggested the complex obstacles, now and in the future, to keeping the lake swimmable and fishable.
Smeltzer outlined new work to understand the source of phosphorus that drives algae blooms in Missisquoi Bay, a part of the lake where blooms have been a major problem. He said his models indicate that a “major amount” of phosphorus in the water is released by sediments on the bay floor in the summer. That is likely to delay water quality improvements, even if land sources of phosphorus are reduced."
Excerpts from article written by Candace Page of the Burlington Free Press published here: http://www.burlingtonfreepress.com/article/20100608/NEWS02/100607020/Study-shows-drop-in-phosphorus-entering-Lake-Champlain
Unfortunately, after years of overfertilizing land and accelerating drainage of farmland, the damage has been done, and generations to come will have to deal with algae blooms, a slimy lake bottom underfoot while swimming and closed beaches.