Interpreting Dead Zone Data

Background

A dead zone is an area that has little or no oxygen and cannot support marine life. One large dead zone (about the size of New Jersey) exists off the Gulf of Mexico. Similar dead zones exist in marine and fresh waters around the world, though the one in the Gulf of Mexico is particularly significant due to its large size. A dead zone is one example of eutrophication, which means an ecological imbalance that occurs as a result of excess nutrients.

It was in August 1972 that scientists began investigating the dead zone in the Gulf of Mexico, finding severe oxygen depletion at shelf depths of 10-20 meters. Many studies since then have revealed dissolved oxygen (DO) levels lower than the normal seawater level of about 6 milligrams per liter (mg/L) (See Figure 2). Marine life needs DO to live. When conditions become hypoxic (this means DO is less than 2mg/L) shrimp, crabs, fish and other marine life suffocate and die. In some cases, there is no DO (0mg/L). When there is no DO, conditions are known as anoxic.

Activity:

Use what you’ve been learning about eutrophication and dead zones to analyze the graphs at the end of the assignment and answer the following questions:

  1. What units are used to measure dissolved oxygen? ______
  1. At what concentration of dissolved oxygen will fish, crabs, and shrimp begin to suffocate and die? ______
  1. Use a mathematical expression to explain how much less oxygen is in a hypoxic area, than in an area with the normal concentration of dissolved oxygen.
  1. Which month has the longest period of anoxic conditions? ______Givea possible explanation using information from previous activities as support your ideas.
  1. Are there any months during which anoxic and hypoxic conditions don’t exist? ______Again explain why that makes sense using information from previous activities as support.
  1. Use the nitrogen cycle to explain why before human interference, there was significantly less nitrogen entering our water ways.
  2. If we could travel back in time to the days before Americans began to intensively settle and farm the Midwest – how do you think those graphs would be different?
  1. Which part of the nitrogen cycle can sewage treatment plants take advantage of to reduce the amount of nitrogen compounds in the water discharged from sewage treatment plants?
  1. Which part of the nitrogen cycle is responsible for taking the oxygen out of the water?
  1. 2012 was a significant drought year for the Midwest. How would you expect graphs from that year to differ from the graphs of the more typical year used in this assignment? Pick 2 months and draw what they might look like here:
  1. Imagine that you belong to the fishing industry in the Gulf of Mexico. What regulations would you want the government to put in place to help avoid the massive dead zone formation in the Gulf of Mexico?
  1. If you are a corn farmer in the mid-west, come up with several possible solutions to help reduce the amount of nitrogen compounds entering the Mississippi watershed.

Station DO content readings (mg/L) in 1993 August – November