Lesson 5: The Dead Zone
Chemical Oceanography
Last class we learned about the carbon cycle
What is a biogeochemical cycle? What are the primary components of the carbon
cycle? Besides carbon, what other cycling elements are
important to ocean life?
2
Today we will learn about another element that cycles through Earth’s systems
1. Nitrogen is an important element to marine life.
2. An excess of nitrogen can indirectly lead to a reduction of dissolved oxygen in the water.
3. Low oxygen conditions can result in large marine die-offs.
3
Welcome to the Dead Zone!
The Dead Zone is a region in the Gulf of Mexico that does not support marine life in surface waters
Approximate location shown here
Dead zone
Photo: NASA
Dead zone
4
Welcome to the Dead Zone!
This region cannot support life because there is little to no dissolved oxygen (DO) in the water
Similar zones occur throughout the world
What do you think might cause them?
Too many nutrients like nitrogen can cause them
5
How does nitrogen get in the ocean?
Fertilizers
Wastewater
treatment
Factories
Nitrogen is part of the biogeochemical cycle.
Human activities like those in the picture cause nitrogen toenter the water. For example:agriculture, industryand water treatment.
Let’s see why too much nitrogen reduces DO…
Atmosphere
Photo: USGS
6
Too much nitrogen causes plankton blooms
Nitrogen is important to marine life Plankton, marine organisms that drift through the
ocean (e.g. fish larvae, diatoms), use this nutrient for reproduction
Excess nitrogen from humanactivities allows plankton populationsto grow very rapidly, a phenomenoncalled a plankton bloom
Plankton
Photo: NOAA7
How do plankton blooms lead to dead zones?
When plankton die, they settle to the bottom and decompose
Bacteria use up oxygen as they decompose the plankton
DO
In hypoxic (low DO) or anoxic (no DO) environments, fish and other mobile organisms will leave
Those organisms that cannot or do not leave will die off
Plankton
8
During the student activity, you will examine data from the ocean to analyze oxygen content
Dead zone activity
9
Top Related