1. Lesson 5: The Dead Zone Chemical Oceanography

2. 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?
Teacher’s note:
The biogeochemical cycle is the continuous flow of elements and compounds between organisms and the earth
The biogeochemical cycle involves the movement of elements and compounds among the land (lithosphere), organisms, air (atmosphere) and the oceans (hydrosphere).
Carbon (C), Nitrogen (N), Phosphorus (P), Silicon (Si), Iron (Fe), and Trace metals are all essential to marine life.
Phytoplankton use CO2 for photosynthesis; Some CO2 is given back off through respiration,
As phytoplankton die and decompose, carbon settles to the deep ocean
Carbon moves up the food chain as primary consumers like zooplankton eat phytoplankton
Carbon moves further up to secondary and tertiary consumers

3. Today we will learn about another element that cycles through Earth’s systems
Nitrogen is an important element to marine life.
An excess of nitrogen can indirectly lead to a reduction of dissolved oxygen in the water.
Low oxygen conditions can result in large marine die-offs.

4. 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
Dead zone
Photo:
Accessed: November 2010
Photo: NASA

5. 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

6. How does nitrogen get in the ocean?
Atmosphere
Nitrogen is part of the biogeochemical cycle.
Human activities like those in the picture cause nitrogen to
enter the water. For example:
agriculture, industry
and water treatment.
Let’s see why too much nitrogen reduces DO…
Factories
Fertilizers
Wastewater treatment
Photo:
USGS, Accessed: November 2010
Photo: USGS

7. 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 human activities allows plankton populations to grow very rapidly, a phenomenon called a plankton bloom
Photo:
Accessed: November 2010
Plankton
Photo: NOAA

8. How do plankton blooms lead to dead zones?
Those organisms that cannot or do not leave will die off
In hypoxic (low DO) or anoxic (no DO) environments, fish and other mobile organisms will leave
Plankton
When plankton die, they settle to the bottom and decompose
Bacteria use up oxygen as they decompose the plankton DO

9. Dead zone activity
During the student activity, you will examine data from the ocean to analyze oxygen content