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? 2

3. 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

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 Photo: NASA Dead zone 4

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 5

6. 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 to enter the water. For example: agriculture, industry and water treatment. Let’s see why too much nitrogen reduces DO… Atmosphere Photo: USGS 6

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 Plankton Photo: NOAA 7

8. 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

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