What is dissolved oxygen (DO)?

Question

a) Oxygen chemically bonded to hydrogen in water
b) Oxygen molecules trapped in the water
c) A type of gas emitted by marine animals
d) Oxygen bonded with carbon dioxide

Answer 1

Dissolved oxygen (DO) refers to the amount of oxygen that is present in water and is available for aquatic organisms to use for respiration. It is not chemically bonded to water but exists as free oxygen molecules suspended in the water.

The correct choice is: b) Oxygen molecules trapped in the water

Answer 2

Which of the following factors increases dissolved oxygen levels in water?
a) Increased water temperature
b) Decreased water flow
c) Photosynthesis by aquatic plants
d) Pollution from chemicals

Answer 3

The factor that increases dissolved oxygen levels in water is:

c) Photosynthesis by aquatic plants

During photosynthesis, aquatic plants and algae produce oxygen as a byproduct, which increases the amount of dissolved oxygen in the water. In contrast, increased water temperature and decreased water flow generally decrease dissolved oxygen levels, and pollution often negatively affects oxygen levels.

Answer 4

Which method is commonly used to measure dissolved oxygen in water?
a) Secchi disk
b) Colorimeter
c) Winkler titration method
d) Turbidity meter

Answer 5

The method commonly used to measure dissolved oxygen in water is:

c) Winkler titration method

The Winkler method is a classic titration technique that involves adding reagents to a water sample to determine its dissolved oxygen concentration. Colorimeters can also be used for measuring dissolved oxygen but the Winkler method is more traditional and widely recognized for this purpose.

Answer 6

Which of the following negatively impacts low dissolved oxygen levels in water?
a) Increased fish population
b) Eutrophication
c) Death of aquatic organisms
d) Improved water clarity

Answer 7

The option that negatively impacts low dissolved oxygen levels in water is:

b) Eutrophication

Eutrophication is the process where water bodies become enriched with nutrients (often due to runoff from fertilizers), leading to excessive growth of algae. When the algae die and decompose, it consumes a significant amount of dissolved oxygen in the water, thereby reducing the levels available for aquatic organisms and potentially leading to hypoxic conditions (low oxygen levels). Increased fish population and the death of aquatic organisms can be a consequence of low dissolved oxygen rather than a direct cause, while improved water clarity typically does not contribute negatively to dissolved oxygen levels.

Answer 8

What is the typical range of dissolved oxygen in healthy freshwater ecosystems?
a) 0-2 mg/L
b) 4-6 mg/L
c) 6-12 mg/L
d) 12-16 mg/L

Answer 9

The typical range of dissolved oxygen in healthy freshwater ecosystems is:

c) 6-12 mg/L

Healthy freshwater ecosystems generally require dissolved oxygen levels within this range to support the diverse aquatic life, including fish and invertebrates. Levels below 4 mg/L can be stressful for many aquatic organisms, while levels above 12 mg/L can be found in very oxygen-rich conditions.