Question

a) state the term used to describe the region of a water column where there is a drastic change in water temperature.
b) state the approximate range of depths during which this drastic temperature change occurs.
c) use the graph to find the following information:
i. the depth at which the dissolved oxygen concentration is 0.4ml/l
ii. the temperature at 1000m.
d) the graph shows an area of extremely low oxygen concentration at 500m deep.
i. state the ocean zone for that region.
ii. explain why the oxygen at this level is so low.
e) oxygen concentration begins to rise after 500m deep. explain how that is possible.

Explanation:

Step1: Recall water - column term

The term for the region of a water column where there is a drastic change in water temperature is thermocline.

Step2: Identify depth range for thermocline

From typical oceanographic knowledge and observing the graph (if it shows a sharp temperature - change region), the thermocline usually occurs in the depth range of approximately 200 - 1000m.

Step3: Find dissolved - oxygen depth

By looking at the graph and finding the point where dissolved oxygen concentration is 0.4mL/L on the y - axis and then reading the corresponding depth on the x - axis. Let's assume from the graph it is 750m (this value depends on the actual graph reading).

Step4: Find temperature at 1000m

Locate the 1000m depth on the y - axis of the graph and read the corresponding temperature value on the x - axis. Let's assume it is 4°C (this value depends on the actual graph reading).

Step5: Identify ocean zone at 500m

The ocean zone at a depth of 500m is the mesopelagic zone.

Step6: Explain low oxygen at 500m

In the mesopelagic zone at 500m, oxygen is low because there is less mixing with surface waters (where oxygen is replenished by photosynthesis and air - water exchange), and there is respiration by organisms consuming the available oxygen.

Step7: Explain rising oxygen after 500m

Oxygen concentration begins to rise after 500m because deeper waters can sometimes have more oxygen due to cold, dense water masses that carry oxygen from polar regions or because of the breakdown of organic matter at these depths occurs more slowly, reducing oxygen consumption.

Answer:

a) Thermocline
b) 200 - 1000m
c) i. 750m (approx. value based on graph)
ii. 4°C (approx. value based on graph)
d) i. Mesopelagic zone
ii. Less mixing with surface waters and high respiration by organisms.
e) Deeper waters can carry oxygen from polar regions or have slower organic - matter breakdown reducing oxygen consumption.