Study Notes: Dissolved Oxygen, Oxygen Minimum Layer, Density, and Thermocline

Oxygen Levels and the Oxygen Minimum Layer

  • Oxygen levels change a lot below the ocean's surface.

  • As you go deeper, the amount of dissolved oxygen (DO) goes down until it reaches the Oxygen Minimum Layer.

  • This layer is usually around 500 m500\ \mathrm{m} deep, but can be anywhere from 100−1000 m100-1000\ \mathrm{m} depending on the location.

  • In the Oxygen Minimum Layer, DO levels are almost 00 because very little new oxygen gets in, and organisms are still using it up through respiration.

Organisms in the Oxygen Minimum Layer

  • Some creatures can live here because they have special adaptations.

  • These organisms are not very active, so they need less oxygen.

  • Fish in this layer have gills that are very good at getting oxygen from water, even when there's not much.

  • They also have special hemoglobin (a protein in blood that carries oxygen).

Dissolved Oxygen Levels

  • The more wave action there is, the more oxygen gets mixed into the water.

  • Photosynthesis, which makes oxygen, only happens in the upper ocean layer where light can reach (the photic zone).

  • Photosynthesis increases DO, while respiration (done by all living things) decreases DO.

  • DO levels also change with temperature. For example, warm tropical waters have less DO than cold polar waters, where more oxygen can dissolve.

After the Oxygen Minimum Layer

  • Below the Oxygen Minimum Layer, DO levels actually start to go up again with increasing depth.

  • Bacteria that break down dead organic material use oxygen for aerobic respiration.

  • Organisms deeper than the Oxygen Minimum Layer need less food (since there's less available), so they survive with less oxygen because they respire less.

  • Colder temperatures mean more DO.

  • Higher pressure also increases how much oxygen can dissolve in water.

DENSITY AND PRESSURE - Chapter 1 Water
Density

  • Density is how much "stuff" (mass) is packed into a certain space (volume) of water.

  • Formula: Density=mV\text{Density} = \frac{m}{V}

  • Units example: Density (kg cm−3^{-3}) = mass (kg) / volume (cm3^{3}).

Density Formula (alternate forms)

  • Density formulae:

    • ρ=mV\rho = \frac{m}{V}

    • m=ρ Vm = \rho\,V

    • V=mρV = \frac{m}{\rho}

  • These formulas show how density, mass, and volume are related.

Density in practice

  • Denser (heavier) water sinks, while less dense (lighter) water floats to the surface.

  • Partner Discussion prompts: Which color is more dense? Which color is less dense? How do you know?

What Determines Density of Seawater

  • Temperature

  • Salinity (amount of salt)

  • Pressure

Temperature

  • Temperature is the main factor that changes water's density!

  • As temperature goes up, density goes down.

  • Warmer water floats near the surface because the sun heats it, making it even less dense.

  • This warm surface layer is usually shallow and sits on top of colder, denser water.

  • The area between the warmer surface water and the colder deep water is called the thermocline.

  • Thermocline in a lake (mentioned in the lecture).

Thermocline

  • Thermocline: A place where the temperature changes very quickly.

  • Surface water in the tropics might reach 25∘C25^{\circ}\mathrm{C} or higher, but in the thermocline, it could drop to 1∘C1^{\circ}\mathrm{C}.

  • In polar seas, the temperature change with depth (the gradient) is not as sharp.

  • Partner Discussion - Why do you think this is so?

  • In polar areas, the surface water is already close to freezing, and the temperature stays fairly constant as you go deeper.