🌊 Marine Science Study Notes: Effects of Increasing Depth (Syllabus 2.5)

Hello future marine biologists! This chapter is fascinating because we are diving into the abyss—the hidden world beneath the waves. Understanding how conditions change as you go deeper into the ocean is essential, as these changes dictate where and how marine organisms can survive. Don't worry if the deep sea seems daunting; we'll break down these environmental factors one by one!

This topic explores how five crucial physical and chemical conditions in the ocean change as the depth increases.

1. The Five Key Conditions That Change with Depth (2.5.1)

When you move from the surface down to the deep ocean, the environment rapidly transforms. The syllabus requires you to know the five major conditions that are affected:

1. Light penetration
2. Pressure
3. Temperature
4. Salinity (The concentration of dissolved salts)
5. Dissolved Oxygen (DO)

Analogy: Think about going to the top of a very tall mountain. The air changes (less oxygen), the temperature drops, and the pressure changes. The ocean is similar, but far more extreme!

2. Light Penetration (2.5.2 & 2.5.3)

Light is the energy source for almost all life in the surface ocean. As soon as sunlight hits the water, it starts being absorbed.

How Light Changes with Depth

The penetration of light decreases rapidly with depth.

Explanation:

  • The water itself absorbs light energy.
  • Particles and organisms suspended in the water scatter and absorb light.

This rapid decrease creates distinct zones:

1. Sunlight Zone (Photic Zone or Epipelagic Zone): This is the surface layer (typically 0m to 200m). Enough light penetrates here to allow for photosynthesis (the process where producers like phytoplankton make food).

2. Twilight Zone (Dysphotic Zone): Below 200m. Some light is present, but it is too dim for photosynthesis. Organisms here may use this faint light for vision.

3. Midnight Zone (Aphotic Zone): Below 1000m. No sunlight penetrates at all. The only light present is produced by the organisms themselves (bioluminescence).

Measuring Light Penetration: The Secchi Disc (2.5.3 PA)

A simple way scientists measure how deep light goes is by using a Secchi disc.

What it is: A flat, circular disc (usually 30 cm in diameter) painted with alternating black and white quadrants.

How it works:

1. The disc is lowered into the water attached to a measuring line.

2. The depth at which the observer can no longer see the disc is recorded.

3. The disc is lowered slightly further and then slowly raised.

4. The depth at which the observer first sees the disc reappear is recorded.

5. The average of these two depths (disappearing and reappearing) is the Secchi depth. This depth is an inverse measure of the water's clarity and light penetration.

Key Takeaway: Light is highly available at the surface but disappears almost entirely below 1000m, greatly limiting plant life.

3. Pressure (2.5.4)

Imagine standing underneath a huge stack of water. That weight pressing down causes pressure.

How Pressure Changes with Depth

Pressure increases continuously and significantly with increasing depth.

Explanation:

Pressure is caused by the weight of the water column (and the atmosphere) pressing down.

  • At the surface (sea level), the pressure is approximately 1 atmosphere (atm).
  • For every 10 meters of depth you add, the pressure increases by approximately 1 atm.

This means that at the bottom of the Mariana Trench (approx. 11,000m), the pressure is over 1,000 times greater than at the surface!

Why this is challenging for life:

Organisms living deep down need special adaptations, often having flexible, gelatinous bodies or internal pressures balanced exactly with the external pressure to avoid being crushed.

Memory Trick: P is for Pressure, and P is for Pushing down. More depth means more pushing!

Key Takeaway: Pressure increases at a constant and high rate (1 atm per 10m) as depth increases.

4. Temperature (2.5.5)

Temperature changes dramatically in the first few hundred meters, but then becomes incredibly stable.

How Temperature Changes with Depth

Temperature decreases sharply in the mid-layer, leading to constant cold temperatures in the deep ocean.

We can divide the water column into three temperature layers:

1. Surface Layer (Mixed Layer):

• Heated by the sun.
• Mixed by wind and waves.
• Temperature is variable (changes seasonally and daily).

2. Thermocline:

• A layer where the temperature drops rapidly over a short change in depth.
• This layer separates the warm surface water from the cold deep water.

3. Deep Ocean Layer:

• Below the thermocline (typically below 1000m).
• Temperature is very low (often near freezing, around 4°C).
• Temperature is extremely stable (does not change seasonally or yearly).

Did you know? Because cold water is denser than warm water, the colder water naturally sinks and remains at the bottom, creating a stable, cold layer across the entire world ocean.

Key Takeaway: Temperature drops fast in the thermocline and remains cold and stable in the deep water.

5. Salinity and Dissolved Oxygen (2.5.1 & 2.5.6)

Salinity (Concentration of Dissolved Salts)

While light, pressure, and temperature show huge changes, salinity is relatively stable in the open ocean, especially below the surface.

Surface Variation: Salinity can fluctuate slightly near the surface due to:

• High Evaporation: Makes water saltier (higher salinity).
• High Precipitation/Run-off: Dilutes the water (lower salinity).

Deep Ocean Stability: Below the mixed layer, salinity is very stable, typically around 35 parts per thousand (ppt).

Dissolved Oxygen (DO) Concentration (2.5.6)

The concentration of dissolved oxygen (DO) changes significantly, creating a specific pattern with depth.

DO Profile: Step-by-Step

Step 1: The Surface (High DO)

• Water is in contact with the atmosphere, allowing oxygen to diffuse into the water.
• Producers (phytoplankton) are actively photosynthesizing, releasing oxygen.

Step 2: Mid-Depth (Oxygen Minimum Zone, OMZ)

• Around 500m to 1000m.
• Oxygen concentrations drop sharply to a minimum.
• Why? Respiration by marine organisms and decomposers consumes oxygen, but there is no light for photosynthesis to replace it, and the deep, cold, oxygen-rich water hasn't mixed up yet.

Step 3: The Deep Ocean (Slightly Higher DO)

• DO concentration increases slightly again.
• Why? The primary source of oxygen here is deep, cold water that has sunk from the polar regions (this process is called the global conveyor belt). This cold water originally absorbed oxygen at the surface and carries it to the deep sea. Furthermore, there are fewer organisms at this depth, meaning less respiration to use up the oxygen.

Key Term: The Oxygen Minimum Zone (OMZ) is the region of lowest oxygen concentration, typically found between the high-oxygen surface and the slightly higher-oxygen deep layer.

Quick Review Box:

Quick Summary of Depth Effects

Light: Decreases rapidly (Total darkness below 1000m)
Pressure: Increases constantly (1 atm per 10m)
Temperature: Decreases rapidly in the thermocline, then remains cold and stable.
Salinity: Stable, with slight variations near the surface.
Dissolved Oxygen: High at the surface, lowest in the OMZ, then increases slightly in the deep sea.