Marine Science (0697) | Open-ocean ecosystem

🌊 Your Open-Ocean Ecosystem Study Notes (Marine Ecology)

Welcome to the most massive ecosystem on Earth—the open ocean! Don
’t worry if the deep sea seems intimidating; we will break down this vast area into smaller, manageable layers. This chapter is vital because the open ocean drives global processes like nutrient cycling and provides the foundation for most marine food webs. Let's dive in!

1. Defining the Zones: Pelagic vs. Benthic

The open ocean is traditionally divided into two main components based on location:

The Pelagic Zone

This is simply the whole column of open water, away from the bottom or the shore. Think of it as everything floating or swimming above the seabed. Organisms here are called pelagic species (like tuna or jellyfish).

The Benthic Zone

This refers to the seabed or the bottom surface of the ocean. Organisms living here are called benthic species (like flatfish, crabs, or clams).

2. Vertical Layers of the Open Ocean (The Three Zones)

The open-ocean water column (the Pelagic Zone) is further divided into three vertical zones, mainly determined by light penetration.

Imagine you are traveling down in a specialized deep-sea elevator. Here are the three floors you will stop at:

1. The Sunlight Zone (or Epipelagic Zone)

   • Depth: Between 0 m and 200 m.
   • This is the surface layer where photosynthesis occurs.

2. The Twilight Zone (or Mesopelagic Zone)

   • Depth: Between 200 m and 1000 m.
   • Some light filters down, but not enough for photosynthesis.

3. The Midnight Zone (or Bathypelagic Zone)

   • Depth: The region of the ocean below 1000 m.
   • Complete darkness.

Section A: The Sunlight Zone (0–200 m)

3. Conditions and High Productivity

This zone is the engine of the entire ocean food web.

Key Conditions in the Sunlight Zone:

• Light available: Plenty of light for photosynthesis.
• Dissolved Oxygen: High content (due to contact with the atmosphere and photosynthetic organisms).
• Biomass: High (because of the abundant producers).
• Pressure: Relatively low pressure (compared to deeper zones).
• Temperature: Variable temperature (it changes depending on latitude, season, and time of day).

Why does the Sunlight Zone have the highest Biomass?

Biomass is the total mass of living organisms. Since sunlight is available, photosynthetic organisms (the producers) can flourish. Producers form the base of the food chain, allowing a massive population of consumers to thrive above them. Therefore, high light = high productivity = high biomass.

4. Life in the Sunlight Zone: Plankton

Plankton is a collective term for organisms that drift in water currents. They form the base of the open-ocean food web.

There are two main types of plankton:

Phytoplankton (Producers)

• They are microscopic producers (meaning they photosynthesize).
• Examples include diatoms (with silica shells), dinoflagellates, and cyanobacteria.
• They are responsible for most of the primary production in the ocean.

Zooplankton (Consumers)

• They are consumers (they eat phytoplankton or other zooplankton).
• They include tiny organisms like copepods, larvae of fish and invertebrates, and larger drifters like jellyfish.

Section B: The Twilight and Midnight Zones (200 m and below)

As we sink below 200 m, the environment becomes harsher, colder, and darker.

5. Conditions in the Twilight Zone (200–1000 m)

In this zone, there is some very dim light, but not enough for producers to photosynthesize effectively.

• Light availability: Low light availability.
• Dissolved Oxygen: Some dissolved oxygen remains.
• Temperature: Stable temperature (and usually quite low, dropping rapidly after the surface layer).
• Pressure: Higher pressure (it increases by about 1 atmosphere for every 10 meters of depth).

6. Conditions in the Midnight Zone (>1000 m)

This is the true abyss—an extreme environment.

• Light availability: No light (complete darkness).
• Dissolved Oxygen: Little dissolved oxygen.
• Temperature: Very stable low temperature (near freezing).
• Pressure: Very high pressure.

Section C: Adaptations of Open-Ocean Species

7. Adaptations in the Sunlight and Twilight Zones

Organisms here often need speed and stealth to survive, as there is little physical shelter.

• Migration: Many species (like plankton, squid, shrimp, and some fish) are migratory. They undertake daily vertical migration, coming up to the sunlight zone at night to feed, and retreating to the darker twilight zone during the day to avoid visual predators. (This is for food and predator avoidance.)
• Food Source: Species are able to use a wide range of food sources, maximizing opportunities in a nutrient-poor environment.
• Speed: Fish (like tuna) are fast swimmers as there is no shelter available to hide.
• Coloration: Many use counter-shading for predator avoidance. They are dark on top (blending with the dark depths when viewed from above) and light on the bottom (blending with the light surface when viewed from below).

8. Adaptations of Deep-Sea Species (Midnight Zone)

Life below 1000m requires specialized features to handle extreme pressure, darkness, and scarce resources.

• Bioluminescence: The production of light by a living organism. It is used to attract mates or prey, or sometimes to illuminate predators to scare them off (like a burglar alarm).
• Color: Organisms are often dark brown or black. This makes them almost invisible, even against the light produced by bioluminescence, which is usually blue.
• Metabolism: They are typically slow moving with a long lifespan due to the low oxygen and extremely stable low temperature, which slows down chemical reactions (metabolism).
• Body Structure: Many have gelatinous bodies (like jellyfish or deep-sea squid) to cope with the high pressure without being crushed.
• Feeding: Many fish species have large backward-facing teeth and massive mouths. Once prey is caught, the backward-facing teeth ensure it cannot escape. They also use a wide range of food sources.

9. Adaptations of Benthic Species (Seabed Life)

These organisms live on or in the seabed (the benthic zone). Their adaptations focus on hiding and surviving on the substrate.

• Hiding: Many have the ability to camouflage on, or burrow into, the seabed to avoid predation.
• Flatfish (e.g., Flounder, Sole): These fish lie flat on one side. During development, one eye moves across the head so both eyes are on the same side. This allows them to bury themselves while still being able to look for predators or prey.
• Skates and Rays: They are dorsally flattened (flat from top to bottom) with elongated pectoral and pelvic fins, making them look like a diamond or circle shape, which helps them stay low and concealed.
• Invertebrates: Some, like sea stars or sea urchins (echinoderms), use legs or tube feet on their ventral surface (underside) to move and often have a hard spiny covering on the dorsal surface for protection.

Key Takeaway for Open Ocean: The availability of light is the single most important factor determining the conditions, productivity (biomass), and adaptations found in the different open-ocean zones.