Marine Science (9693) AS Level Study Notes: Chapter 3 – Interactions

Welcome to the "Interactions" chapter! This section is vital because it explains how marine life survives—who eats whom, how organisms help or harm each other, and how the essential building blocks (nutrients) are cycled through the ecosystem. Understanding these relationships is the foundation for studying marine ecology!

3.1 Symbiotic Interactions

In a marine ecosystem, many different species live in close association with one another. When two different species live in a close physical relationship, it is called a symbiotic relationship (or symbiosis).

There are three main types of symbiosis, categorized by who benefits and who is affected (harmed, helped, or neutral).

Types of Symbiosis

  1. Mutualism (The Win-Win: + / +)

    Definition: Both organisms (the host and the symbiont) benefit from the relationship.
    Analogy: It’s like two friends helping each other with homework—you both get better grades!

    Example: Boxer crabs and anemones.
    The boxer crab carries small, stinging anemones in its claws. The anemones get transport (helping them catch food) and the crab uses the anemones for defense against predators. Both benefit.


  2. Commensalism (The One-Sided Benefit: + / 0)

    Definition: One organism (the symbiont) benefits, while the other organism (the host) is neither helped nor harmed.
    Analogy: Imagine riding on a public bus. You benefit from the transport, but the bus itself (the host) is unaffected by your presence.

    Example: Whales and barnacles.
    Barnacles attach themselves to the skin of a whale. They benefit by getting free transport to nutrient-rich areas and filter feeding opportunities. The whale is generally unaffected (though very heavy barnacle loads might slightly increase drag, this is usually negligible).


  3. Parasitism (The Takeover: + / -)

    Definition: One organism (the parasite/symbiont) benefits at the expense of the other organism (the host), which is harmed.
    Common Mistake: A parasite doesn't want to kill the host immediately, because then it loses its food source. Parasites usually just weaken the host.

    Example: Copepods and marine fish.
    Copepods (tiny crustaceans) can attach to marine fish (the host) and feed off their blood, tissues, or mucus. This harms the fish by draining energy and potentially causing infection or disease.

Quick Review: Symbiosis

Mutualism: Make friends (both benefit).
Commensalism: Chill and ride (one benefits, one is neutral).
Parasitism: Painful experience (one benefits, one is harmed).

3.2 Feeding Relationships

The flow of energy through an ecosystem is determined by who eats whom. These relationships define the structure and stability of marine communities.

Key Terms in Feeding Relationships (Trophic Levels)

  • Trophic Level: The position an organism occupies in a food chain.
  • Producer: Organisms that produce their own food (organic substances) from inorganic substances, using energy from sunlight (photosynthesis) or chemicals (chemosynthesis). They form the base of the food chain.
  • Consumer: Organisms that obtain energy by feeding on other organisms.
    • Primary Consumer: Eats producers (always a herbivore).
    • Secondary Consumer: Eats primary consumers (can be a carnivore or omnivore).
    • Tertiary Consumer: Eats secondary consumers.
    • Quaternary Consumer: Eats tertiary consumers.
  • Herbivore: Eats plants/algae (producers).
  • Carnivore: Eats other animals.
  • Omnivore: Eats both plants/algae and animals.
  • Predator: An animal that hunts and kills other animals (prey) for food.
  • Decomposer: Organisms (like bacteria and fungi) that break down dead organic matter and waste, recycling nutrients back into the ecosystem.

Food Chains and Food Webs

A food chain shows the direct path of energy transfer (e.g., Phytoplankton \(\rightarrow\) Zooplankton \(\rightarrow\) Sardine \(\rightarrow\) Tuna).
A food web shows the complex, interconnected feeding relationships among all organisms in an ecosystem. Food webs are much more realistic than food chains.

Producers: Photosynthesis vs. Chemosynthesis

Producers convert inorganic carbon (like CO₂) into usable organic compounds (biomass).

Photosynthesis

This process uses the energy from sunlight to create glucose. It is the basis of most marine food chains (especially in the surface zones).

Word Equation:
carbon dioxide + water \(\xrightarrow{\text{light/chlorophyll}}\) glucose + oxygen

What happens to the glucose?

  1. Some glucose is used immediately in respiration to release usable energy (ATP) for the producer's survival.
  2. The rest is used to create biomass (growth and repair).
Respiration

This process releases energy from glucose (organic compounds), making it available to all organisms (producers and consumers).

Word Equation:
glucose + oxygen \(\rightarrow\) carbon dioxide + water ( + usable energy/ATP)

Chemosynthesis

In environments without light (like deep-sea hydrothermal vents), producers use chemical energy from dissolved inorganic substances (e.g., hydrogen sulfide, methane, iron) to fix carbon and produce organic material. These chemosynthetic bacteria support entire ecosystems where sunlight never reaches.

Productivity and Energy Transfer

Productivity

Definition: Productivity is the rate of production of biomass per unit area or volume per unit of time.
High primary productivity (a fast rate of biomass production by producers) means more energy is available for primary consumers, influencing the size of the whole food chain.

Energy Loss in Food Chains

Energy transfer between trophic levels is highly inefficient.

  • Only about 10% of the energy from one trophic level is successfully converted into biomass at the next level.
  • The remaining 90% is lost, mainly as:
    • Heat during respiration.
    • Uneaten parts (e.g., bone, shell).
    • Waste materials (excretion and egestion).

Think of it like money: If you earn $100 (energy), you have to pay $90 in taxes and bills (losses), leaving you only $10 to save (biomass for the next level).

Ecological Pyramids

Ecological pyramids graphically represent the structure of an ecosystem.

  1. Pyramid of Energy

    Shows the rate of energy flow at each trophic level (e.g., kJ m⁻² yr⁻¹). It must always be upright because energy is lost at every transfer, and therefore cannot decrease as you move up the food chain.

  2. Pyramid of Biomass

    Shows the total dry mass of organisms at each level (e.g., g m⁻²). While usually upright, it can sometimes be inverted (upside down).
    Example: During an algal bloom, the primary consumers (zooplankton) might briefly have a greater total biomass than the producers (phytoplankton) because the tiny phytoplankton reproduce and are eaten so rapidly. The *rate* of production is high, even if the standing *mass* is low.

  3. Pyramid of Numbers

    Shows the total number of individual organisms at each level. This pyramid is often upright but can be inverted or irregular.
    Example of Irregularity: One large tree (producer) supports thousands of insects (primary consumers).
    Including Parasites: Since one large host can be parasitized by millions of microscopic parasites, including parasites often makes the pyramid of numbers inverted.

Key Takeaway: Energy Flow

Energy flows (is lost) along the food chain, while nutrients cycle within the ecosystem. The low efficiency of energy transfer (10% rule) means marine food chains usually have a maximum of 4 or 5 trophic levels.

3.3 Nutrient Cycles

Nutrients are substances required by organisms for growth, repair, energy, or normal metabolism. Their availability fundamentally limits the size and type of populations in the ocean.

The Chemical Building Blocks

Nutrients include gases (like CO₂) and ions (like NO₃⁻ and PO₄³⁻).

Essential Elements and their Roles
  • Nitrogen (N): Essential for making proteins, DNA, and chlorophyll.
  • Carbon (C): Used to make all organic compounds (carbohydrates, lipids, proteins).
  • Phosphorus (P): Essential for making DNA and bones/teeth.
  • Calcium (Ca): Used to make bones, shells (e.g., molluscs), and coral skeletons (calcium carbonate).
  • Magnesium (Mg): Used to make chlorophyll (vital for photosynthesis).

The Nutrient Reservoir

The main reservoir of nutrients is the large pool of these substances dissolved in the ocean water. This dissolved reservoir is readily available for uptake by producers (like phytoplankton).

Processes Affecting Nutrient Availability

1. Nutrient Replenishment (Adding Nutrients)

The dissolved nutrient reservoir is constantly being topped up by:

  1. Run-off: Nutrients (like nitrates and phosphates) are washed from terrestrial (land) ecosystems into the ocean via rivers and streams.
  2. Upwelling: Deep ocean water is rich in nutrients (because dead organic matter sinks and decomposes there). Upwelling is the movement of this deep, cold water up to the surface, making those nutrients available to surface producers.
  3. Tectonic Activity: Hydrothermal vents release hot, nutrient-rich dissolved minerals from the Earth's crust directly into the deep ocean.
  4. Dissolving of Atmospheric Gases: Gases like carbon dioxide (CO₂) dissolve directly from the atmosphere into the surface water.
  5. Excretion and Decomposition: Waste products (excretion) and the decay of dead organisms (decomposition) by bacteria release nutrients back into the water column.
2. Nutrient Depletion (Removing Nutrients)

Nutrients are removed from the surface reservoir primarily by:

  • Uptake into Organisms: Producers absorb dissolved inorganic nutrients to build their biomass. These nutrients are then transferred up the food chain to consumers.
  • Sinking of Organic Matter: When organisms die or excrete waste, the organic matter sinks to the deep ocean, effectively removing the nutrients from the photosynthetic surface layer.
  • Harvesting: Nutrients are removed from the marine ecosystem when humans catch fish or shellfish and take them away (harvesting).

Marine Snow

Marine snow refers to the continuous shower of organic material (mostly dead plankton, faecal pellets, and mucus) falling from the surface waters down to the deep ocean. This is the crucial mechanism by which energy-containing organic material is transferred from the primary producers in the epipelagic zone to the deep-sea ecosystems.

Limitation of Productivity

Productivity may be limited by the availability of dissolved nutrients.
In surface waters, producers quickly use up essential nutrients like nitrate (NO₃⁻) and phosphate (PO₄³⁻). Even if sunlight is abundant, the ecosystem cannot produce more biomass without these key nutrients. This is why upwelling areas (which bring up deep, nutrient-rich water) are areas of exceptionally high productivity.

The Carbon Cycle (Marine Focus)

The carbon cycle describes how carbon moves between the atmosphere, oceans, rocks, and living organisms.

Key Processes Involving Carbon:

  1. Photosynthesis: Producers fix atmospheric or dissolved CO₂ into glucose (organic carbon).
  2. Respiration: All organisms break down organic carbon (glucose) and release CO₂ back into the water/atmosphere.
  3. Decomposition: Decomposers break down dead organic matter, releasing CO₂.
  4. Combustion: Burning fossil fuels releases CO₂ into the atmosphere.
  5. Formation/Weathering of Rocks: Carbonate ions (CO₃²⁻) are used by organisms (like corals and molluscs) to form shells and skeletons (calcium carbonate). Over geological time, these form carbonate rocks. The dissolution (weathering) of these rocks releases CO₂/carbonate back into the system.
  6. Oceanic Sink: The ocean acts as a massive carbon sink, absorbing vast amounts of atmospheric CO₂ by dissolving it into the water.
Key Takeaway: Nutrient Cycles

Nutrients are not unlimited! They must be constantly recycled or replenished (especially through upwelling) to maintain high productivity in the marine environment.