Marine Science (9693) | Nutrient cycles

Welcome to Nutrient Cycles in Marine Ecosystems!

Hello future marine scientists! This chapter might seem a bit like chemistry mixed with biology, but it is fundamentally about how the ocean recycles the ingredients of life.

Why is this important? Because the availability of these ingredients—the nutrients—controls where marine life thrives and how productive the entire ocean is. Think of nutrients as the essential vitamins and minerals that keep the marine ecosystem running!

3.3 Defining Marine Nutrients

3.3.1 What Exactly is a Nutrient?

In Marine Science, a nutrient is a generic term for any substance required by an organism for essential biological functions, including:

• Energy supply
• Growth and repair of tissues
• Normal metabolism

3.3.2 & 3.3.5 Essential Elements and Their Roles

The most important nutrients in the ocean fall into different categories: gases, ions, and organic compounds. Organisms use these to build critical biological structures.

Key Nutrient Types (Ions and Gases)

Nutrients can include:

Ions:

• Nitrate ($\text{NO}_3^-$)
• Phosphate ($\text{PO}_4^{3-}$)
• Carbonate ($\text{CO}_3^{2-}$)
• Magnesium ($\text{Mg}^{2+}$)
• Calcium ($\text{Ca}^{2+}$)

Gases:

• Carbon Dioxide ($\text{CO}_2$)

Organic Compounds:

• Carbohydrates
• Lipids (fats/oils)
• Proteins

Biological Roles of Essential Elements

Each element plays a unique role:

• Nitrogen (N): Essential for building proteins, chlorophyll (for photosynthesis), and DNA. Nitrogen is often the key limiting factor in surface waters.
• Carbon (C): The backbone of all organic compounds (carbohydrates, lipids, proteins).
• Phosphorus (P): Used to make DNA and bones (or phosphate-based structures).
• Calcium (Ca): Vital for making bones, shells (like in molluscs), and coral skeletons.
• Magnesium (Mg): A core component necessary for chlorophyll production.

Quick Review: Think of N, P, and C as the "big three" nutrients that control phytoplankton growth!

3.3.3 & 3.3.4 Building the Big Molecules

Large biological molecules (polymers) are built from smaller units (monomers).

Analogy: Think of these molecules like building blocks (Legos). The small block is the monomer, and the finished castle is the large molecule.

The Elements that Make Us

Here are the elements found in the major organic compounds:

• Carbohydrates and Lipids contain: Carbon (C), Hydrogen (H), Oxygen (O).
• Proteins contain: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).
• (Note: Lipids and Proteins sometimes contain Phosphorus (P) or Sulfur (S), but C, H, O, N are the primary requirements.)

Monomers and Polymers (Small to Large)

The syllabus requires you to know how these large molecules are formed from small molecules:

1. Glucose (monomer) is linked together to form Starch and Cellulose (polymers/large carbohydrates).
2. Amino Acids (monomers) are linked together to form Proteins (polymers).
3. Fatty Acids and Glycerol (monomers) are linked together to form Lipids.

3.3.6 – 3.3.10 The Nutrient Reservoir: Sources, Sinks, and Removal

The Ocean Reservoir and Availability

Most essential nutrients are soluble in water. This means there is a huge, dissolved store or reservoir of nutrients readily available in the ocean to be taken up by producers (like phytoplankton) and consumers.

3.3.8 Depletion: This reservoir is constantly depleted (used up) in the surface layers by the uptake into organisms during primary production and growth. When nutrients are taken up, they become part of the living biomass.

3.3.7 How is the Reservoir Replenished? (The Sources)

If the surface nutrients are constantly used up, how do they return? Through several key processes:

1. Upwelling
This is perhaps the most important process.

1. Winds push surface water away from the coast or away from the equator.
2. Deeper, cold, nutrient-rich water rises to replace the surface water that moved away.
3. This deep water contains dissolved nutrients from decomposed organisms that sank earlier, effectively bringing "fertilizer" back to the surface layer.

2. Run-off
Rivers and streams carry nutrients (often from weathered rocks, agriculture, or sewage) from the land into the ocean. This process primarily replenishes coastal nutrient levels.

3. Excretion and Decomposition
When organisms excrete waste products (e.g., urea, ammonia) or when they die and their bodies break down (decomposition) due to bacteria, the organic material is broken back down into dissolved inorganic nutrients (like nitrate and phosphate). This recycling happens throughout the water column.

4. Tectonic Activity
Hydrothermal vents, created by tectonic plate activity, release superheated water rich in dissolved minerals and gases (like sulfide) from the Earth's crust into the deep ocean. While often fueling chemosynthesis, they also contribute to the overall nutrient reservoir.

5. Dissolving Atmospheric Gases
Gases like carbon dioxide ($\text{CO}_2$) and nitrogen ($\text{N}_2$) dissolve directly from the atmosphere into the surface layer of the ocean.

3.3.9 Transfer to the Deep Ocean: Marine Snow

How do nutrients get from the active surface layer down to the deep reservoir for storage and eventual upwelling?

Marine Snow is the continuous shower of organic material falling from the surface waters down to the abyssal zone.

• It consists of dead organisms, faecal pellets, and aggregates of smaller organic particles (like mucus).
• This process effectively transfers energy-containing organic material from the productive surface waters to the deep ocean, where it provides food for deep-sea communities.
• When this organic material reaches the bottom, it decomposes, replenishing the deep ocean's dissolved nutrient reservoir.

3.3.10 Removal by Harvesting

Nutrients that have been successfully taken up by organisms and integrated into the food chain can be removed from the cycle entirely by harvesting.

• Example: Humans catch and remove fish, marine mammals, or large algae.
• When these organisms are removed from the ocean, the essential elements (N, P, C, Ca) contained within their bodies are taken out of the marine ecosystem cycle.

3.3.11 Productivity Limitation

Why is Productivity Limited?

Productivity is the rate at which biomass (organic matter) is produced, usually by photosynthesis.

Productivity (and thus the growth of phytoplankton) may be limited by the availability of dissolved nutrients.

• In warm, surface waters (like the tropics), there is plenty of sunlight, but often a lack of mixing (no upwelling). Producers rapidly use up all the available nitrate and phosphate.
• Once these nutrients are gone, even if there is abundant light and warm temperatures, productivity stops. This is called nutrient limitation.
• Did you know? In many parts of the ocean, Nitrogen (often in the form of nitrate) is the main limiting factor for primary production.

3.3.12 The Carbon Cycle in the Ocean

How Carbon Moves Through the Marine Environment

Carbon is essential for all life, and the ocean holds huge quantities of it. The carbon cycle describes how carbon moves between the atmosphere, living organisms, the water, and rocks.

The syllabus requires you to know the following processes involving carbon:

1. Photosynthesis (Uptake)
Marine producers (like phytoplankton) absorb dissolved carbon dioxide ($\text{CO}_2$) from the water to convert it into organic carbon compounds (like glucose), using sunlight energy.

2. Respiration (Release)
All living organisms (producers and consumers) break down organic compounds (like glucose) for energy, releasing carbon dioxide ($\text{CO}_2$) back into the water.

3. Decomposition (Release)
When organisms die, decomposers (bacteria) break down their dead organic matter. This process releases carbon dioxide ($\text{CO}_2$) back into the water and eventually the atmosphere.

4. Combustion (Release)
The burning of organic matter (like wood or, critically, fossil fuels) releases large amounts of carbon dioxide ($\text{CO}_2$) into the atmosphere, which then dissolves into the ocean.

5. Formation of Fossil Fuels (Storage)
When dead organisms sink and are buried under sediment over millions of years, the organic carbon they contain can be transformed into oil and gas (fossil fuels), storing the carbon away from the active cycle.

6. Formation and Weathering of Carbonate Rocks (Storage and Release)

i. Formation (Storage): Marine organisms (corals, molluscs) use dissolved carbonate ions ($\text{CO}_3^{2-}$) and calcium to form shells and skeletons of calcium carbonate ($\text{CaCO}_3$). When they die, these shells settle and eventually compress to form sedimentary rocks like limestone, storing carbon long-term.

ii. Weathering (Release): When these rocks (either on land or exposed underwater) are broken down by chemical or physical processes (weathering), the carbonate ions and $\text{CO}_2$ are released back into the water, completing the long-term geological cycle.

Don't worry if this seems tricky at first! The key to the cycles is remembering that carbon is released by "burning" (combustion, respiration, decomposition) and taken up by "building" (photosynthesis, shell formation).

Key Takeaway: Nutrient cycling is the fundamental recycling process that keeps marine ecosystems productive. Upwelling is essential for replenishing surface nutrients that are otherwise quickly used up by primary producers.