Marine Science (0697) | Eutrophication

Marine Science (0697) Study Notes: Eutrophication (Section 6.7)

Welcome to Eutrophication!

Hi there! This chapter might sound complicated, but it’s really about what happens when the ocean gets *too much* of a good thing—nutrients! We are studying how human activities on land can drastically change the marine environment, leading to the formation of underwater "dead zones." Understanding this process is key to seeing how our actions influence marine ecology.

Eutrophication is defined simply as the excessive richness of nutrients in a body of water, frequently due to run-off from the land, which causes a dense growth of plant life (producers) and the eventual death of animal life from lack of oxygen.

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6.7 Sources of Excess Nutrients

Eutrophication doesn't happen naturally very quickly; it’s usually caused by human activity adding massive amounts of nutrients to coastal waters. These nutrients act like super-strong fertiliser for marine producers.

A. Agricultural Fertilisers (6.7.1 & 6.7.2)

Modern farming uses fertilisers to improve crop yields. These chemicals are rich in essential elements that plants need for rapid growth, particularly nitrogen (N) and phosphorus (P).

• The Problem: Fertilisers are often water soluble.
• The Movement: When it rains, the excess fertiliser not absorbed by crops is washed into rivers and drainage systems. This movement of chemicals from land into waterways is called run-off.
• The Destination: This nutrient-rich run-off eventually flows into the sea, usually in coastal areas or estuaries.

B. Untreated Sewage (6.7.4)

Another major source of excess nutrients is untreated sewage (wastewater). Sewage is full of organic waste products from humans, which are rich in nitrogen and phosphorus. If this sewage is released directly into the ocean or rivers without proper treatment, it immediately increases nutrient availability, leading directly to eutrophication.

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6.7 The Step-by-Step Process of Eutrophication

The explanation of the process (6.7.5) is critical for exam success. Memorise these steps in order!

Step 1: Nutrient Overload (The Trigger)

When run-off and sewage enter the water, the availability of nutrients (especially nitrogen and phosphorus) suddenly and rapidly increases. Previously, these nutrients might have been the limiting factors for producer growth.

Step 2: Algal Bloom (Increased Growth of Producers)

The producers in the water, such as microscopic phytoplankton (algae), use these abundant nutrients to grow and reproduce extremely quickly. This results in a massive population explosion known as an algal bloom (a thick layer of algae covering the surface).

• These blooms often block sunlight from reaching deeper water, killing submerged plants like seagrasses, which also use oxygen at night.

Step 3: Death and Decomposition (The Crash)

Because the bloom is so dense, the algae eventually die, either because they run out of nutrients (temporarily) or because the surface layer blocks out the light needed for photosynthesis.

• The massive amount of dead organic matter begins to sink through the water column. This sinking organic matter is food for decomposers (like bacteria).
• There is an increased decomposition rate due to the huge amount of dead algae.

Step 4: Oxygen Depletion (Increased Respiration & Reduced Dissolved Oxygen)

This is the most crucial step leading to environmental damage (6.7.5).
The decomposers (bacteria) break down the dead algae using aerobic respiration, which requires a lot of oxygen:

\( \text{Oxygen} + \text{Glucose} \rightarrow \text{Carbon Dioxide} + \text{Water} \)

• The decomposers consume oxygen faster than the surrounding water can replenish it.
• This leads to a massive reduction in dissolved oxygen levels in the water (hypoxia or anoxia).

Step 5: Death of Marine Organisms (The Final Impact)

Marine animals (fish, crabs, molluscs) need dissolved oxygen to respire and survive. Since the decomposers have used up all the oxygen, organisms that cannot move away from the area suffer death of organisms requiring dissolved oxygen (6.7.5).

• These areas are often called "dead zones" because high concentrations of marine life cannot survive there.

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6.7 Environmental Effects of Eutrophication

The primary effect of eutrophication on the marine environment (6.7.3) is oxygen depletion, which severely impacts the ecosystem structure.

A. Formation of Dead Zones

As described above, the lack of dissolved oxygen leads to areas of hypoxia (very low oxygen) or anoxia (no oxygen). Coastal regions, which are prone to receiving large volumes of run-off, are most susceptible. These dead zones cannot support complex marine life, leading to a drastic loss of biodiversity and mass fish kills.

B. Disruption of Food Webs

If producers (algae) die off too quickly, the primary consumers that rely on them suddenly lose their food source, causing further instability throughout the food web.

C. Loss of Habitats

In addition to blocking light for submerged plants like kelp and seagrass, the decomposing matter can sometimes build up, altering the sediment and making the habitat unsuitable for benthic (seabed) organisms.