Marine Science (9693) | Ecological impacts of human activities

Welcome to Chapter 9.1: Ecological Impacts of Human Activities

Hello future marine scientists! This chapter is crucial. It connects everything we've learned about healthy ecosystems (like coral reefs and food webs) with the reality of how human actions, both on land and at sea, cause damage.
Understanding these impacts isn't just about passing the exam; it’s about knowing how to protect the ocean we rely on. Don't worry if some topics seem alarming—we'll also cover solutions!

Section 1: Pollution and Habitat Degradation (9.1.1)

Human activities introduce a massive range of substances and physical changes into the marine environment. We'll break down the key sources of pollution and their specific ecological effects.

1.1 Impacts from Industrial and Land-Based Activities

The Oil Industry (Spills and Chronic Discharge)

Oil spills are dramatic, but chronic leakage (smaller, regular releases) from industry and transport can also be damaging.

• Water Quality: Oil is a hydrocarbon mixture that forms a slick on the surface. This reduces light penetration, severely limiting photosynthesis by phytoplankton and submerged plants.
• Habitats & Organisms: Oil physically smothers organisms on the surface (like seabirds and marine mammals), reducing insulation and buoyancy. In coastal habitats (e.g., mangroves and rocky shores), oil coats surfaces, blocking gas exchange and causing suffocation.
• Food Webs: Toxic components of oil can kill primary consumers, disrupting the base of the food web.

Agriculture (Run-off)

When rain washes over farmland, it carries substances into rivers and, eventually, the sea.

• Nutrient Load: Fertilizers containing nitrates and phosphates cause eutrophication.
• Eutrophication explained: High nutrient levels trigger massive, rapid growth of algae (an algal bloom). When these large quantities of algae die, decomposers (bacteria) use up huge amounts of dissolved oxygen during respiration. This creates hypoxic (low oxygen) or anoxic (no oxygen) 'dead zones', where fish and other mobile organisms cannot survive.
• Toxins: Pesticides and herbicides (used to kill pests and weeds) are often toxic to marine life, especially larval stages.

Sewage and Refuse Disposal

Poorly treated sewage and general waste significantly affect coastal waters.

• Sewage: Similar to agricultural run-off, raw sewage introduces high levels of nutrients, leading to eutrophication and oxygen depletion.
  It also introduces pathogens (disease-causing bacteria and viruses), posing health risks to humans and marine organisms (e.g., causing disease in shellfish).
• Refuse Disposal: Non-biodegradable waste (plastics, metals, glass) can physically harm organisms through ingestion (mistaking it for food) or entanglement (e.g., seals caught in discarded fishing nets).

Desalination Plants

Desalination removes salt from seawater to produce fresh water, often used in dry coastal areas.

• Impact: The waste product is highly concentrated salt water, called brine. This brine is often hotter than the ambient seawater and contains chemical pre-treatment agents.
  When discharged back into the ocean, the brine creates a plume of water with abnormally high salinity and temperature, which can kill or stress benthic organisms that cannot tolerate these rapid environmental changes.

1.2 Impacts from Marine Installations and Fishing Practices (9.1.1)

Renewable Energy Installations

The construction and presence of structures like offshore wind turbines or tidal energy generators alter the marine habitat.

• Habitat Change: The physical presence of turbine foundations acts as an artificial reef substrate, attracting species (like mussels) and changing local community structures.
• Physical Impact: Construction involves noise and vibration, which can disrupt sensitive species like whales and dolphins.

Damaging Fishing Practices

Certain non-sustainable fishing methods cause severe, immediate ecological damage.

• Dredging: This technique involves dragging a large metal frame or net along the seabed to catch shellfish (like scallops).
  

  Impact: It causes physical destruction of benthic habitats, scraping away organisms and substrates like seagrass beds or slow-growing corals, and stirring up massive amounts of sediment (increasing turbidity).

• Blast Fishing (Dynamite Fishing): This illegal method uses explosives to stun or kill fish for easy collection.
  

  Impact: It causes catastrophic physical damage to habitats like coral reefs, often reducing decades of coral growth to rubble instantly. It also kills non-target organisms indiscriminately.

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Section 2: Toxins in the Food Chain – Bioaccumulation and Biomagnification (9.1.2)

Some pollutants are particularly dangerous because they don't break down easily (they are persistent) and they accumulate in living tissues. This is especially true for certain heavy metals.

2.1 Understanding Accumulation

Bioaccumulation

Definition: The build-up of a non-biodegradable toxin in the tissues of an organism over its lifespan.

Example: A small fish eats constantly, and every time it consumes food containing trace amounts of mercury, that mercury stays stored in its body fat or organs, increasing its concentration over time.

Biomagnification

Definition: The process where the concentration of a toxin increases at successively higher trophic levels in a food chain.

Analogy: Imagine a toxin particle is one marble. A plankton eats one marble. A small fish eats 10 plankton, so it now has 10 marbles. A large predator eats 10 small fish, accumulating 100 marbles.

• Mechanism: Since energy transfer is inefficient (around 10% between levels), an organism at a high trophic level must consume a large mass of prey from lower levels to sustain itself. If the prey contains accumulated toxins, the predator receives a massive, concentrated dose.
• Examples of Toxins:
  • Mercury: Released into the environment from the combustion of fossil fuels (like coal). It bioaccumulates in fish.
  • Heavy Metals in Antifouling Paint: Historically, paints used on ship hulls contained toxic compounds (like organotin compounds) to prevent organisms (fouling) from attaching. These compounds leach into the water and enter the food chain, affecting high-level predators.

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Section 3: The Threat of Plastics and Microplastics (9.1.3, 9.1.4, 9.1.5, 9.1.6)

3.1 Defining Microplastics

Plastics are perhaps the most pervasive pollutant. We categorize plastic fragments based on size.

• Microplastics: Defined as any plastic particle with a diameter less than 5 mm.

Categories of Microplastics:

1. Primary Microplastics: These are plastics that were manufactured to be small.
   Example: Microbeads used in exfoliating body washes or small fibers shed from synthetic clothing during washing.
2. Secondary Microplastics: These fragments result from the breakdown of larger plastic items.
   Example: Pieces of a plastic bottle or fishing net that have fragmented over time.

3.2 The Non-Biodegradation Process (9.1.4)

Most plastics are non-biodegradable. This means they cannot be broken down chemically by living organisms (like bacteria) into simple, harmless substances (like CO₂ and water).

Instead, large plastics are broken down into smaller and smaller secondary fragments through physical and chemical weathering, which includes:

• UV Radiation (Sunlight): Breaks the chemical bonds in the plastic polymer, making the material brittle.
• Wind Action and Wave Action: Physical grinding and abrasion break the brittle plastic into tiny pieces.
• Temperature: Higher temperatures increase the rate of these breakdown processes, accelerating the formation of secondary microplastics.

3.3 Impacts of Plastics and Microplastics (9.1.5)

The impact of plastics is widespread, affecting organisms from the very bottom of the food web to the top.

• Uptake by Plankton: Plankton (especially zooplankton) are tiny organisms that form the base of the marine food chain. They can mistake microplastics for food, consuming them directly.
• Transfer along the Food Chain: Once plankton consume microplastics, they are transferred to primary consumers (e.g., small fish or filter feeders) and then up the food chain, similar to biomagnification (though the plastic itself doesn't necessarily magnify, the quantity ingested accumulates).
• Absorption of Toxic Compounds: Plastics (especially microplastics due to their high surface area-to-volume ratio) can absorb toxic compounds (like DDT or PCBs) from the seawater. When an organism ingests the plastic, these toxins are released into the organism’s tissues.
• Ingestion and Entanglement:
  • Ingestion: Larger plastic pieces fill the stomachs of organisms (like sea turtles, whales, and seabirds), giving them a false sense of fullness, leading to starvation.
  • Entanglement: Abandoned fishing gear, often called ghost nets, continues to "fish," trapping and killing marine animals through entanglement.
• Risk to Humans: If fish, shellfish, or crustaceans containing plastics or high concentrations of plastic-associated toxins enter the human food chain, it poses a direct risk to human health.

3.4 Strategies to Limit Plastic Release (9.1.6)

Limiting the flow of plastics into the ocean requires global, multi-faceted strategies:

• Improved Waste Management: Implementing better collection, sorting, and recycling infrastructure, especially in developing coastal nations.
• Legislation and Policy: Banning specific problematic items, such as single-use plastics and primary microplastics (e.g., microbeads in cosmetics).
• Extended Producer Responsibility (EPR): Making manufacturers responsible for the life cycle of their products, including disposal and recycling.
• Education and Awareness: Promoting consumer changes, such as reducing consumption of plastic products, using reusable items, and participating in clean-up efforts.
• Innovation: Developing biodegradable alternatives and improving filtration systems (e.e., washing machine filters to catch microfibers).

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Chapter Summary: Key Takeaways

The ecological impact of human activities is primarily categorized into Physical Damage (dredging, blast fishing, oil spills), Chemical Pollution (heavy metals, oil toxicity), and Nutrient Overload (eutrophication from agriculture/sewage).
Toxins pose a double threat through bioaccumulation (in one organism) and biomagnification (up the trophic levels).
Microplastics, specifically, threaten ecosystems by being ingested by plankton and acting as carriers for toxic substances, disrupting the entire marine food web.