Biological Characteristics of Oceans

I. Marine Ecosystems and Biodiversity

• Oceans host a vast range of ecosystems: coral reefs, mangroves, estuaries, seagrass beds, and open oceans.
• Marine biodiversity includes plankton, nekton (e.g., fish, squid), benthos (organisms living on the ocean floor), and marine mammals.
• Oceans support about 80% of all life forms on Earth.
• Marine ecosystems provide crucial services: oxygen production, carbon sequestration, food supply, and climate regulation.

II. Photic Zone

• Also called the euphotic zone, it is the upper layer of the ocean where sunlight penetrates.
• Depth: up to 200 meters depending on water clarity.
• Supports photosynthesis by phytoplankton.
• Most marine life is concentrated in this zone.
• Base of the marine food web originates here.

III. Aphotic Zone

• Zone below the photic zone where sunlight does not reach.
• Devoid of photosynthesis; organisms depend on marine snow (organic matter falling from upper layers) or chemosynthesis.
• Includes mesopelagic, bathypelagic, abyssopelagic, and hadal zones.
• Home to adapted species like anglerfish, giant squid, and bioluminescent organisms.

IV. Primary Productivity in the Ocean

• Primary productivity is the rate at which phytoplankton convert sunlight into chemical energy via photosynthesis.
• Influenced by light availability, nutrient supply, and temperature.
• High productivity areas:
  • Coastal zones
  • Upwelling regions (e.g., Peru and Benguela currents)
  • Estuaries
• Low productivity in open ocean gyres (nutrient-poor zones).

V. Coral Reefs and Their Ecological Importance

• Coral reefs are biodiverse marine ecosystems built by calcium carbonate-secreting corals.
• Located in shallow, tropical waters (between 30°N and 30°S).
• Called the “Rainforests of the Sea” due to their rich biodiversity.
• Ecological roles:
  • Support 25% of all marine species
  • Buffer coastlines from waves and storms
  • Provide food and livelihood to millions
• Sensitive to temperature rise and acidification.

VI. Deep-Sea Ecosystems

• Located at great ocean depths where light, temperature, and food availability are extremely low.
• Organisms are adapted to high pressure, darkness, and cold.
• Includes hydrothermal vent ecosystems, where chemosynthetic bacteria form the base of the food chain.
• Unique species: tube worms, deep-sea anglerfish, and vent crabs.

Human Impact on Oceans

I. Overfishing

• Exploitation of fish stocks beyond sustainable limits.
• Leads to collapse of species like Atlantic cod, bluefin tuna.
• Disrupts marine food chains and biodiversity.

II. Depletion of Marine Resources

• Excessive extraction of fish, shellfish, oil, gas, and minerals.
• By-catch and destructive fishing practices (e.g., bottom trawling) harm non-target species and habitats.
• Marine ecosystems suffer from reduced resilience.

III. Marine Pollution

Pollution from land-based and ocean-based sources severely affects marine health.

a. Oil Spills

• Result from tanker accidents, offshore drilling.
• Cause massive damage to marine life and coastal ecosystems.
• Oil coats the bodies of marine birds and mammals, leading to death.

b. Plastic Waste

• Over 8 million tons of plastic enter the ocean each year.
• Leads to entanglement, ingestion, and eventual death of marine animals.
• Microplastics have entered the marine food web and even human consumption.

c. Chemical and Heavy Metal Contamination

• Industrial discharge introduces mercury, lead, cadmium, and other toxins.
• These bioaccumulate in fish and biomagnify in the food chain.
• Can cause neurological and reproductive issues in both marine life and humans.

IV. Climate Change and Its Effects on Oceans

a. Ocean Warming

• Caused by increased greenhouse gas concentrations.
• Leads to:
  • Coral bleaching
  • Sea-level rise (thermal expansion)
  • Changes in species distribution
  • Reduced dissolved oxygen levels

b. Coral Bleaching

• Occurs when corals expel symbiotic algae (zooxanthellae) due to stress from rising temperatures.
• Leads to coral starvation, disease, and eventual death.
• Mass bleaching events are becoming more frequent.

c. Destruction of Marine Habitats

• Coastal development, bottom trawling, dredging, and pollution degrade habitats.
• Loss of mangroves, seagrass beds, coral reefs, and estuaries.
• Reduces biodiversity and ecosystem resilience.