16.3 Aquatic and Marine Ecosystems

Aquatic and marine ecosystems are vital to Earth's biodiversity and climate regulation. From freshwater lakes to deep ocean trenches, these environments support unique life forms adapted to water's challenges. They're divided into zones based on depth, light, and other factors.

These ecosystems provide crucial services like oxygen production, carbon storage, and food for billions. However, they face threats from pollution, overfishing, and climate change. Understanding their complexity is key to preserving these essential habitats for future generations.

Aquatic Ecosystem Characteristics

Categories and Zones

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• Aquatic ecosystems are divided into two main categories
  • Marine ecosystems contain salt water
  • Freshwater ecosystems contain non-saline water
• Marine ecosystems are divided into distinct zones, each with unique characteristics
  • Intertidal zone (area between high and low tide)
  • Neritic zone (shallow ocean waters over the continental shelf)
  • Oceanic zone (open ocean waters beyond the continental shelf)
  • Benthic zone (bottom of the ocean, including sediment surface and subsurface layers)
  • Abyssal zone (deep ocean waters, typically below 2,000 meters)
• Freshwater ecosystems include
  • Lentic (still water) systems (lakes, ponds)
  • Lotic (flowing water) systems (rivers, streams)
  • Wetlands, which can be freshwater or saltwater (swamps, marshes, bogs)

Factors Influencing Aquatic Ecosystems

• The availability of nutrients, light, and oxygen in aquatic ecosystems is influenced by several factors
  • Water depth affects light penetration and vertical stratification
  • Turbidity (cloudiness or haziness of water) affects light penetration
  • Mixing of water layers affects nutrient and oxygen distribution
• Aquatic ecosystems are characterized by their primary producers and consumers
  • Primary producers include phytoplankton, algae, and aquatic plants
  • Consumers include zooplankton, fish, and other aquatic animals
• Temperature and salinity also play important roles in structuring aquatic communities and influencing organismal physiology

Marine Ecosystem Zonation and Productivity

Pelagic Zones

• Marine ecosystems are divided into distinct pelagic zones based on depth and light availability
  • Epipelagic zone (surface to ~200 m) receives the most sunlight and has the highest primary production by phytoplankton
  • Mesopelagic zone (~200-1,000 m) receives less sunlight and has lower productivity; many organisms migrate vertically to feed in the epipelagic zone at night
  • Bathypelagic zone (~1,000-4,000 m), abyssopelagic zone (~4,000-6,000 m), and hadopelagic zone (>6,000 m) are characterized by complete darkness, high pressure, and low temperatures; very low productivity and specialized organisms adapted to extreme conditions
• Productivity in pelagic zones is influenced by nutrient availability and upwelling
  • Upwelling brings nutrient-rich deep waters to the surface, stimulating primary production
  • Regions of high productivity (coastal upwelling, polar regions) support large populations of fish, seabirds, and marine mammals

Benthic Zones

• Marine ecosystems are also divided into benthic zones based on depth and substrate type
  • Intertidal zone is exposed to air during low tide and submerged during high tide; organisms adapted to withstand physical stresses (wave action, desiccation)
  • Subtidal zone extends from the low tide line to the edge of the continental shelf; supports diverse communities of algae, seagrasses, and invertebrates
  • Deep-sea benthic zones include the continental slope, abyssal plain, and hadal trenches; characterized by high pressure, low temperature, and absence of sunlight; support unique communities of organisms adapted to these conditions
• Benthic productivity is influenced by the availability of organic matter sinking from the pelagic zone and the presence of chemoautotrophic organisms that produce energy from chemical compounds

Aquatic and Marine Adaptations

Adaptations for Living in Water

• Aquatic and marine organisms have evolved various adaptations for living in water
  • Streamlined body shapes and fins for efficient swimming (fish, dolphins)
  • Gills for extracting oxygen from water (fish, some invertebrates)
  • Swim bladders for buoyancy control (fish)
  • Adaptations for diving, such as large oxygen-storing muscles, collapsible lungs, and tolerance to high carbon dioxide levels (whales, seals)
• Osmotic regulation is crucial for maintaining water and ion balance
  • Freshwater organisms constantly excrete water and conserve ions
  • Marine organisms conserve water and excrete excess ions

Adaptations to Specific Zones and Habitats

• Deep-sea organisms have adaptations for living in dark, high-pressure environments
  • Large eyes or bioluminescence for seeing in the dark (lanternfish, anglerfish)
  • Enzymes that function under high pressure and low temperature
  • Slow metabolic rates and energy-efficient locomotion (deep-sea cucumbers, tripod fish)
• Intertidal organisms have adaptations for withstanding physical stresses
  • Strong adhesive structures for attachment to rocks (barnacles, mussels)
  • Ability to close shells tightly to prevent desiccation during low tide (limpets, snails)
  • Tolerance to wide ranges of temperature and salinity (sea stars, crabs)
• Coral reef organisms have adaptations for living in nutrient-poor, clear waters
  • Symbiotic relationships with photosynthetic algae (zooxanthellae) for energy (corals, giant clams)
  • Efficient nutrient recycling and conservation (sponges, tunicates)
  • Specialized feeding adaptations, such as mucus nets for capturing plankton (corals, feather stars)

Ecological Importance of Aquatic and Marine Ecosystems

Ecosystem Services

• Aquatic and marine ecosystems provide crucial ecological services
  • Primary production by phytoplankton and other aquatic producers, contributing to global oxygen generation
  • Nutrient cycling and water purification by wetlands and coastal ecosystems (mangroves, salt marshes)
  • Habitat provision for a diverse array of species, including commercially important fish and invertebrates
  • Carbon sequestration by marine sediments, seagrasses, and marshes
• Coral reefs are particularly important ecosystems
  • Support the highest biodiversity of any marine ecosystem
  • Serve as nurseries for many fish and invertebrate species
  • Provide coastal protection from waves and storms
  • Generate significant economic benefits through tourism and recreation

Human Benefits and Impacts

• Aquatic and marine ecosystems provide valuable economic and cultural services
  • Fisheries and aquaculture supply food and livelihoods for millions of people worldwide
  • Coastal tourism and recreation generate significant revenue and employment
  • Many cultures have deep spiritual and traditional connections to aquatic and marine environments
• However, these ecosystems face numerous anthropogenic threats
  • Overfishing and destructive fishing practices (bottom trawling, dynamite fishing)
  • Pollution from land-based sources (nutrients, chemicals, plastics)
  • Habitat destruction and modification (coastal development, dredging, filling of wetlands)
  • Climate change impacts (ocean acidification, sea level rise, warming temperatures)
• Protecting and restoring aquatic and marine ecosystems is essential for maintaining their ecological integrity and the services they provide to humans and other species