open ocean and deep sea

Aims & Learning Outcomes

• Define terminology and depth ranges
• Characterize gradients from sea surface to deep seafloor
• Explore epipelagic, mesopelagic, and bathypelagic realms
• Explore bathyal, abyssal, and hadal benthic realms
• Introduce chemosynthetic ecosystems
• Characterize key physiological trends with depth

Ocean Profile

• Provinces of the Ocean:
  • Neritic Province
  • Oceanic Province
• Key Factors Influencing Depth:
  • Light penetration
  • Pressure (in Bars)

Depth Zones

1. Continental Shelf
   • Littoral and Sublittoral Zones
   • Epipelagic Zone (Sunlit Zone): 0 to -200 m
     • Pressure: 21 bars
2. Mesopelagic Zone (Twilight Zone): -200 m to -1000 m
   • Pressure: 101 bars
3. Bathypelagic Zone (Dark Zone): -1000 m to -3000 m
   • Pressure: 301 bars
4. Abyssopelagic Zone: -3000 m to -6000 m
   • Pressure: 501 to 601 bars
5. Hadal Zone (Abyssobenthic Zone): -6000 m to -11000 m
   • Pressure: 701 to 1101 bars

Key Characteristics and Measurements

• Light Absorption:
  • ~90% light absorbed in upper 200 m
  • ~99% absorbed in upper 1000 m
• Temperature at Depths:
  • Below mesopelagic zone (1000 m): 1-5 °C
  • Temperature affects metabolism—animals adjust to temperature, resulting in slower metabolic rates.
• Pressure Increase with Depth:
  • Pressure increases by 1 atmosphere (atm) or 14.7 psi every 10 m
  • Mean ocean depth: 3800 m (381 atm = 5600 psi)

Vertical Gradients in the Ocean

Gradient 1: Light, Temperature, and Pressure

• Visual Summary:
  • Significant gradients of light, temperature, and pressure as depth increases in the ocean.

Biological Gradients and Life in the Deep

Gradient 2: Abundance and Biomass

• Abundance and Biomass Trends:
  • Decrease with depth
  • Most deep-sea life relies on sinking organic material, known as particulate organic carbon (Marine Snow)
  • Approximately 1-5% of surface productivity reaches the deep-sea floor (average of 3%)
• Food Limitation:
  • Less food = lower biomass, abundance, and size of organisms
  • Reduced food coupled with low temperatures leads to slower metabolisms (Rex et al. 2006, Mar Ecol Prog Ser 317: 1-8)

Epipelagic Zone (0-200 m)

Characteristics

• Overview:
  • Open ocean, far from land, limited nutrients
  • High visibility (deep blue water), low food density for predators
• Nutrient Dynamics:
  • Limited iron (from dust, rivers) suppresses photosynthetic productivity, termed 'high nutrient, low chlorophyll' (HNLC) regions

Visual Predation

• Effects of High Visibility:
  • Prompts evolution of fast swimming and good vision; leads to
  • Arms Race: Evolution of hydrodynamic, streamlined shapes for speed
  • Long-range foraging due to low food density
  • High temperature and selective pressures result in high metabolisms

Predation Countermeasures

• Defensive Mechanisms Against Visual Predators:
  • Schooling: Makes it difficult for predators to select an individual prey.
  • Silvering of Scales: Acts as a mirror, creating an 'invisibility cloak.'
  • Countershading:
  • Dark dorsal sides make organisms invisible from above
  • Pale ventral sides provide camouflage from predators below

Mesopelagic Zone (200-1000 m)

Characteristics

• Twilight Zone:
  • Defined by steep temperature gradient and minimal light
  • Approximately 20% of surface productivity reaches mesopelagic zone
  • Home to gelatinous fauna and 'ugly' fish
  • Biomass can rival that of global fisheries
  • Plays major role in carbon cycling

Largest Daily Migration: Diurnal Vertical Migration (DVM)

• Migration Patterns of Zooplankton:
  • Zooplankton migrate to surface waters at dusk and return at dawn
  • Reasons include predator avoidance and energy dynamics involving feeding and digestion
  • Enhances downward carbon transport to the deep-sea floor

Predation in Mesopelagic Zone

• Despite low light, this zone remains a site of active predation, including numerous fish and cephalopods. Common features:
  • Ambush Predation: Common due to energetic reasons
  • Fish are small, have large upward-facing eyes, bioluminescent lures, and large mouths

Eye Adaptations

• Eye Characteristics in Twilight Zone:
  • Large, upward-pointing eyes detect silhouettes of prey above
  • Examples include the Pacific barreleye fish (Macropinna) which has evolved additional retina for ventral views

Colouration in the Mesopelagic Zone

• Many fish adopt silvering to blend in with downwelling light, while deeper species may appear red, absorbing blue light from the surface.

Bioluminescence in the Mesopelagic Zone

Overview

• Emission of light from specialized cells (photophores) is seen across various marine taxa.
  • Light is produced via oxidation of luciferin with luciferase enzyme or through symbiotic relationships with bacteria/dinoflagellates (Haddock et al. (2010) Ann Rev Mar Sci 2: 443-493).

Function of Bioluminescence

• Counterillumination:
  • Used for camouflage against predators and prey; only blue light penetrates the depth, making this an effective strategy.

Bathypelagic Zone (1000-4000 m)

Characteristics

• Marked by extreme darkness and cold (approximately 4 °C) with food limitations.
  • Predation strategies involve ambush and lure due to the scarcity of energy resources.

Eye and Colour Adaptations

• Eyes are optimized for detecting point source of bioluminescent light rather than colour images, often leading to black or dark brown coloration for absorption of bioluminescent light.

Buoyancy Mechanisms

• Gas Bladders: Useless Below:
  • Gas bladders, typically used by teleosts for buoyancy, become compressed under depth, which leads to reduced buoyancy.
  • Deep-sea fish utilize lipids and less dense tissue for neutral buoyancy.

Reproductive Strategies

• Adaptations to Darkness:
  • Hermaphroditism (presence of both reproductive organs) may occur; bioluminescence can attract mates.
  • Male parasitism (e.g. in anglerfish) involves permanent attachment of males to females, fusing into her circulatory system for reproductive purposes.

Comparison of Fish Traits from Surface to Depth

Benthic Realm: Beyond the Continental Shelf

Configuration

• Structure:
  • Denotes the ecological zones, demarking depths.
  • Distinction made between neritic, oceanic, littoral, sublittoral zones—all contributing to the ecological diversity observed.

Upper Bathybenthic Zone (200-2000 m)

• Conditions characterized by declining light, temperature, and food supply.
• Habitat complexity promotes higher biodiversity, especially around hard surfaces where filter feeders like sponges and corals thrive.

Lower Bathybenthic Zone (2000-4000 m)

• More sedimented with lower complexity, less food input due to the distance from the photic zones; currents are slower.

Abyssal Plains (4000-6000 m)

Characteristics

• Comprised of 50% of the planet's surface and extremely flat. Sedimentation rates are slow (2-3 mm-cm per 1000 years).
• Abyssal fauna exhibit lower diversity compared to bathyal fabrics due to food limitation.
  • Organisms characterized by extreme sparsity and adaptations to low energy environments.

Hadal Zone: Trenches (6000-11000 m)

Definition and Position

• Comprised of deep trenches found in subduction zones with high pressure and low diversity.
  -Home to unique scavenging organisms such as certain amphipods and sea cucumbers, and species like the snailfish.

Deep-sea Summary

• Nutrient Comparison: Open ocean is nutrient-deprived relative to coastal areas.
• Deep-sea fish exhibit adaptations such as large mouths, absence of gas bladders, and bioluminescence.
• Scavenging behavior becomes prevalent at greater depths, alongside phenomena like gigantism in certain invertebrates.