Ocean Water and Ocean Life Notes

Ocean Water Composition

  • Seawater Composition:
  • Approx. 3.5% dissolved minerals by weight.
  • Salinity:
  • Definition: Total amount of solid material dissolved in water.
  • Expressed in parts-per-thousand (‰).
  • Average salinity is approximately 35‰.
  • Major constituent: Sodium chloride (NaCl).

Sources of Sea Salts

  • Chemical weathering of rocks.
  • Outgassing:
  • Gases released from volcanic eruptions contribute to ocean salinity.

Processes Affecting Seawater Salinity

  • Decrease in Salinity (add water):
  • Precipitation.
  • Runoff from land.
  • Melting: Icebergs and sea ice.
  • Increase in Salinity (remove water):
  • Evaporation.
  • Formation of sea ice.
  • Surface salinity ranges from 33‰ to 38‰ in the open ocean.

Temperature and Density Variations with Depth

Temperature:

  • Low-latitudes:
  • Warm surface temperatures; rapid decrease in temperature with depth (thermocline).
  • High-latitudes:
  • Cooler surface temperatures; gradual change in temperature with depth.

Density:

  • Density: Mass per unit volume—indicating how heavy something is for its size.
  • Factors Influencing Density:
  • Salinity and Temperature (greatest influence).
  • Density is low at surface in low-latitudes, increasing with depth (pycnocline).
  • High-latitudes have high-density (cold) water at the surface with little change with depth.

Ocean Layering

  • Ocean Structure: Layered based on density.
  • Surface Mixed Zone: Sun-warmed zone, often within 300 meters depth.
  • Transition Zone: Between the surface and deep zone.
  • Deep Zone:
    • No sunlight; temperatures near freezing; high-density water.

Variations in Temperature and Salinity with Latitude

  • Temperature and salinity varies with geographic latitude, showing a gradient from the equator to the poles.
  • Higher temperatures and lower salinity found at the equator compared to the tropics.

Diversity of Ocean Life

  • Marine Environment: Home to diverse organisms.
  • Photosynthesis: Most marine organisms thrive in sunlit surface waters.

Types of Marine Organisms:

  • Plankton:
  • Phytoplankton (algae) and Zooplankton (animals).
  • Nekton:
  • Swimmers; animals capable of moving independently of ocean currents.
  • Benthos:
  • Bottom dwellers; diverse species exist, especially along shallow coastal floors.

Marine Life Zones

Based on Light Availability:

  • Photic Zone: Sunlit upper part of the ocean; includes the euphotic zone near the surface where light is strongest.
  • Aphotic Zone: Deep ocean area without sunlight.

Based on Distance from Shore:

  • Intertidal Zone: Where land and ocean meet.
  • Neritic Zone: Extends from the low tide line to the shelf break.
  • Oceanic Zone: Beyond the continental shelf.

Based on Water Depth:

  • Pelagic Zone: Open ocean of any depth.
  • Benthic Zone: Includes all sea-bottom surfaces.
  • Abyssal Zone:
  • Deep, high-pressure region; no sunlight; sparse life; food mainly from decaying organic matter and hydrothermal vents.

Exam Questions

  • Common Salt in Seawater:
  • Most abundant salt: Sodium Chloride (NaCl).
  • Salinity Variations:
  • Water near the equator has lower salinity compared to water near the tropics.
  • Thermoclines:
  • Best developed at lower latitudes, indicating the presence of a rapid temperature change with depth.
  • Importance of Density:
  • Determines the vertical position of the water within the ocean.