Topic: Fisheries
Theme: Systems and Structures

Key Concepts:

Abiotic Factors

  • Non-living (abiotic) factors that influence the patterns of ocean life include:
    • avorable physical and chemical enviornments,
    • Large scale circulation,
    • Near shore slopes,
    • Local tides, and
    • Storm events
    • Available sunlight

Nutrient Cycle

  • Most surface waters are deficient in nutrients and cannot sustain growth unless upwelling resupplies nutrients.
  • The base of marine life is a large complex group of microscopic organisms known as plankton. Plankton can be broadly divided into:
    • Phytoplankton - the plants or photosynthetic organisms,
    • Zooplankton - the animal component, and
    • Bacteria - decomposers of organic matter into its abiotic constituents.
    • Picoplankton
  • Nutrients, such as nitrogen and phosphorus, found mostly in colder upwelled water are needed to fertilize phytoplankton that grow only in near-surface waters of the ocean.
  • Phytoplankton (microscopic plants) is the principal source of food supporting all animal life in the oceans. Phytoplankton is sometimes referred to as the grass of the sea.
  • Fish population density and productivity are higher in the coastal areas, especially in upwelling regions, than in the open ocean areas where there is no upwelling.

Food Chain

  • Photosynthetic organisms are eaten by herbivorous zooplankton, which are eaten by carnivores, which are eaten by large carnivores, and so on. Each step in this food chain is a trophic level.

Seasonal Blooms

  • Intense blooms of phytoplankton occur naturally, but infrequently, in many parts of the coastal oceans, particularly in temperate climates.
  • Intense blooms of dinoflagellates occur in some regions, especially the tropics.
    • The frequency of such blooms appears to be increasing and
    • They are appearing in areas where they have not been seen before.
    • Dinoflagellate blooms can be caused by a variety of mechanisms:
      • Large increase in freshwater runoff which creates a shallow layer of low - salinity water in a previously well-mixed water column.
      • Run-off that contains inorganic or organic compounds react with toxic substances, such as copper and mercury, and reduces their toxicity to phytoplankton.
  • The dense concentration of phytoplankton in bloom colors the surface water yelllow, green, brown, or reddish, depending on the species. Many dinoflagellates have a reddish color, so their blooms are called red tides.
  • Toxins produced by dinoflagellate blooms often kill large numbers of fish.

Zones of the Ocean

  • The near-surface water layer, where there is light, and photosynthesis is possible, is called the photic zone.
  • Life is found everywhere in the ocean--on the ocean floor, on the continental slopes, and floating or swimming in the water. Life is found thousands of meters deep in the ocean, far from the reach of sunlight. Special types of life are found surrounding chimney-like vents that release hot water rich with minerals.
  • The photic zone layer is where most marine life exists.
  • The depth of the photic zone layer is not constant because the angle of the incoming sunlight varies from day to day and seasonally. The photic zone extends to the seafloor in many areas of the coastal ocean.
  • Productivity is greatest in zones of high temperature gradients called ocean fronts.