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The Euphotic Zone

Characteristics of The Euphotic Zone

Characteristics of The Euphotic Zone
Plants and Animals
Density Independent and Dependent Factors
Parasitism, Mutualism, Coevolution
Trophic Levels
Human Interaction


Vertical Stratification



Sunlight Penetration







Food Chain

I. Types of pelagic oceanic food chains

A. Gyre-center, nutrient-poor

1. I'll use the food-chain framework of this section to introduce the different types of plankton.

2. At the base are the photosynthetic autotrophs (phytoplankton=single-celled); nannoplankton (20 - 200 microns).These are the most important marine plants and these PHYTOPLANKTON produce 99% of the food for marine animals

a. Dinoflagellates (Often nannoplankton) = algae, one or  more flagella, no hard exoskeleton so don't sink so fast   some bioluminescent, tropics, temperate summers, centers of gyres in mid-latitudes. Can also ingest organic matter as a food source which gives them a competitive advantage in these nutrient-poor environments.

1) Cause of red tides and paralytic shellfish poisoning.  During a red tide up to 2 million of them can be found in a quart of seawater.

a) Produce water-soluble toxins, some of which kill shellfish and some of which are just passed on to, and affect the human consumers in the form of PSP.

b) Are known to have caused 300 deaths world-wide.

c) Symptons akin to drunkeness

b. In temperate regions the nature of the phytoplankton population changes with the season varying between diatoms during periods of high nutrients and mixing intensities and dinoflagellates later during low-nutrient, low-mixing times.

c. Coccolithophores (relatives of diatoms) = nannoplankton with calcite plates, warmer, nutrient-deficient tropical & temperate waters where their extremely small size makes them very efficient producers and slow sinkers. (<.06mm (.002in) in diam.) Their hard parts are the sediments that form chalk, such as makes up the White Cliffs of Dover.

3. Next up the food chain are the microzooplankton which are unicellular, amoeba-like  zooplankton (KINGDOM PROTISTA, eukaryotes). In the Linnean Classification these are not in the Animal Kingdom, but since they are not primary producers, we will call them animals.

a. These protozoans (Phylum Protozoa) can also be benthic  & favor warmer, organic-rich water. The commonest forms have calcareous (foraminifera) or siliceous (radiolarians,) exoskeletons from which they can extrude protoplasm to trap food (i.e., nannoplankton) in the surrounding water.

4.  Next macrozooplankton, which are dominated by the copepods

a. single most important zooplankton- bulk of animal mass in oceans,

b. tiny shrimp-like creatures (.5-5mm), limited vert. migr, graze on phytoplankton

c. Largely responsible for deep scattering layer (accumulations of organisms in a thin band extending horizontally along a density boundary or at a preferred light intensity or food  resource level, which are capable of partially reflecting sound waves from depth sounders).

d. Members of:  Phylum Arthropoda-jointed legs & external skeletons.  Largest group of animals on Earth (includes the insects). 

1) Class Crustacea -  majority of all marine animals

5. Next megazooplankton

a. Euphasids or Krill (3-5 cm, Class Crustacea)   

1) Also herbivorous

2) Dominant zooplankton at high latitudes and may be  main food for baleen whales.

b. Chaetognath (arrow worms)-voracious carnivores   

1) In some cases a specific species is found only in a certain water mass, to the extent that they can used to identify the origin of the water in which found.

c. Various forms of what we normally group as jellyfish. 

1) (Phylum Coelenterata or Cnidaria) Organisms with a  gut and tentacles at one end, meat-eaters (predatory, although also feed on organic detritus),stinging cells in tentacles, some colonial forms = Portugues Man-of-War (tentacles can be > 100 ft long).

2) Ctenophores (Phylum Ctenophora) - "comb jellies", small (<few cms) and transparent, bioluminescent, plankton/nekton (i.e., entirely pelagic), also meat-eating but have adhesive cells on their tentacles instead of stinging cells.

3) About 97% water so they cannot support their own body weight and have no muscles or bones. Nor do they have a brain, heart or eyes.

a) Instead they have a network of nerve cells that help them move and react to food and danger. They have simple sensors that tell them if they aremoving up or down (i.e., toward or away from the light).

4) Supremely successful group in all the oceans.

5) The Sea Wasp in Australia has a deadly sting that can kill a person in minutes.

6) In some reginos of the oceans that are being overfished by commercial fishermen, jellyfish have moved in and replaced the overexploited bony fish.

7) Sea turtles, some fish and some sea birds will eat them.

6. Planktivores = fish and other organisms that eat plankton

a. Stomias and lantern fish in the deep ocean make-up most of these

7. Piscivores =  fish and other organisms that eat fish 

a. Many mammals, squid, game fish and popular table fish

Geographic Distribution
        The Photic Zone is located in every ocean and it is the part of the ocean that recieves sunlight.

Topographic Features That Affect Climate
       There isnt really a topograhpic feature that affects the climate becasue the photic zone is in open ocean. 
Seasonal Variation
       Seasons change in open ocean just like they do on land. The photic zone experiences water spouts, hurricanes, tsunamis and other natural occurances just like anywhere else on land.