EOSC 315 Chapter 3: Module C
10 Apr 2019
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Module C:
1. Describe the major zones of the ocean; Differentiate b/w the benthic, pelagic,
neurotic & the oceanic realms
-PELAGIC Zone = consists solely of
the H2O column = vast area of open ocean between surface and bottom of ocean!
-Organisms that live here don’t always have shelter for rest/protection (bc in open), but
^pressure depth often restrict organisms to particular depths!
-Plankton = can’t swim, but rely on currents to carry it from place to place!
-Phytoplankton = most important primary producers in many marine ecosystems !
-Zooplankton = heterotrophic plankton!
-Nekton = animals that can swim well enough to oppose the currents ie) vertebrates, fish
& mammals
-Cab be further divided into 2 separate horizontal sections:!
-(1) NERITIC = shallow H2O column from the coast to the edge of the continental shelf!
-(2) OCEANIC = deep H2O column beyond the continental shelf!
-Further divided by depth into 4 vertical zones that correspond to the amount of light:!
-(1) EPIPELAGIC Zone = (0-200m) portion of pelagic from the surface photic zone
to the depth at which 1% of surface life penetrates!
-Bc it’s ~depth of continental shelf, nearly all neritic waters lie in the epipelagic!
-Food webs in this zone share 1 simple feature = primary prod. by phytoplankton
is the base => all org. depends on primary prod.!
-Primary prod here deepen on regenerated nutrients (ie. N, Fe, P)!
-If nutrient-limited production => ocean is limited in the limiting nutrient ie) N
(see Text pg 355 for details)!
-(2) MESOPELAGIC Zone = (200-1000m) b/w depth of 1% surface light & the level
at which light is undetectable!
-Not enough light for photosyn, so primary prod. can’t grow!
-Enough light to see by = Twilight zone!
-(3) BATHYPELAGIC Zone = (1000-4000m) region in which no light is detected!
-(4) ABYSSALPELAGIC Zone = (4000-6000m) contains v deep benthic
communities near bottom of ocean!
-(5) HADALPELAGIC Zone = (6000-11000m) includes deepest trenches!
-BENTHIC Zone = encompasses the bottom portion of the ocean (whether it be shallow or
1000s of m deep)!
->95% of marine life live on or just above benthos!
-In SHALLOW areas (<200m) food provided directly by photosynthesizes either attached to
bottom (seaweeds) or in H2O-column (phytoplankton)!
-In DEEP areas, food provided by fall of dead org. material in form of MARINE SNOW
(=small org. particles that aggregate together to form “fluffy” particles that fall uniformly to
ocean floor)!
-INTERTIDAL Zone = area b/w the tides that’s exposed to the air when the tide is out but
underwater at high tide!
-Makes up small fraction of continental shelf!
-Most of the shelf is never exposed to air, even tat the lowest of low tides
-SUBTIDAL Zone = where benthic organisms that inhabit the continental shelf below the
intertidal live in !
2. Differentiate b/w the photic & dysphotic zones; State which areas will have the most
organisms & why
-The marine environment can be divided into 3 diff zones based on light!
-(1) PHOTIC / EUPHOTIC Zone = area that incl. the sunlit portion of the upper layers of
ocean!
-Well-lit area of surface ocean where photosyn. occurs (see Module A)
-Tropical oceans: surface oceans are nutrient limited => limits photosyn. => minimal
suspended solids => clear H2O => light penetrates deeper => euphoric zone extends to
as deep as 200m!
-Temperate oceans: ^productivity in H2O => phytoplankton are abundant => ^suspended
solids in H2O column => euphoric zone only 100m or shallower due to SHADING = Bc
they absorb light for photosyn, the phytoplankton cut down the amount of light available
to phytoplankton living below!
-(2) APHOTIC Zone = deeper area where there’s no distinguishable surface light!
-Deepest & largest portion of the pelagic!
-Here the environ. is in permanent & complete darkness!
-(3) DYSPHOTIC Zone = aka “twilight zone” = area in b/w where light does penetrate but is
less than 1% of surface light, which is NOT high enough to support photosyn!
-Visible light can be seen by looking up while there’s complete darkness when looking
down!
-Can reach depths of up to 1000m!
3. Understand the importance of the compensation depth & the critical depth & why the
mixed layer has to be equal to or shallower than the critical depth for a phytoplankton
bloom to occur
-COMPENSATION Depth = depth at which there’s a BALANCE b/w RATE of photosyn. &
respiration processes of the phytoplankton community existing at that depth (as if they were
in a fixed position there & not being mixed through the mixed layer)!
-Thus net photosyn, and net phytoplankton growth rate is 0!
-AKA, if you were to hold a bottle of phytoplankton at the compensation depth, they’d
never grow nor sink in # or mass!
-
-CRITICAL Depth = depth where total gross photosynthetic production (ie. photosyn of all
organs being mixed around) is EQUIVALENT to total grow phytoplankton respiration (ie. the
respiration that’s occurring in all phytoplankton) per unit of surface area (measured from the
surface to this depth)!
-Thus over the depth interval above the critical depth, net photosyn. per square meter is
zero for the mixing phytoplankton pop!
-IF mixed layer depth is DEEPER than critical depth => the phytoplankton are being mixed
too deep => light limited & cannot form a bloom => photosynthesis < respiration for this
community!
-IF mixed layer depth is SHALLOWER/EQUAL than critical depth (ie. phytoplankton are
getting ^^^light at all times) => phytoplankton bloom => photosynthesis > respiration,
(given that nutrients aren’t limiting)
-Occurs as winter turns to spring & storms decrease => not much disturbance of ocean
surface => mixed layer is shallower!
