Factors affecting oceanic ecosystems:
Net Primary Productivity (NPP) = the amount of carbon fixed by photosynthesis that
exceeds respiration demands of the plant and goes into growth.
Light
Most intense over the equator. With increasing distance from the equator, light is spread
over a greater surface area because of lower angles. Light decreases with depth, so
photosynthesis occurs in the photic zone (shallow layer of water that light penetrates to
about 50m).
Compensation depth = where light levels are reduced to about 1% of surface value. Below
this NPP stops.
Temperature
Closely related to sunlight. As depth increases, temp decreases. Regional sea surface temps
are influenced by strong winds blowing away surface water and allowing deeper cold water
to rise.
Tropical oceans are unproductive despite high insolation due to stable water column and
sharp permanent thermocline preventing overturning. Poor nutrient renewal in photic zone
and no upwelling of cold, nutrient rich deep water.
Cold Antarctic waters are productive as they are isothermal (little variation in water temp)
which enables continuous overturning.
Nutrient supply
Fluvial transport = Dissolved nutrients from rocks are carried to oceans by rivers.
Aeolian transport = Dust (mineral aerosols) enter oceans near continents.
Up/downwelling = redistributes nutrients.
Marine snow = Nutrients returned to abyssal plain creating basis of deep-water ecosystems.
Anthropogenic flux = sewage and fertiliser.
Plankton (a pelagic organism that floats) assimilate nutrients and are passed through food
chain. They are producers which mean they capture energy from the sun and store it as
organic matter. Nutrients are returned to the water as waste from organisms or when dead
organisms decompose. Nutrient levels relatively low at the surface and quickly used. Cold,
dense, deep water holds nutrients near ocean floor. Exceptions include at areas of upwelling
where nutrients are also high at the surface creating a high NPP, such as the Southern
Ocean and Antarctica.
Macro-nutrients = required in large quantities. N, P and Si.
Micro-nutrients = required in trace quantities. Fe, Cu, Zn.