Detailed notes on further study on Ocean Productivity from the Fisheries module in a level environmental science. Created by an A grade a level environmental science student and based on the AQA specification but with more information.
Ocean Productivity
Ocean productivity (also known as marine productivity) refers to the primary production of
single-celled phytoplankton suspended in the ocean.
Phytoplankton are single-celled organisms that live in aquatic
environments. They are autotrophs, meaning that they
produce their own food via photosynthesis.
This organic carbon is then supplied to heterotrophs.
Heterotrophs are organisms that live by consuming compounds produced by other
organisms.
Ocean Productivity Examples
Marine heterotrophs encompass a wide range of organisms. They are categorised into three
main types.
Seagrasses
Benthos Organisms that live on the sea floor.
Anglerfish.
Most of the organic matter produced by phytoplankton is respired back into dissolved
organic forms in the surface ocean; recycled for use again by phytoplankton. Only a fraction
is exported to the deep ocean (through death)
Benthic plants are also primary producers in oceans. However, these organisms only
contribute to around 5 - 10% of marine plant material. This is because they only grow on the
, fringes of the world's oceans, in shallow areas with sufficient light for photosynthesis. Thus,
phytoplankton carry out most of the the ocean's productivity.
Measuring Ocean Productivity
The total productivity of a region or system is the gross primary productivity.
Some of this organic material is used to sustain phytoplankton. The amount leftover from
this is known as the net productivity.
Net productivity is the amount of organic material available to support the heterotrophs of
the ocean.
Therefore, net primary production is the gross primary production minus energy used for
growth and development of the plant.
Primary productivity is calculated by measuring the uptake of CO2, or the output of
oxygen.Production rates are expressed as grams of organic carbon per unit area per unit
time. Typically, grams of carbon per metre squared per year is used - shortened to g C
m−2 yr−1.
The annual primary productivity of the oceans is estimated to be approximately 50 ×
1015 grams (50 × 109 metric tons) of carbon per year, roughly half of the global total.
Factors Affecting Ocean Productivity
Primary productivity is limited by either nutrient availability or light.
Nutrient Availability
The most important nutrients for phytoplankton are nitrogen (N), phosphorus (P), iron (Fe)
and silicon (Si).
Dissolved inorganic carbon is highly abundant in the oceans, so is not typically listed among
the other nutrients.
Nitrogen and Phosphorus
Phytoplankton require relatively uniform amounts of N and P.
This can be quantified using the Redfield Ratio. Plankton build their biomass using C:N:P
ratios of 106:16:1.
The dissolved N:P ratio in the deep ocean is close to the 16:1 ratio of plankton biomass.
Iron and Silicon
Iron is found in biomass in trace amounts, but it is used for essential purposes in organisms.
Scarcity often affects productivity in oceans, especially in regions of upwelling.
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