The Amazon Rainforest is nicknamed 'Lungs of the Earth'. Millions of trees photosynthesise all over our earths lands, converting carbon dioxide into oxygen. However, the Amazon is not the only important producer of oxygen on Earth. Trillions of tiny organisms, too small to see, produce around half of the world's oxygen from within the oceans.
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.
| Type of Heterotrophs | Definition | Examples |
Zooplankton | Floating animals. | - Krill
- Dinoflagellates
- Jellyfish
|
Nekton | Swimming organisms. | |
Benthos | Organisms that live on the sea floor. | |
The majority 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.
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 the majority of 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.
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.
Read more about upwelling in a heading lower down.
Silicon is only an essential nutrient for certain taxa of plankton, such as diatoms.
Diatoms are a diverse group of single-celled algae, which have a unique glass wall made out of silicon dioxide, which provides protection from the crushing jaws of predators.
However, silicon availability is a major factor in the broad ecology of surface waters, as diatoms typically dominate in Si-rich waters.
Light
Sunlight is the energy source for phytoplankton. However, very little sunlight penetrates below a depth of 80m. In fact, in some highly productive coastal regions, light can only penetrate 10m below the ocean's surface.
As a result, photosynthesis is restricted to the upper, light-penetrated part of the ocean - known as the photic zone. Beneath this zone is the aphotic zone, where photosynthesis does not occur due to insufficient light.
Sunlight causes surface water to be much warmer than underlying water. Warm water has a lower density than colder water, so it stays above it, creating a thermocline.
A thermocline is a vertical temperature gradient driving density stratification across the ocean.
Density stratification occurs when here is a cold layer of dense water at the bottom and a warmer layer on top.
Seasonal and Local Conditions
In temperate regions, plankton abundance peaks in spring as temperature and light intensity increase. Secondary abundance peaks occur in the autumn. Additionally, strong winter storms mix the water column, distributing nutrients, facilitating phytoplankton growth.
Heavy rainfall in coastal areas (especially in those that experience monsoons) can result in nutrient-rich, turbid plumes.
Turbid is a word often used to describe liquids that are cloudy or thick due to suspended matter within them.
Primary Productivity in Marine Ecosystems
Primary productivity varies in different parts of the oceans.
Open Ocean
The open ocean typically has a low productivity. Warm sunlit water is separated from the colder, nutrient-rich interior by a strong density difference. Mixing is restricted, thus reducing nutrient supply.
Light also affects the productivity of open oceans. The vast majority of the open ocean is in the aphotic zone, where there is not enough light for photosynthesis.
The combination of restricted mixing and low light means that the majority of open ocean biomass (phytoplankton and heterotrophs) are found within 200m of the ocean's surface.
Coastal Zones
Along the coasts, the seafloor is shallow. Sunlight can often penetrate to the bottom, enabling benthic organisms to photosynthesise. Furthermore, sinking organic matter is intercepted by the seabed, where it supports benthic communities, before being recycled back into dissolved nutrients available for primary production.
Coastal zones are highly productive ecosystems due to proximity to land and nutrients, interception of sinking matter and propensity for coastal upwelling.
Upwellings
Regions of upwelling are the most productive waters in the world. Although they occupy less than 2% of the oceans, they support high biodiversity and provide 20% of the world's fish harvest.
Upwellings are areas where cold, nutrient-rich deep water flows upwards towards the surface.
Due to their high biodiversity, regions of upwelling are hugely important for fisheries, providing social and economic benefits.
Locations and intensities of upwellings are influenced by changes in atmospheric circulation, such as the El Niño climate pattern.
The exceptional warming of surface waters in the eastern tropical Pacific Ocean is referred to as El Nino. It is a climate pattern. It is part of a bigger phenomena known as the El Nino-Southern Oscillation (ENSO). El Nino is the 'warm phase' of this. The unusual cooling of the area's surface waters is referred to as La Nina, the "cold phase" of ENSO. The Southern Oscillation represents the fluctuations in ENSO's atmospheric component, whilst El Nino and La Nina are thought to be its ocean components. El Nino affects local weather from Australia to South America and beyond as well as ocean temperatures, the strength and speed of ocean currents, the condition of coastal fisheries, and more.
Upwellings can cause phytoplankton blooms.
I hope that this article has clarified biological ocean productivity for you. Remember that the limiting factors of ocean productivity are light and nutrient availability; and the thermocline, which separates the nutrient-rich waters from the photic zone, limits primary productivity.
The Biological Productivity Of the Ocean - Key takeaways
- Ocean productivity is the primary production of single-celled phytoplankton suspended in the ocean.
- Phytoplankton supply organic matter to heterotrophs. Most organic matter is respired back into dissolved inorganic forms, available to be used again by phytoplankton.
- Primary productivity is limited by nutrient and light availability. Colder, nutrient-rich waters are trapped below warmer, sunlit water.
- Open ocean has a low rate of productivity, compared to coastal zones and areas near upwellings.
1. Biology Dictionary, Benthos, 2017
2. Biology Dictionary, Zooplankton, 2018
3. Britannica, Marine Ecosystems, 2018
4. Britannica, Primary Productivity, 2022
5. Daniel Sigman, The Biological Productivity of the Ocean, Nature Education, 2012
6. Geomar, Research on the ocean’s most productive areas, 2019
7. Neil Campbell, Biology: A Global Approach Eleventh Edition, 2018
8. Rebecca Lindsey, What are Phytoplankton?, 2010
Explanations 
Textbooks 
Exams 
Magazine 
