The Biological Productivity Of the Ocean

Ocean Productivity Examples

Marine heterotrophs encompass a wide range of organisms. They are categorised into three main types.

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.

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.

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.

Silicon is only an essential nutrient for certain taxa of plankton, such as diatoms.

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.

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.

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.

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.

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.

^{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}