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Plant growth in the oceans: Resources and controls

Plant growth Resource limitation Summary Grazing control

Plant growth Resource limitation Summary Grazing control

These tiny, single-celled plants grow simply by dividing into two new cells The most important plants in the ocean are phytoplankton

If the conditions are good for growth, the number of cells will double each day

DAY 1 1 cell

DAY 2 2 cells

DAY 3 4 cells

DAY 4 8 cells

Population (number of cells) Starts with one cell, one cell division per day

This growth is exponential, and is described by a model of the type: where N is number of cells and t is time, and the constant k is related to the doubling time:

DAY million cells The exponential model is obviously correct in describing the process of continuous cell division, but it predicts an unrealistic ever-increasing biomass

In reality, there is a limit to the amount of phytoplankton biomass that a habitat will support, so the rate of population increase starts to decline

This logistic model is more complex, but makes a more realistic prediction Starting population Equilibrium population Rate constant Time

Plant growth Resource limitation Summary Grazing control

What can limit phytoplankton growth? Too little light Lack of materials to build new cells

WATER OXYGEN CARBOHYDRATE CARBON DIOXIDE LIGHT STEP Like all plants, phytoplankton algae grow by photosynthesis They build complex organic materials from carbon dioxide and water, using light as an energy source

Light decreases as you go deeper into the ocean

The decrease in light with depth is also described well using an exponential model, this time a decay: Light at depth ‘z2’ Light at depth ‘z1’ Absorption coefficient Difference in depths

Photosynthesis is dependent on light

There is more photosynthesis where water is clear than where it is turbid Depth (metres) Photosynthetic rate

Too dark for growth surface 100 m 500 m 1000 m 3500 m Enough light here for growth

So phytoplankton will grow well when they are close to the surface... … but phytoplankton that are deeper down will not receive enough light for growth

The light available for phytoplankton is controlled by a number of different environmental factors

Phytoplankton production Phytoplankton biomass LIGHT ENERGY

Phytoplankton production Phytoplankton biomass TurbidityWeather Ocean circulation Ice coverClimate Incident solar radiation Mixed layer temperature Average light in surface mixed layer Wind mixing

In addition to light, phytoplankton need a wide range of chemical elements to build organic material

Carbon is the most abundant element in organic material However, carbon is in plentiful supply in the surface oceans Carbon dioxide is readily soluble in water, and can diffuse in from the atmosphere

Other elements are needed to build phytoplankton biomass Some elements are required in large amounts, whilst others are needed only in minute quantities

These elements are usually called nutrients Nutrients needed in large amounts include nitrogen, silicon and phosphorus

CHEMICAL NUTRIENTS Phytoplankton production Phytoplankton biomass TurbidityWeather Ocean circulation Ice coverClimate Incident solar radiation Mixed layer temperature Wind mixing Average light in surface mixed layer

Again, there are complex environmental controls on the amount of nutrients available to phytoplankton

Phytoplankton production Phytoplankton biomass TurbidityWeatherIce coverClimate Incident solar radiation Mixed layer temperature Nutrient availability Wind mixing Ocean circulation Chemical nutrient supply Biogenic regeneration Average light in surface mixed layer

Trace elements are elements that are needed in very small amounts Many are metals, such as iron, zinc and cobalt. They may form parts of enzymes, so that trace elements can be important controls on growth and uptake of nutrients

Phytoplankton production Phytoplankton biomass TurbidityWeatherIce coverClimate Incident solar radiation Mixed layer temperature Nutrient availability Wind mixing Ocean circulation Chemical nutrient supply Biogenic regeneration Trace elements Average light in surface mixed layer

Plant growth Resource limitation Summary Grazing control

So far, we have looked at controls on phytoplankton growth rates

Removal of phytoplankton is an important control on both biomass and production

BIOMASS LOSSES Phytoplankton production Phytoplankton biomass TurbidityWeatherIce coverClimate Incident solar radiation Mixed layer temperature Nutrient availability Wind mixing Ocean circulation Chemical nutrient supply Biogenic regeneration Trace elements Average light in surface mixed layer

Herbivorous animals that graze phytoplankton are small, ranging in size from fractions of a millimetre to a few centimetres These animals grow and reproduce rapidly, so that their population increases fast when phytoplankton biomass increases This means that grazing is an effective control on phytoplankton

The overall picture of control on phytoplankton involves many factors, often inter-related

Sedimentation TurbidityWeatherIce coverClimate Incident solar radiation Mixed layer temperature Average light in mixed layer Nutrient availability Wind mixing Ocean circulation Chemical nutrient supply Biogenic regeneration Trace elements Phytoplankton production Phytoplankton biomass Grazing

Plant growth Resource limitation Summary Grazing control

Cell division gives rapid population increase Phytoplankton does not increase exponentially in nature You have seen that -

Light decreases with depthPhotosynthesis is determined by light, so decreases with depth and turbidity You have seen that -

Chemical nutrients are also important for phytoplankton growth Grazing by herbivores can control the abundance of phytoplankton You have seen that -

Sedimentation TurbidityWeatherIce coverClimate Incident solar radiation Mixed layer temperature Average light in mixed layer Nutrient availability Wind mixing Ocean circulation Chemical nutrient supply Biogenic regeneration Trace elements Phytoplankton production Phytoplankton biomass Grazing

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