Phytoplankton

Phytoplankton. | © NIOZ (www.nioz.nl)

Plankton is the collective name for all free-floating organisms in water. While zooplankton refers to the animals, phytoplankton encompasses the plants. They range from one-celled algae to multicellular floating plants. Although most phytoplankton are too small to see with the naked eye, scientist can tell whether or not the water contains lots of plankton by the color of the sea. One-celled algae are responsible for producing 70% of the oxygen in the atmosphere. Without these microscopic plants, we would not be able to survive! They also form the basis of the food pyramid in the sea.

A plankton calendar

Although there is much less plankton activity in the darker months of the year, there is always some species of plankton in the North Sea throughout the year. And even though you may see a certain species in July of one year doesn't mean you will be see it necessarily in July of the next year. However, there is a certain sequence of when the various species appear so it is possible to make a kind of plankton calendar. Scientists at the Royal Dutch Marine Institute for Sea Research (NIOZ) did just that in 2008 for plankton found in the Marsdiep, the channel between Texel and Den Helder. There is a reproduction of that calendar displayed elsewhere in this encyclopedia.

Phytoplankton, iron and carbon dioxide

Phytoplankton uses carbon dioxide to grow. In that way, it fixes this greenhouse gas so that less remains in the atmosphere. In theory, more algal blooms result in a decrease in the greenhouse effect. There are various substances which are believed to cause an algal bloom, such as iron. For years now, there have been discussions concerning the effects of fertilizing the sea with iron. Recent research published in Nature in 2012 showed that iron does indeed promote algal blooms. However, it is unknown what other effects this may have on the marine environment. Therefore, the authors of the article do not recommend large-scale iron fertilization for combating climate change.

Decline in nutrients due to phytoplankton growth

Decrease in nutrients in the Marsdiep 1990-2000 | © Ecomare | Source: NIOZ (www.nioz.nl)

In order to grow, phytoplankton is dependent upon sunlight and nutrients such as nitrate, phosphate and silicic acid. The amount of light and nutrients can vary throughout the year. In the winter, there is too little light for the phytoplankton to grow. Although there are enough nutrients in the seawater, they are not used. Therefore, few phytoplankton are found in the winter.
An increase in sunlight is the most important factor for determining the increase of phytoplankton in the spring. The diatoms are the first to start multiplying. These species require relatively little light and grow well at low temperatures. This growth can occur quickly, with a cell division every day. They consume the  dissolved nitrate, phosphate and silicic acid. The silicic acid is used for forming the shell, while nitrate and phosphate are used for forming building materials (organic materials such as carbohydrates, proteins and fats).

After a period of time, when there is no more silicic acid in the water, diatom production ends and they slowly die off. In the meantime, the penetration of light and the temperature in the upper water column has increased. Other species of algae which do not need silicon, such as the flagellates, now have their chance.
Growth ends in the summer when the amount of nitrate and phosphate is used up by these species. The remaining algae die off and sink to the sea bottom, where they are eaten by benthic animals or decomposed by bacteria. Nutrients are released as the algae decompose, becoming available again for new phytoplankton. This process takes place in the shallow parts of the North Sea until the autumn. At that point, the light periods have become so short that further growth is just about impossible.

Overabundance

Phaeocystis | © Ecomare

Eutrophication occurs when there is an overabundance of nutrients in the water. Lots of nutrients means lots of phytoplankton reproduction, resulting in massive death, the so-called algae bloom. With the removal of certain phytoplankton species, room is made for other phytoplankton species. For example, a decline in diatoms allows flagellates such as Phaeocystis to increase. When taking the food web into account, this means that diatom consumers will decline while flagellate consumers will profit.

Small but indespensible

Total year production of phytoplankton in the Marsdiep | © Ecomare | Source: data from NIOZ |

If you were to count the number of plankton in a liter of North Sea water, you would find between 100,000 and 100 million individuals. These phytoplankton are capable of producing organic material using the sun as an energy source (photosynthesis). They grow rapidly, multiplying in many fold per year. In that way, phytoplankton provide a huge source of nutrition at the base of the food chain. In the process of photosynthesis, oxygen is also released. It either mixes with the water or enters the atmosphere. This source of oxygen makes up 70% of the available oxygen for living organisms, whether they live in the oceans or on land. There are also other plants in the sea, such as seaweed, but they are so much in the minority compared to the phytoplankton that their contribution as sources of oxygen and food plays a much less significant role.

Suspended in water

© Jolanda van Iperen, NIOZ

In order to grow, phytoplankton is dependent upon sunlight and nutrients such as nitrate, phosphate and silicic acid. The amount of light and nutrients can vary throughout the year. In the winter, there is too little light for the phytoplankton to grow. Although there are enough nutrients in the seawater, they are not used. Therefore, few phytoplankton are found in the winter.
An increase in sunlight is the most important factor for determining the increase of phytoplankton in the spring. The diatoms are the first to start multiplying. These species require relatively little light and grow well at low temperatures. This growth can occur quickly, with a cell division every day. They consume the  dissolved nitrate, phosphate and silicic acid. The silicic acid is used for forming the shell, while nitrate and phosphate are used for forming building materials (organic materials such as carbohydrates, proteins and fats).

After a period of time, when there is no more silicic acid in the water, diatom production ends and they slowly die off. In the meantime, the penetration of light and the temperature in the upper water column has increased. Other species of algae which do not need silicon, such as the flagellates, now have their chance.
Growth ends in the summer when the amount of nitrate and phosphate is used up by these species. The remaining algae die off and sink to the sea bottom, where they are eaten by benthic animals or decomposed by bacteria. Nutrients are released as the algae decompose, becoming available again for new phytoplankton. This process takes place in the shallow parts of the North Sea until the autumn. At that point, the light periods have become so short that further growth is just about impossible.

The amazing world of plankton

Most phytoplankton are only visible with a microscope. They often have fantastic shapes, from round to oval, sometimes with elegant protrusions. The cells appear green and brown from the chlorophyll granules. Most are made up of one cell, however colonies of multi-celled species also exist. If you look more closely, then you can distinguish two groups of phytoplankton: cells with and cells without flagellates. A flagellate is a kind of thread used for propulsion. The small algae with flagellates belong to the flagelates, the others are diatoms.

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