Phytoplankton

Plankton is the collective name for all free-floating organisms in water. Phytoplankton is the plant category. One will find between 100,000 and 100 million plankton (algae) per liter seawater in the North Sea. All life in the sea lives either directly or indirectly off of these algae. They are capable of producing organic materials using the sun as an energy source. Phytoplankton grows rapidly, multiplying in many fold per year. This provides a large source of nutrients at the base of the food chain.

Oxygen is released during photosynthesis and mixes with the water or air. Other living organisms in sea are not capable of making organic materials and therefore are totally dependent upon phytoplankton as a source of food. The released oxygen is an important supplement to the oxygen supply needed by the animals.

Phytoplankton is not the only plant life in the sea. Besides one-celled algae, all kinds of seaweed and salt marsh plants can be found along the coasts. Since there are many more phytoplankton than large seaweed and salt marsh plants, the latter plays a minor role in the food pyramid in the sea. They are only eaten by a few animals in the tidal zone.

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 one looks more closely, then one 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 without flagellates belong to the diatoms, and those with are called flagellates.

Decline in nutrients due to phytoplankton growth

In order to grow, phytoplankton is dependent upon light 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, phytoplankton is not found much 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 begin to multiply. These species require relatively little light and grow well at low temperatures. This growth can occur quickly, with a cell division every day. The dissolved nitrate, phosphate and silicic acid is hereby consumed. 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 the silicic acid is used up in the water, the production ends and the diatoms 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 algae which have not been eaten by the animals in the area die off and sink to the sea bottom. There they are eaten by benthic animals or decomposed by bacteria. The earlier mentioned nutrients are released during the decomposition of the algae, becoming available once again for the phytoplankton.

In the shallow parts of the North Sea, growth and death of the algae will occur in this manner until the autumn. At that point, the light periods have become so short that further growth is just about impossible.

A tremendous diversity in plants and animals live in the sea. They cannot survive without carbohydrates, proteins and fats. Animals cannot produce these building materials themselves; plants, including phytoplankton, can. Animals receive their building materials either directly or indirectly by eating algae or algae-eaters. Some zooplankton eat phytoplankton. In turn, the rest of the zooplankton, benthic animals and fish eat the zooplankton, and thereby indirectly receive their building materials, while they in turn are eaten by larger fish, seabirds and marine animals. This series of eat and be eaten is called the food chain. The relationship is usually more complicated than a simple chain so therefore a better term is food web.

Floating in water

It is important for the phytoplankton to stay afloat in the upper water column, and not sink to the bottom. 'Staying afloat' is a life requirement. Without sunlight they will die. The planktonic plants are not small and light for nothing. The lighter one is, the larger is the upper surface area in relation to the weight. And the larger the surface area, the slower one will sink (drag factor).

The floating ability by diatoms is increased through all kinds of protrusions in the form of hairs and needles. Some types have a drop of oil in their encasement to increase the floating ability. Furthermore, the complicated pattern of grooves, pits and perforations in the encasements also serve to increase the surface area and thereby the floating ability. Finally, a flat cell has a larger surface area than a round cell.

Flagellates have similar adaptations. The body of the planktonic plant Pyrocystis elegans has an armour with large protrusions to increase the floating ability.

Phtyoplankton and nutrients

When there are too many nutrients in the water, one speaks of eutrophication, and an algae bloom will result. This is probably also the cause of a shift in species composition by the phytoplankton. The percentage of diatoms will decline while the flagellates such as Phaeocystis will increase. When taking the food web into account, this means that diatom consumers will decline while flagellate consumers will profit.

Source: de Vleet, Ecomare