Dinoflagellates are all unicellular and the cell is naked or has a cell wall (of organic material) composed of a set of plates. The cell is divided into two halves by a broad horizontal groove connected with a groove running across. Each groove has a flagellum and it is the wave-motion of the flagella that drives the cell forward in a spiral. Dinoflagellates are common i fresh water, but we find their greatest diversity in the sea.

Diatoms. Diatoms have a dominant role in the primary production in sea- water. They are also to be found in freshwater. The diatoms have a wall, which is made of silica, and it is built of two parts, an upper and a lower scale with net of pores. The cell is yellow or brown in colour. They live single or are attached to each other in various ways to form colonies.

Example: Asterionella

Green-algae  (Chlorophytes) comprise one of the major groups of algae when one considers the abundance of species and their frequency of occurrence. Most of the green-algae are grass-green, but sometimes the green pigment are masked by other pigments in the chloroplasts. The group has both unicellular species and colonial species, some with flagella and some without. The plankton forms only occur in fresh water. In the sea the green-algae are mostly benthic.

Example: Volvox

Conjugate algae is a subgroup of green-algae. They are to be found in marches and fresh water. They do not occur in the sea and very few have been recorded from brackish water. A wall is enclosing the living content in the cell and the wall is composed of organic matter. The wall is divided in two symmetric parts and the two parts separates into two new cells. The conjugate algae are relatively large and easy to study. Some of them can reproduce sexually.

Example: Micrasterias

Golden algae  - (Chrysophytes) have got the name because of the predominance of golden-brown pigments covering the colour of chlorophyll. The cell is free-swimming with two flagella. Some are living alone and others are together in colonies. They are common only in fresh water.

Example: Dinobryon

Small flagellates. This group includes a very different kind of organisms, all with photosynthesis. They live solitary and all have one or two flagella. They live both in freshwater and in seawater and play a dominant role in water blooming in special time of the year. They are all very small and often it is necessary to study the living cells short after they are collected. The smallest autotrophic organisms are now thought to be of greatest importance in the biomass production.

Example: EmilianIa huxleyi