Dear shruti,

 You can identify using there morphological characters.

There are lot of literatures and monographs available for reference all you need is a compound research microscope.. 

Look for characters like color, cell wall, cell type, presence of flagella, location of flagella,number of flagella, motile or non-motile, chloroplast shape..

With regards

Perumal

Okay..thank you sir..! 😊 

Dear Shruti:

There is a very good Manual for starting to study the marine phytoplankton: 

Tomas, C.R. (ed.) 1996. Identifying Marine Phytoplankton. Academic Press, San Diego, California.

See it.

Good luck.

Eugenia

Okay..thank you mam..! 😊