From MarineBiotech Infopages
Definition of Algae:
Collective name of a large group of chlorophyll-containing plants, comprising the sea-weeds and various freshwater forms, ranging in size from single cells to long stems.
This is the common definition for Algae, other definitions can be discussed in the article
Kelp is known as the largest marine algae. A relatively rapid increase in the population of (usually) phytoplankton algae in an aquatic system can lead to algal blooms and the undesirable condition of eutrophication.