 The arrival of spring often presents a problem for owners as the beautiful, clear water of their ponds disappears beneath a blanket of green, slimy algae. String algae tends to bloom or grow excessively in the early months of spring, spreading over the surface in an unsightly manor.
There are thousands of species of pond algae, but for simplicity they can be classified in three categories: microscopic, filamentous , and attached-erect string algae.
Microscopic algae, also called phytoplankton, are tiny, free-floating cells that give the water its characteristic green color. This type can undergo excessive blooms during mid-summer months, rising to the surface as a layer of yellow-green or reddish scum. A sudden die-off of microscopic algael blooms, caused by a change in water temperature or a stretch of several overcast days, can deplete dissolved oxygen levels in ponds to a critical level for the survival of aquatic organisms.
Green filamentous algae gives owners the most headaches. Many species of this type of algae are tolerant of cold water temperatures and undergo blooms in early spring. Ponds having recurring problems begin to exhibit blooms as early as March, although some algae blooms in late February have been reported.
Blooms begin in clear water in shallow areas where sunlight can penetrate the water to reach the soil of the bottom. The string cells join together in long stands resembling green hairs, which grow in fur-like clumps along the bottom and edges, breaking off and floating to the surface to form dense mats. Sudden die-offs of dense blooms can create serious water quality problems, not to mention unattractive conditions as the dead algae decays.
There are a variety of options for controlling this problem. Algae growth is stimulated by light penetration in water and the availability of nutrients needed for plant growth, such as carbon, nitrogen and phosphorus. Light penetration to the bottom occurs in the shallow areas. These shallow areas are where growth of algae and weeds typically starts. Carbon and nitrogen are generally abundant nutrients in ponds. The lack of available phosphorus is usually a factor that keeps blooms at bay. Ponds that receive loads of nutrients, especially phosphorus, tend to experience chronic problems with algae blooms. | 
