Microalgal biodiversity or the variation in life forms within a given ecological region is often considered the measure of a specified ecosystem. Biodiversity indicates not only the species abundance of the area, but also the sum of the genera, species, and ecosystems of a region. The water bodies in these diverse habitats harbor a wide vari­ety of microalgae that appear, disappear, and reappear during the changing seasons.

2.4.1 Freshwater

Algae present in various freshwater habitats such as ponds, puddles, lakes, agricultural lands, oxidation ponds, streams, canals, springs, water storage tanks, reservoirs, and rivers are described and enumerated. Sub-aerial algae that transpire on moist tree barks, the walls of buildings, and dripping rocks are also considered freshwa­ter algae by various algologists. The information on freshwater algae is vast, yet remains scattered. Blue-green algae, green algae, diatoms, and euglenoid flagellates are the main components of freshwater habitats. There are also other types, such as planktons (free floating), benthons (attached to sediments), epiphytic algae (attached to larger algae and hydrophytes), epilithic algae (on stones and rocks of reservoirs and lakes), epipelic algae (attached to sand and mud), endophytes (living inside the tissue of other plants), epizoic algae (on shell of snails), and endozoic forms (inside sponges). Considerable research has been carried out on various water bodies with reference to hydrological conditions and the periodicity, abundance, and seasonal variation of algae. Members of the green algae dominate in summer and become less dominant in winter. High temperatures and organic matter support the occurrence of euglenoids, while high phosphates and low organic matter facilitate the abundance of diatoms. An increase in pH, organic matter, and nutrients sometimes leads to the for­mation of blooms. Microcystis aeruginosa forms blooms in several highly alkaline and nutrient-rich ponds. Diurnal variations of microalgae have also been noticed. Lakes and reservoirs have been found to show highest algae occurrence during sum­mer and winter. The shallow waters near the shores contain epiphytic filamentous green algae, and the deeper waters support only deep euplanktonic organisms such as desmids and diatoms. Blue-green algae occur in low dissolved oxygen, abundant organic matter, and high temperature, and become possible indicators of pollution, while green algae occur with exactly the opposite conditions. Diatoms show seasonal periodicity, always reaching a maximum during summer, and correlate with the sili­cates. Desmids are very sensitive to pollution. Hydrobiological studies on polluted waters have led to spotting microalgal indicators by calculating the species diversity index. Studies on the relationship of microalgae and their nutrient requirements in some lakes have in certain cases led to developing approaches for conservation and prevention from pollution.