Open systems include natural waters, lakes, and dams where the growth of the microalgae of interest either occurs naturally or is encouraged through addition of nutrients. Harvesting is carried out in situ; for example, Spirulina is harvested com­mercially from Lake Texcoco in Mexico. Although naturally harvested microalgae incur very little cost in cultivation, the productivity and product quality (biologically and toxicologically) cannot be assured (Lee, 2001).

5.3.1.1 Circular Ponds

The first mass culture of microalgae was carried out in circular ponds (Lee, 2001). They are generally simple, round, concrete ponds or dams, mixed by a rotating cir­cular arm fixed in the center of the pond, or by manual stirring. The size of the pond is limited by the strain of the water resistance against the rotating motor. The largest reported pond is 50 m in diameter (Lee, 2001). They are commonly used in Japan, Taiwan, Indonesia, and Ukraine for Chlorella cultivation (Lee, 2001; Pulz, 2001).