When their environment becomes unsuitable, akinetes, which are spore-like bodies, are produced asexually by Zygnema colonies. Their very thick cell walls allow these algae to withstand droughts and harsh winters. Sexual reproduction may also commence in poor environments as two side-by-side filaments grow conjugation tubes toward each other. DNA from each plant moves toward the center of the tube, combines, and forms a zygospore. The zygospore sinks into the sediments of the still waters, awaiting more favorable habitat conditions before emerging as new algal strands. However, when nitrates and phosphates are available in sufficient quantities, most frequently vegetative reproduction, or fragmentation of Zygnema, occurs. As with Spirogyra, thick mats and green clouds of Zygnema can indicate over-fertilization or "enrichment" of water bodies, often by contaminated storm water runoff.