Water weed. Clumps of water weed, Spirogyra sp. infest a ditch. Otherwise known as Mermaid's tresses" or "water silk", Spirogyra is a green alga which forms slimy threads in stagnant or slow- moving water. It is particularly abundant in spring and autumn. Spirogyra is capable of accumulating large amounts of phosphate from the surrounding water. Water draining from farm land is often rich in this chemical, and can cause the weed to grow rapidly. This growth blocks light from other plants which then die. The bacteria decomposing the dead vegetation can consume most of the water's oxygen, leading to extermination of many of the ditch's resident species."