Researchers are concerned about the appearance of a "dead zone" covering about one-third of Lake Erie, according to this article in The Marion Star. The dead zone, a low-oxygen area of the lake in which fish cannot survive, appeared in the late 1990s, after years of improvement in water quality in this once heavily polluted Great Lake. The primary suspect at this point is the Quagga Mussel (Dreissena bugensis), a European native that is closely related to the Zebra Mussel (D. polymorpha). Quagga mussels have been known in the Great Lakes since the 1990s, and can cause problems by digesting organic material from the lake bottom while releasing phosphorus. The phosphorus contributes to algal growth, which can severely deplete dissolved oxygen levels in the water. Other potential contributors to the problem include increased sewage and agricultural runoff, and a drop in lake water levels.