Further problems with the environment arose when a vast algal bloom appeared in one-fifth of Lake Okeechobee in 1986, the same year cattails were discovered overtaking sawgrass marshes in Loxahatchee National Wildlife Refuge. Scientists discovered that phosphorus, used as a fertilizer in the EAA, was flushed into canals and pumped back into the lake. When the lake drained, the phosphorus entered the water in the marshes, changing the nutrient levels. It kept periphyton from forming marl, one of two soils in the Everglades. The arrival of phosphorus allowed cattails to spread quickly. The cattails grew in dense mats—too thick for birds or alligators to nest in. It also dissolved oxygen in the peat, promoted algae, and prohibited growth of native invertebrates on the bottom of the food chain.