As the population along the Australian coast increased, so did agricultural activity. Fertilizers from farms run off into the reef, resulting in high nutrient concentration—an environment where algae takes over, killing the coral beneath. In addition, forty drainage basins comprise the Great Barrier Reef Catchment area—a place where water collects as it drains into the Great Barrier Reef. According to a report issued by the Great Barrier Reef Marine Park Authority, urban development and agricultural expansion has altered the flow of water in these catchment areas, causing reefs to flood. Combined with rising sea levels, this can result in reef drowning, where the water becomes too deep that the coral can't get the sunlight they need.