Greenish growths on lakes, ponds and rivers are common across the U.S., typically blamed on excess nitrogen in runoff water carrying fertilizer from farmland. The problem in Prospect Park and Central Park is different: the artificial lakes and ponds, designed by Frederick Law Olmsted and Calvert Vaux, are filled with the same water that New Yorkers drink. The potassium-enriched tap water helps the scum flourish.  The mild winter and hot summer have exacerbated the problem this year.