The large natural river supply of phosphorus and nitrogen, essential nutrients for marine plants and algae, has always made the Black Sea very fertile. The tiny floating marine plants known as phytoplankton which form the base of the marine food chain are either eaten or die and gradually fall to deeper waters where bacteria take care of decomposing them, almost completely. Replenishment of the bottom waters of sea with new seawater from the Mediterranean takes hundreds of years. The bacteria in the bottom waters quickly consume all the oxygen and the sea is virtually dead below a depth of about 180 metres. The Black Sea is the biggest natural anoxic basin in the world. Despite this situation, for millennia, its surface waters supported a rich and diverse marine life. Its coastal inhabitants prospered from the abundant fisheries and, more recently, from the millions of tourists who flocked from all over eastern and central Europe to bathe in its warm waters and enjoy the beauty of its shorelines, plains and mountains.