Nutrients cause excessive algae blooms. When there's more algae than can be consumed by predators, the excess die, sink to the bottom and are decomposed by bacteria in a process that depletes the water of oxygen. The amount of nutrients entering the Bay is strongly tied to precipitation. Rains flush large amounts of nitrogen and phosphorus off fields, lawns, golf courses and other land and into waterways. U.S. Geological Survey figures show just how strongly nutrients and flows go together.
High flows present a sort of one-two punch to the Bay.
That stratification prevents the top and bottom layers from mixing, meaning that when bottom oxygen is depleted, it is difficult for it to be replenished by the oxygen-rich surface waters. Compounding the picture further is that flows into the Bay increase at a faster rate than rainfall. That amount stays pretty constant - if 30 inches fall, Mother Nature still takes her 20 inches, or if 50 inches falls, she is still pretty content with the same 20. What that means is that any increased rainfall from climate change will go straight to the Bay, unless the rate of evaporation increases dramatically. As a result, said Ray Najjar, an oceanographer at Penn State, a 10 percent increase in rainfall actually translates into a 20 percent increase in flow reaching the Bay.