For the past few decades dead zones have become a yearly occurrence. Dead zones form in summers when higher temperatures reduce the oxygen holding capacity of the water, the air is still and especially in years of heavy rains that carry excess nutrient pollution from cities and farms. The excess nutrient pollution combined with mild weather encourages the explosive growth of phytoplankton, which is a single-celled algae. While the phytoplankton produces oxygen during photosynthesis, when there is excessive growth of algae the light is chocked out and the algae die and fall below the interface between the warmer fresh water and fall into the colder sea water. The phytoplankton is decomposed by bacteria, which consumes the already depleted oxygen in the lower salt level, leaving dead oysters, clams, fish and crabs in their wake.
In a wedge estuary such as Chesapeake Bay where the layers of fresh and salt water are not well mixed, there are several sources of dissolved oxygen. The most important is the atmosphere. At sea level, air contains about 21% oxygen, while the Bay’s waters contain only a small fraction of a percent. This large difference between the amount of oxygen results in oxygen naturally dissolving into the water. This process is further enhanced by the wind, which mixes the surface of the water. Scientists are just beginning to study the impact of the winds in delivering oxygen to various water layers. The other important sources of oxygen in the water are phytoplankton and aquatic grasses which produce oxygen during photosynthesis, but when they die consume oxygen during decomposition by bacteria. Finally, dissolved oxygen flows into the Bay with the water coming from streams, rivers, and the Atlantic Ocean. http://www.dailypress.com/news/science/dead-rise-blog/dp-scientists-say-wind-reduces-chesapeake-bay-dead-zones-20110725,0,7046457.story?track=rss
Water temperature and total river flow are linked to the size of the summer dead zone. The peak of oxygen depletion typically occurs in July or August. Water temperatures are highest during these months and the days are longest accelerating the growth of phytoplankton that ultimately consumes all the dissolved oxygen. The dead zone is typically gone by November. Cooler air temperatures at this time of year chill the surface waters, while the deeper water remains warm and allows more mixing of the layers during storms. Cooler water also will hold more oxygen. The size and shape of the dead zone is variable from month to month during the summer. Typically, the largest dead zones occur in years with the highest spring snow melts and rains. Though this year may surpass it, 1993 had the largest cumulative spring river flows of the past 22 years and the largest average summer dead zone with an average of 5.2% of the channel.
River flow volume is linked to increased size of the dead zone because the heavy rains and snow melt that create the river flow carry excess nutrients of nitrogen and phosphorus from agriculture, septic systems, overflows from sewage treatment plants and runoff from lawns, gardens and paved surfaces. These nutrients fuel the out of control grow of the phytoplankton that overwhelms the natural system. The decomposing phytoplankton, combined with higher water temperatures, can cause large areas of the deepest parts of the Chesapeake Bay's deep channel which is the ancient Susquehanna riverbed to have little or no oxygen to support marine life.
These excess nutrients are washed from agricultural fields and animal / feed lots as well as from suburban yards. The Chesapeake Bay Foundation has emphasized the importance of agricultural nutrient management plans and the Commonwealth is phasing out the use of phosphorus in ornamental lawns. However, an additional significant contribution to the excess nutrient contamination during storms is from waste water treatment plants and over taxed sewer systems. The sewer system in Baltimore (where the dead zone begins) still overflows with frightening regularity despite a consent order with the EPA signed in 2002. In a single storm in mid March of this year 4.7 million gallons of untreated but diluted sewer water overflowed from city sewer lines. That was just one of many spring storms. The Baltimore public works department is in the 8th year of a $1 billion rehabilitation of the city's aging, leaky sewer system, which won't be finished for several more years. In Washington DC one third of the sewer system is a combined system that is also subject to direct release of sewage during storms and snow melt. Combined sewer systems are sewers that are designed to collect rainwater runoff and domestic sewage, and industrial wastewater in the same pipe. http://cfpub.epa.gov/npdes/home.cfm?program_id=5
The combined efforts of state and local governments with conservation organizations have made much progress since 1978 in improving the health of the Bay, but are still short of the regional goal. According to the indices created by the Chesapeake Bay Foundation, The Chesapeake Bay Program and Chesapeake EcoCheck, there has been little if any progress in the past decade in the health of the estuary. The size of this summer’s dead zone puts a big exclamation point on the work that still needs to be done to restore the health of the Chesapeake Bay estuary. http://stateofthecoast.noaa.gov/hypoxia/dead_zone.html
The Chesapeake Bay is not the only estuary with a dead zone. Dead zones have become common summer events caused by man, human waste, and the waste and excess nutrients from agriculture necessary to feed us and ornamental gardens to please us. It has be predicted by Researchers from Texas A&M University that the Gulf of Mexico dead zone currently estimated at 3,300 square miles will exceed the typical summer average of 5,600 square miles. The scientists are predicting more than 9,400 square miles of dead zone in the coastal waters of the estuary due to the record flooding in the Mississippi valley that flooded fields and towns up and down the river during the spring carrying with the flood waters the excess nutrients from farms, yards, septic systems and sewage treatment plants in its wake. The Gulf of Mexico Dead Zone is not expected to peak until late August.