Dead zones begin to form when excess nutrients, primarily nitrogen and phosphorus, enter coastal waters and help fertilize blooms of algae. Major nutrient sources include fertilizers, wastewater, and the burning of fossil fuels. When these algae die and sink to the bottom, they provide a rich food source for bacteria, which in the act of decomposition consume dissolved oxygen from surrounding waters. If stratification of the water column prevents the mixing or dissolution of atmospheric oxygen into these waters, they will remain oxygen poor.
Stratification occurs frequently in the deeper waters of Chesapeake Bay during the summer doldrums, when calm conditions minimize physical mixing of the water column by surface waves. Density differences also encourage stratification. In Chesapeake Bay, input from freshwater tributaries often forms a low-density surface layer atop the saltier, denser bottom waters that enter the Bay mouth from the Atlantic Ocean.
Shallow waters are much less likely to stratify compared to deep waters, and are thus less likely to develop hypoxia. First, shallow waters tend to be well-mixed by winds and tides. Second, waters that are shallow and clear enough to allow light to reach the bottom can support primary producers such as phytoplankton, algae, and seagrasses that release oxygen during photosynthesis.