The Chesapeake Bay is approximately 300 kg long and is divided into upper, middle, and lower bay regions. The Susquehanna River provides more than half the flow of the estuary. River flow drives circulation in the estuary and lower-layer counter-flow creates a relatively long residence time of 90–180 days. It also controls stratification from spring to fall. Episodic wind-mixing events periodically break down stratification, but it is quickly reestablished within a few days.
Hypoxia develops in the bottom layer in the upper bay in late spring and then expands southward during the summer. The initial spring decline is primarily caused by increased organic deposition from higher freshwater inflow that produces increased oxygen demand. DO declines in late spring, and throughout the summer is more closely related to symptoms of eutrophication.
Increasing development in the watershed has increased nutrient loading to Chesapeake Bay 2.5 fold from 1945 to 1990. Nutrient concentrations are highest in the tidal freshwater upper bay and then rapidly decrease seaward. Light attenuation in the upper Chesapeake Bay is mainly caused by suspended sediments, which tend to limit algal production. In the middle and lower bays, both phytoplankton and suspended sediments affect light attenuation. Sharp declines in water clarity are usually caused by algal blooms triggered by high levels of nutrients. Eutrophication-induced shifts in the phytoplankton community has caused increases in harmful algal blooms.
Because of turbidity, submerged aquatic vegetation cover in most portions of the bay system has decreased. Important benthic habitat also has been lost because of hypoxia, and oyster abundance has dramatically declined. It appears the hypoxia has become more severe in recent years even at equivalent levels of nitrogen loading. This implies that the bay has become less able to assimilate nitrogen inputs without developing hypoxia.
Water quality stations established by various research groups are located throughout Chesapeake Bay and are used to collect nutrient, Secchi disk, temperature, DO, and other data. Those data are used to examine spatial and temporal trends and relationships among parameters such as the percentage of seagrass cover over time (National Estuarine Experts Workgroup 2010).
Research:
National Estuarine Experts Workgroup. 2010. Nutrients in Estuaries: A Summary Report of the National Estuarine Experts Workgroup 2005–2007. U.S. Environmental Protection Agency. Accessed October 2016. https://www.epa.gov/sites/production/files/documents/nutrients-in-estuaries-november-2010.pdf.