Flows tend to higher in the spring and lower in the summer. In deeper areas of the Bay, this difference in density can lead to the formation of a pycnocline, which is a layer in the water column separating two areas of different density. The pycnocline can be a physical barrier to exchange between surface and bottom waters and is a contributing factor in the depletion of dissolved oxygen from bottom waters. This natural process has been occurring in the Bay for thousands of years. Nutrients are a necessary component of the Bay's food web.
Excess phytoplankton dies, consuming oxygen needed by other living resources.
Many species of fish and shellfish cannot tolerate low oxygen for a great length of time. So, areas below the pycnocline of low to no oxygen can become useless, reducing available habitat in the Bay. Also, when dissolved oxygen is depleted from bottom waters, complex chemical and biological interactions cause nutrients to be released from sediments. Disruptions to the pycnocline by wind allow nutrient-rich bottom waters to escape to the surface, where they fuel further phytoplankton growth and start the process again. The Chesapeake Bay Program keeps tab on this process by monitoring nitrogen, phosphorus and dissolved oxygen levels in the Bay.