What factors lead to low dissolved oxygen levels during the summer?
Low summer dissolved oxygen in the mainstem of the Bay is controlled primarily by the quantity of nutrients flowing out of the Susquehanna River during the preceding spring. The excess nutrients lead to blooms of phytoplankton that settle to the bottom of the Bay when they die. Bacteria decompose the dead phytoplankton, a process which consumes dissolved oxygen. During summer, water temperatures increase and this enhances the rate of decomposition and subsequent reduction in dissolved oxygen levels. During summer, factors such as wind and river flow also play an important role in affecting dissolved oxygen levels.
