Chesapeake Bay - Indicator Details:

This map shows how often (% of sampling times) dissolved oxygen levels were above or below threshold between June and September 2009. Good water quality is characterized by having dissolved oxygen levels that are frequently above the threshold (indicated by shades of green), while poor water quality tends to have dissolved oxygen that is frequently below the threshold (indicated by shades of yellow and orange). In 2009, many regions of the Bay had dissolved oxygen levels that were frequently above the threshold. Areas of the Bay that frequently had poor dissolved oxygen levels in relation to the thresholds include the Pataspco and Back Rivers, portions of the Mid Bay, and the Elizabeth River.

This map shows average bottom dissolved oxygen levels for summer (June to September) 2009. The most extensive areas of low average dissolved oxygen occurred in the deeper channels of the mainstem Bay and lower Potomac River. However, small areas of low dissolved oxygen were also observed in the lower reaches of many smaller tributaries including the Rappahannock, Patapsco, and Chester Rivers. Low dissolved oxygen in the Bay's mainstem is controlled primarily by the quantity of organic matter and nutrients flowing out of the Susquehanna River during the preceding spring. While this map provides a broad-level assessment of dissolved oxygen in the Bay, it does not accurately capture small and short-term low dissolved oxygen events that are often attributed to causing fish kills.