The average daily flow into the Chesapeake remained at a record-setting pace through July, and the flows were taking a toll on Bay water quality, monitoring showed.
Through July, flows into the Chesapeake averaged 100.5 billion gallons per day, about 12 percent higher than 1996 — the previous record — when flows averaged 90 billion gallons per day, according to the U.S. Geological Survey.
This year’s flows are 157 percent higher than the 48-year average compiled since records have been kept.
As a result, record low levels of dissolved oxygen were set or tied in the deepest parts of the Chesapeake this May, according to Bay Program monitoring data.
While parts of the Bay go anoxic — devoid of oxygen — almost every summer, the onset of low-oxygen conditions appeared to start earlier than usual this year. Oxygen is critical for many Bay species, and when oxygen levels drop too low, species that live in affected areas either have to move or, if they can’t, suffer added stress or even die.
High flows contribute to low oxygen because they set up strong “stratification” between the fresh, top layer of the Bay and saltier, bottom waters. This prevents oxygen in the top layer from replenishing the bottom.
In addition, high flows typically carry added nutrients, which spur algae blooms. Much of the algae ultimately die and sink to the bottom where they are decomposed by bacteria, a process that depletes the water of oxygen.
Monitoring data also showed that water clarity was worse than normal in many areas, a reflection of algae growth as well as sediment flushed into the water by the high flows. Cloudy water blocks sunlight from reaching Bay grasses which provide food and habitat for crabs, fish, waterfowl and other species.
In July, the flow from the Bay’s tributaries was 41.6 billion gallons a day, or 15 percent above the average of 36.3 billion gallons a day, according to the USGS.