The surveys of the Chesapeake Bay rated the oxygen levels as near average in 2025, but there were sharp variations - particularly in July. In our weekly series with the Bay Journal Delmarva Public Media's Don Rush talks with editor-at-large Karl Blankenship about what the surveys revealed.
RUSH: The overall oxygen conditions in the Chesapeake Bay may have been judged average, but there were areas that were notably worse. This is Don Rush. Between rains and a hot summer, one study found that around 20% of the main portion of the Bay saw serious conditions. In our weekly series of Bay Journal, we talked with editor at large Karl Blankenship about how serious this is.
BLANKENSHIP: It was kind of a mixed result last year, overall scientists said the size of the dead zone fell within what they consider to be the normal range, but there was a lot of variation during the year and early July, which is a key growing season, it was quite a bit larger than normal. But early and late in the year, it tended to be less severe than normal. So it's kind of average, but average kind of masks a lot of variations that saw during the year.
RUSH: In terms of the main portion of the Chesapeake Bay, what are we looking at there?
BLANKENSHIP: In Maryland's portion of the bay in early July, the Department of Natural Resources found that almost 20% of its portion of the bay was hypoxic, which means it had too little oxygen for most aquatic life, which was the fifth worst in 40 years. Bay-wide, the Virginia Institute of Marine Science does a bay-wide estimate. The monitoring is done like every two weeks, and the Virginia Institute of Marine Science has a computer model, which is based on monitoring that tries to fill in the gaps of what's going on in between when water sancos are actually drawn and its conclusions was that bay-wide, in early July, about 12% of the bay was hypoxic, which again, bay-wide was one of the worst early Julys on record. The bay-wide figure is always better than the Maryland figure because when you get to the lower bay, there's a lot of water mixing from the ocean, and so that tends to relieve the low oxygen conditions. But they both agreed that in early July it was worse than normal.
RUSH: So what have been the factors then in this kind of variation, in terms of the dead zone?
BLANKENSHIP: A lot of factors contribute to that. When nutrients spur too much algae growth, the excess sinks to the bottom and it's decomposed by bacteria and a process that draws oxygen out of the water. So, when there's higher than normal river flows into the bay early in the year, it creates a larger than normal dead zone because those high flows wash in lots of nutrients. But the high flows also create really strong stratification between the surface waters where oxygens recharge from the atmosphere and bottom waters, which are losing oxygen because the algae is being decomposed by bacteria. And so it's those high flows that set up conditions that in large part determine the extent of hypoxia. But other factors during the summer are really important too. When it's hotter than normal like it was last year, that increases the stratification and prevents the mixing of top and bottom waters in the bay. On the other hand, when there's a lot of wind, which started to happen last year in August, when there's more wind than normal, it mixes the water. And again, that reduces the extent of the Dead Zone
RUSH: Bay Journal editor at large, Karl Blankenship on the oxygen conditions in the Chesapeake Bay. This is Don Rush for Delmarva Public Media.
