EASTON — It was an average year for dissolved oxygen conditions in the Maryland portion of the Chesapeake Bay, according to a report the Department of Natural Resources released this week.
The DNR, along with teams in Virginia with the Institute of Marine Science, monitors hypoxic water conditions twice a month during the summer, taking the last sampling in early September. Virginia’s teams monitor May through October.
According to the Summer 2018 Hypoxia Report, overall conditions were normal — dissolved oxygen volumes averaging a little less than a cubic mile — compared to the long-term average, measured from 1985 through 2017.
The largest displacement in the oxygen levels occurred in late July, when hypoxic conditions decreased to the Maryland record low. This was followed by a record high in hypoxic conditions in September.
Resource Assessment Service Director Bruce Michael said this fluctuation was largely a response to the extreme rain events in late June, prior to the cruise.
“Oxygen is trapped in the Bay. ... In the springtime, we have higher flows, which deliver more nutrients to the Bay,” Michael said.
Michael said oxygen is trapped throughout water columns in the top and bottom layers of the Bay. He said the bottom layer, being saltier and usually not mixing between the upper layer, often is broken down by prolonged winds. This can circulate oxygenated water to the bottom column.
“We want oxygen throughout the water column, it’s a natural phenomenon,” Michael said.
Michael said with more nutrients available, algae forms, dies and consumes oxygen, making less available for other organisms. The impact of the change in oxygen on the oysters, a chief filterer of water in the Bay, still is being studied.
“Oysters need some salt water, some fresh water. ... The fresh water probably killed more oysters,” Michael said. “Impacts we had to oysters are probably going to be more toward freshwater flows. That said, areas lower in the Bay with less salty water probably helps with the diseases, like MSX and Dermo.”
Michael said the Virginia Institute of Marine Science, which also studies hypoxic levels in the Bay, has similar findings. Some results differ due to varying locations and amount of studies, he said.
“Our results are largely the same,” he said.