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Scientists: Oxygen levels in Inland Bays remain low

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Officials have been trying to reduce pollution in the Inland Bays watershed to restore the health of the bays. But, after decades of efforts to reduce pollution, scientists say the health of the bays has been slow to recover. One measure of bay health, dissolved oxygen, remains low in many parts of the bays.

Pollution reductions are supposed to help restore the health of the Inland Bays, including increasing the amount of oxygen available for fish. The reductions are in place, but there is no instant gratification as far as restoration is concerned.

Scientists in Delaware have a lot of data on dissolved-oxygen levels produced by an assortment of sampling methods, said Chris Bason, science coordinator for the Center for the Inland Bays. Low levels of dissolved oxygen and the need to restore seagrass were the primary reasons for establishing limits on pollution entering the bays and for the state pollution-control strategy for the Inland Bays, he said.

Scientists use the number of summer mornings in which dissolved oxygen is Jess than the level considered healthy for aquatic life as one indicator of the overall health of the bays, Bason said. He gave a presentation of 11 years of oxygen-testing data to the center's Scientific and Technical Advisory Committee Friday, July 16.

"The take-home message is that what we've seen at the site level, low dissolved oxygen is pretty much widespread," he said. Many areas in and around the bays aren't even close to meeting dissolved-oxygen level goals, he said. In samples taken before 9 a.m., 41 percent of Re-hoboth Bay, 47 percent of Indian River Bay and 12 percent of Little Assawoman Bay failed to meet the minimum standard of dissolved oxygen

"Low dissolved oxygen is moderate to severe in almost all our tributaries. The tributaries are still highly degraded by pollution," said Bason.

Dissolved-oxygen levels haven't improved over the last decade, even though nutrient pollution entering the bays has been reduced. Bason said the bays now receive less nutrient pollution than they once did, which eventually will lead to better oxygen levels and better fish habitat. But, oxygen levels have not yet recovered.

Dissolved oxygen changes according to a pattern. It's very low in the morning, in some places near zero, and rises throughout the day. "Fish leave the tributaries in the morning and when dissolved oxygen rises during the day, they return," said Bason.

"It's a trade-off for them. Food for young fish is in the tributaries. They expend a lot of energy to go back up to eat," said Bason. The theory is that expending that energy to return to tributary streams reduces the growth rate of the fish and even their reproduction, he said.

The Citizens Monitoring Program coordinated by the Univer-sity of Delaware takes water samples from the Inland Bays. So does the Department of Natural Resources and Environmental Control.

Bason said examining all the data taken over more than a decade, there is no evidence of an overall trend; in fact samples taken at each site show a lot of variation

Bason recommended some ad-justments to monitoring systems, including adding more continuous monitoring stations and increasing the number of spot sampling sites to give scientists more data to study.'





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