A low oxygen “dead zone” has been killing fish in the lower Neuse River for more than a week, resulting in roughly 150,000 dead menhaden so far, according to Lower Neuse River Keeper Travis Graves.
The fish have been washing up on Flanners Beach in the Neuse River Recreation Area in Craven County. Fishers setting crab pots in the estuary have also reported pulling up dead blue crabs, Graves said.
While fish kills are not uncommon at that location, Graves said they are not a sign of healthy ecosystem.
“This isn't something that should be happening on the river,” he said. “But unfortunately, we do see it almost every year.”
Dead zones happen in the wake of large algal blooms. After the algae die, bacteria move in and multiply, feeding on the vegetation and causing it to decompose. Eventually, the bacteria use up most, or even all, of the dissolved oxygen in the water. Without enough oxygen, marine life will either leave the area, weaken or die.
Algal blooms can occur naturally, but they are more likely to happen – and happen on a large scale – when the water is spiked with excess nutrients, especially nitrogen. Fertilizer and animal waste running off the land are a major source of nitrogen in rivers and streams.
Massive fish kills in the late 1990s spurred the state and the U.S. Environmental Protection Agency to set maximum limits on nitrogen levels in the waterways, as instructed by the Clean Water Act. The state also adopted the goal of reducing overall nitrogen input into the Neuse estuary by 30 perecent.
Since then, wastewater treatment facilities and large industries that discharge directly into the water have “all done a really commendable job of reducing their nutrient discharge into the water,” said Graves. But non-point source pollution, such as runoff from fields, farms and houses, is much more difficult to control.
Riparian buffers – the strips of vegetation along rivers and streams – absorb pollution and slow runoff from entering the water, Graves said. “Riparian buffers really are the best bang-for-the-buck protection that we have,” he said.
Proposed legislation that would reduce the width of riparian buffers under the Neuse River rules and allow more disturbance near the water would further stress the system, Graves said. Sen. Trudy Wade, a Guilford County Republican sponsoring the legislation, could not be reached for comment Thursday.
A relatively wet winter and early spring delivered a lot of fresh water to the estuary and with it nitrogen-containing runoff, said Hans Paerl, professor of marine and environmental science at the UNC Institute of Marine Sciences in Morehead City.
“The scenario that we’ve had from the spring into the summer is kind of a perfect storm scenario,” said Paerl.
The heavy dose of freshwater sits atop the denser, ocean saltwater. Like oil and water, Paerl said, these two types of water don’t mix, and without mixing, the water at the bottom of the estuary cannot be replenished with oxygen from the air.
Paerl and colleagues have been monitoring the lower Neuse River since the early 1990s, as part of a partnership between UNC and the N.C. Department of Environmental and Natural Resources.
His team samples dissolved oxygen, chlorophyll, temperature, and other parameters in the estuary every two weeks and has gotten better at predicting fish kills, he said.
They last sampled Monday, when they noted conditions were ripe for a kill. Reports of dead fish began coming in late Tuesday and early Wednesday, he said.
Graves says he began noticing dead fish about 10 days ago.