Scientists predict severe dead zone, but are hopeful about underwater grass beds
By Karl Blankenship, BayJournal.com
The downpours that soaked 2018 have spilled into this year, with three of the first five months reporting higher-than-normal freshwater flows into the Chesapeake.
That will likely mean worse-than-normal oxygen conditions in the Bay. Scientists are predicting the fourth largest summertime dead zone in the last two decades.
Still, the often record-setting rains that commenced a year ago have not been a total washout for the estuary.
Underwater grass beds — a closely watched indicator of Bay health — appear to have survived last year’s influx of muddy water. Field reports from this spring made scientists cautiously optimistic that the beds may not suffer the extensive dieback they had feared.
But scientists say this spring’s high flows may have hurt the spawning run of shad in many places — it was the worst-ever on the Susquehanna. And biologists say the freshwater has allowed invasive blue catfish and snakeheads to turn up in places they haven’t been seen before.
Many species, though, will find much of the Bay off-limits this year because of poor oxygen conditions, according to scientists.
“The forecast this year reflects the high levels of precipitation that have been observed across the Bay’s watershed,” said Jeremy Testaof the University of Maryland Center for Environmental Science, which makes the annual Chesapeake dead zone prediction in conjunction with scientists from the University of Michigan.
“The high flows observed this spring, in combination with very high flows late last fall, are expected to result in large volumes of hypoxic and anoxic water,” Testa added.
Heavy rains wash large amounts of nutrients off the watershed and ultimately into the Bay, where they spur large algae blooms. When the algae die, they sink to the bottom and are decomposed by bacteria in a process that removes oxygen from the water, resulting in so-called dead zones in deep areas of the Bay.
Spring rains this year washed an estimated 102.6 million pounds of nitrogen from the Susquehanna River into the Bay, far more than the springtime average of 80.6 million pounds, according to the U.S. Geological Survey. The Potomac River contributed another 47.7 million pounds, compared with the normal flow of
Scientists predict that big influx of nutrients will produce about 2.1 cubic miles of hypoxic water — water with too little oxygen to support most Bay species — or about one-sixth of the Chesapeake. Between 0.49 and 0.63 cubic miles, or about 4 percent of the Bay, will be anoxic, meaning water in that area will have essentially no oxygen.
“The forecast is not surprising considering the near-record high flows in 2018 that have continued into 2019,” said Bruce Michael, director of the Resource Assessment Service of the Maryland Department of Natural Resources. He added, though, that the long-term trend has been toward a slight improvement in oxygen conditions.
USGS figures show that, except for March and April, river flows into the Chesapeake Bay have been higher than normal every month starting last July, one of the longest periods of sustained high river flows on record.
Because persistent high flows drive bloom-feeding nutrients and cloud the water with sediment, they are generally considered bad news for underwater grasses, which depend on clear water to get the sunlight they need to survive.
After measuring a record 104,843 acres of grasses in the Bay in 2017, scientists were braced for a significant retreat. But it appears the decline last year may be limited, though the results won’t be known until surveys are completed later this year.
Bob Orth, the Virginia Institute of Marine Science researcher who oversees the annual underwater grass survey, described his outlook as “cautious optimism.”
Much of last year’s survey was completed before consistently heavy rainfalls began last July. At that point, about 80 percent of the Bay had been surveyed, and results showed it was likely en route to another record-setting year with grass beds being found in some places they had not previously been seen, said Dave Wilcox, a VIMS scientists who works on the survey.
But the persistent high flows and muddy water prevented the survey from being completed.
Scientists say some of the beds died back amidst the high flows, especially in the Upper Bay and upper parts of several tidal tributaries where conditions were the worst.
But scattered satellite imagery and field observations made last year indicate that many beds in hard-hit areas appear to have survived. That includes the bed in Susquehanna Flats which, despite bearing the brunt of high flows from the Bay’s largest tributary, appeared largely intact.
Biologists caution that it is possible that the stress caused by poor water quality could still result in diebacks that will not be evident until this year. While they expect declines, scientists are optimistic it won’t be as catastrophic as they had feared.
This spring, scientists reported seeing apparently healthy beds when they’ve been out on the water, and citizens have reported robust beds in areas such as the Severn River in Maryland.
Brooke Landry, a biologist with the Maryland DNR, said citizens trained to survey underwater grass beds have reported large areas of horned pondweed this spring, one of the earliest underwater plants to emerge in Maryland.
“That is really good news,” she said. “In areas where there may have been a little horned pondweed here and there in earlier years, there’s a lot of it out there now.”
Grasses are considered one of the most important indictors of Bay health because they require clear water to survive. They are also important habitats for many types of fish, juvenile blue crabs, and waterfowl.
But the high flows have been a mixed bag for other Bay species. This spring was the worst year on record for fish passage at the Conowingo Dam on the Susquehanna, where just 4,787 migrating fish were lifted over the 94-foot-high structure.
More than 190,000 have been lifted over the dam by multimillion-dollar elevators in some years. Biologists said high flows in May contributed to this year’s poor performance.
In Virginia, biologists also blamed high flows for contributing to poorer-than-average American shad runs on the James, York and Rappahannock rivers. On the James, the annual VIMS survey of shad and river herring saw just nine American shad this spring, said Pat McGrath, the biologist who oversees the effort.
The good news, he said, is that years with high flows often result in the good survival of young from those fish that do spawn. Last year’s juvenile fish surveys showed better-than-average survival for several anadromous fish — those that live in the ocean but return to rivers to spawn — including shad and striped bass.
“Hopefully, all this water translates to a good ‘year class’ with juvenile shad,” McGrath said.
One species that clearly seemed to benefit, he said, was blue catfish — a fish introduced in the 1970s, whose population has dramatically expanded in the last two decades.
The invasive species is confined to fresh and brackish water, but McGrath said there was so much freshwater in the James this spring that their survey was netting hundreds near the James River Bridge by Newport News, which is normally well beyond the range of blue catfish.
Biologists around the Bay say reduced salinities over the last year have allowed both blue catfish and snakeheads, another invasive species, to expand their range.