Monitoring data is slowly beginning to paint a picture of what happened during the spring 1993 “freshet,” which pumped billions of gallons of fresh water into the Chesapeake Bay.

The amount of water from rain and snowmelt that flowed down the Susquehanna River last April was the highest ever recorded. It reduced salinity levels in much of the Bay through July. And scientists studying the Atlantic Ocean last summer found a plume of low salinity water stretching 50 miles from the mouth of the Bay.

Scientists and water quality managers are still reviewing much of the data, but many say their initial reviews of the data indicate that the Bay held up fairly well, a surprise to many. Fish reproduction for many species was higher than normal, the size of grass beds appear to have expanded in the Bay, and many water quality parameters were not as bad as many had expected.

“It does not appear to be as big an issue as everyone thought it was going to be,” said Chris D’Elia, director of the National Oceanic and Atmospheric Administration’s Sea Grant Program at the University of Maryland. “I guess the bottom line is that estuaries are extremely variable, especially in high flow areas like the Chesapeake’s watershed.”

Indeed, the water flows this spring were huge. In April, the average daily flow down the Susquehanna was 246 billion gallons per day, breaking the record of 210 billion gallons per day set in June 1972 after Tropical Storm Agnes, according to figures from the U.S. Geological Survey.

Flows measured at the Conowingo Dam, 14 miles from the mouth of the river, exceeded 80,000 cubic feet per second — the dam’s maximum turbine capacity — for 42 straight days, from March 25 to May 5, according to USGS. Agnes exceeded 80,000 cubic feet per second for only 16 days. (Peak flows of Agnes were higher, however, topping 1 million cubic feet per second versus 467,000 cfs last spring.)

While flows were up in other tributaries as well, the huge Susquehanna flow is by far the main driving force in the Bay. During April, the Susquehanna contributed 62 percent of the flow to the Bay, the Potomac 18 percent, and the James 8 percent. Normally, the Susquehanna contributes 40 percent of the spring flow.

Last year, according to USGS, those flows carried huge amounts of nutrients and sediments:

  • During an average year, 2.6 billion pounds of sediment wash down the Susquehanna into the Bay. Last spring, twice that amount — 5.3 billion pounds — came down the Susquehanna in just five weeks.
  • During an average year, 52.8 million pounds of nitrogen wash down the Susquehanna. Last spring, more than twice that amount — 112 million pounds — came down the Susquehanna in just five weeks.
  • During an average year, 5.72 million pounds of phosphorus wash down the Susquehanna. Last spring, almost that much — 4.8 million pounds — came down the river in just five weeks.

High nutrient flows are important because they can overfertilize the Bay, creating huge algae blooms. When there is more algae than fish and other predators can consume, the excess clouds the water and prevents Bay grasses from getting the light they need to survive. Eventually, the algae sinks to the bottom and decomposes in a process that depletes the water of oxygen. This reduces the amount of habitat available for fish and other aquatic species.

Last year, though, scientists said the blooms in Maryland’s portion of the Bay appeared to be about normal through much of the summer. The reason, some suggested, was that sediment in the water blocked sunlight needed to spur algae growth, and the high flows pushed much of the nutrients into Virginia waters and even out the Bay.

Indeed, Bruce Neilson of the Virginia Institute of Marine Sciences reported higher than normal chlorophyll concentrations — a measure of algae abundance — as well as higher than normal nitrogen concentrations during April and May at his Virginia monitoring stations.

By late May, however, the bloom was completely gone, he said. The freshet “probably delivered a high nutrient flow,” he added, “but by having such a high flow, some of it probably moved straight through the system.”

Strong flows generally make oxygen conditions in deep portions of the Bay worse than normal. Bottom areas of the Bay contain dense salt water that flows up the estuary while the less-dense fresh water flows down the surface and out into the ocean.

The stronger the flows, the greater the the difference in density between the top and bottom layers. When such a high degree of “stratification” exists, there is little mixing between the layers. While the top layer gets oxygen from the atmosphere, decaying algae consumes the available oxygen in the bottom layers and there is no way to get more. Ultimately, these bottom layers can become anoxic — or totally depleted of oxygen.

Neilson said measurements of dissolved oxygen at his Virginia stations were lower than normal in May, but recovered to better than normal beginning in June.

Some scientists said the oxygen levels probably increased rapidly in the Virginia portion of the Bay because strong fresh water flows out of the Bay trigger strong salt water flows up the bottom. That quick influx of water restored oxygen levels in the bottom layer of the Bay, but by the time the flows reached Maryland, the oxygen was probably depleted.

Neilson, in fact, said his monitoring showed that the closer one got to the Maryland border, the worse the oxygen situation became.

Rob Magnien, of the Maryland Department of the Environment’s Chesapeake Bay and Watershed Management Administration, said dissolved oxygen levels along the bottom of Maryland’s portion of the Bay remained lower than normal through the first half of September. Magnien said the dissolved oxygen levels were “in the low range” of those that had been observed in the spring and summer during the past decade.

Likewise, he said, monitoring showed a higher than normal degree of stratification between the top and bottom layers of the Bay through July.

That points to the role that high flows can play in depleting oxygen even in years where the algae blooms only appear to be average.

“There’s usually enough material formed in a typical bloom, even if it’s not a huge one, to support the system going anoxic,” said Larry Harding, a research associate professor with the Maryland Sea Grant College. “We’re above some threshold level of material required to get to that condition, so the physics [of stratification] take over.”

Originally, many expected conditions to be worse because they thought there would be a series of rebounding blooms throughout the summer. That didn’t happen. In large part, they say, that was because the flows dropped dramatically after early spring. Starting in late May, the flows dropped to below normal, reducing the amount of nutrients available to fuel any additional blooms.

That appeared to benefit the Bay’s submerged aquatic vegetation, which provide important food and habitat for many of the Bay’s fish and shellfish.

Beds of Bay grasses were devastated after Agnes, and remained low for years, beginning a slow but steady comeback in the mid-1980s. Many thought this year’s load of nutrients and sediment would reverse that trend. But the opposite may be true.

“Overall in 1993, my initial estimate is that we will see an increase in the number of hectares of SAV,” said Bob Orth, an associate professor at the Virginia Institute of Marine Sciences. “How much, it’s hard to say.”

Orth based his estimate on a review of aerial photos from an annual survey of Bay grasses. Orth said the photos revealed a number of areas — notably in the York and Chester rivers — where grasses were either seen for the first time or dramatically expanded their coverage. “Even in the Susquehanna flats, where we thought it was going to be horrible, plants seem to have persisted,” he said.

The reason, he said, may be that many of the aquatic grasses are summer plants. The freshet may have been early enough that its impacts had largely worn off by the summer. “The heavy rains were followed by the faucet being turned off for a good month,” Orth said. “And that may have been what perhaps contributed to the healthy beds that we saw.”

Court Stevenson, a professor at the University of Maryland’s Horn Point Environmental Laboratory, whose research team was measuring water quality conditions every three hours throughout the summer in grass beds near the Susquehanna flats, said the turbid water cleared up when the flows declined. “It was near optimal water quality conditions,” he said, “and out in the flats there was more milfoil [a type of Bay grass] in deeper areas than we had ever seen.”

Both Orth and Stevenson said this year will be critical for the grasses. It is possible, they said, that the grasses used most of their energy reserves of carbohydrates to survive any early effects of the freshet along with later stresses from heat. “The more critical thing is next year [1994],” Stevenson said. “The worst thing about Agnes didn’t show up that year for SAV. And I think we need to keep a multiyear perspective.”

Grasses were not the only species that fared well. Striped bass recorded their best spawning year on record.

Steve Jordan, a biologist with the Maryland Department of Natural Resources and director of its Cooperative Oxford Laboratory, said that it has been thought for a number of years that high flows benefit striped bass spawning because the nutrients washed down the river feed zooplankton which, in turn, provide food for young rockfish. “There’s some evidence of that this year,” he said. “In the Choptank, where we had record recruitment for striped bass, we also had record numbers of zooplankton. But there are other conditions that, of course, complicate that whole picture, and they’re not fully understood.”

Indeed, while the high flows may have contributed to the spawn, a variety of other factors play a role, too — particularly water quality and water temperature. Spawning increased for a number of other species, including the white perch, yellow perch, spot, hickory shad, and blueback herring. Again, the spawning processes are not well enough understood to attribute all the increases to the freshet, though it may have played a role, officials say.

More certain is the oyster story, which was both good and bad. High fresh water flows in the Potomac River took a huge toll on oysters, killing more than 90 percent of the bivalves in some places. In Virginia, about half the oysters in the Rappahannock River and between 20 percent and 50 percent of the oysters in the upper James were killed. The oysters were particularly vulnerable to prolonged fresh water flows because the diseases MSX and Dermo — which have devastated oyster populations in recent years — had forced remaining oysters upstream.

But the diseases are slightly more susceptible to fresh water than the oysters, and in the upper Bay it was MSX — not the oyster — that was pushed out. “We are not seeing the levels of MSX that we saw last year [1992],” Jordan said. “What we have seen suggested that a lot of areas that had MSX last year are not infected this year. On the other hand, we haven’t seen any effect on Dermo.”

Few believe that any nutrients are left in the Bay from 1993 to fuel excessive blooms this year. Blooms stemming from Agnes appeared for several years after the 1972 storm. But Harding said that was because the peak flows of Agnes moved large amounts of terrestrial material — such as trees — into the Bay, which take more time to decompose, providing nutrients for years. “Tree limbs and leaves break down a lot slower,” Harding said. “The bacteria that are in the estuary are not used to munching on that kind of thing. It’s not what they use.”

Others agree, noting the absence of any follow-up blooms last summer. “It didn’t even carry over into the summer,” Magnien said, “so I wouldn’t expect it to have any big influence” in 1994.

Many cautioned against drawing major conclusions about what last year’s freshet says about the health of the Bay. The timing of the storm, they say, was critical. Had it occurred at a slightly different time — or had the temperatures been slightly warmer — the impacts on algae and grasses may have been different. The quick switch from high flows to low flows in May was also a factor in how the Bay responded.

“It’s very difficult to predict exactly what’s going to happen with exactly what kind of an event, and it’s one of the big problems we have in environmental science,” D’Elia said. “We spend a lot of time working on this and trying to understand it, and it is baffling at times to us. But it just is very striking how variable the Chesapeake is and how events are hard to assess.”

But some say there may be signs of good news if the apparent lack of lingering ill effects holds up. “I think it shows the resiliency of the Bay,” Magnien said. “It can take a hit like this and recover in fairly short order. And I think that’s a good reason to hope that once we drive some of these nutrient inputs down, we may not have to wait too long to see some of the benefits.