For years, many have thought that a pristine Chesapeake was carpeted with grass beds that may have covered roughly 640,000 acres of the estuary’s bottom.

In fact, the Bay Program in 1993 set a long-term target of restoring roughly that amount of submerged aquatic vegetation in the Chesapeake Bay. The number was based on the approximate amount of potential underwater grass habitat in areas of the Bay less than 2 meters deep.

But new work calls into question whether that amount of grass ever existed in the Chesapeake—even when Capt. John Smith explored the “faire bay” in 1608.

When scientists examined hundreds of historic aerial photographs taken of the Bay since the 1930s, they observed that grass beds used large amounts of habitat—but not at the same time. The beds often migrated from year to year.

In any given year, grasses in a particular area would cover only a portion of their potential habitat. Nothing close to the 640,000-acre level was ever observed.

“I was floored,” said Mike Naylor, a biologist with the Maryland Department of Natural Resources, who reviewed photographs along with scientists from the Virginia Institute of Marine Science. “I thought when we first started this, we might come out with a number like 700,000 acres. It just didn’t work out like that.”

Because the photographs were usually taken for agricultural surveys rather than peering beneath the water surface, grass beds were often obscured, or the photos were taken during the wrong time of year, Naylor acknowledged. But, he added, “even in areas where we had exceptional water clarity and the timing was perfect, we didn’t begin to approach the 600,000-acre number.”

That led Naylor to conduct a review of scientific literature from around the world concerning the coverage of grass beds. That review, he said, “seems to suggest that 100 percent habitat coverage never happens.”

If true, that would mean the Bay Program’s old, long-range target was never attainable. In the historic photos, Naylor and colleagues never observed more than about a third of the available habitat being used by grasses at any one time.

That would mean the most grass that ever grew in areas 2 meters or less in the Bay at any given time was only about 200,000 acres.

But, Naylor said, the Bay likely contained considerably more grass in historic times because its clearer water would have allowed light to reach farther into the water, resulting in grasses growing in deeper areas.

“What I strongly suspect used to be in the Bay was something like 30 percent coverage of whatever the available habitat was at that time, based on water clarity,” Naylor said. “But I suspect that we never had 640,000 acres, even if we had light to 5 meters. If I were forced to guess, I would suspect the number would be more like half of that.”

Others find Naylor’s analysis intriguing. But because John Smith didn’t fly a plane over the Bay and photograph its grass beds, no one will ever know what truly unimpacted Chesapeake grass beds looked like.

“That’s still a tough call because there is so much we don’t know about what went on in the historic past,” said Bob Orth, a VIMS scientist who oversees the Bay’s annual aerial SAV survey. “I do think we need to keep that 600,000 number in the back of our minds and not worry about it as much as we did. Just trying to get ourselves over 100,000 acres would be a great achievement.”

Bill Street, of the Chesapeake Bay Foundation, said it’s hard to know what conditions were like long ago in a Bay very different than it is today. But he said it is likely that the clearer, pre-settlement waters allowed extensive grass coverage in deeper waters, perhaps covering 400,000–600,000 acres.

“I don’t know how we could actually determine that,” he said. “But the fact that on the Gunpowder River we have grasses growing down 12– to 15-feet deep would indicate that under a clean, pristine Bay, you could have had that on a regular basis, Then you would be well over 600,000 acres. I would love to see it. It’s kind of mind-boggling to think about.”