This summer's scorching temperatures appear to have baked eelgrass beds in the lower Bay, causing the second dieback in six years for this important underwater grass species.

Bob Orth, a seagrass expert with the Virginia Institute of Marine Science, said several places he and his VIMS colleagues visited this fall had either completely lost their eelgrass or had very reduced densities compared with what would have been seen in a normal fall.

He has received several reports of an eelgrass dieback from others. But, he cautioned, it appears the dieback may not be as severe as what happened in 2005. A few plants still remained in many places this fall, whereas they were totally absent in many beds in 2005.

The extent and ultimate influence of the dieback won't be known until annual aerial surveys of the Bay's underwater grass beds take place next summer and in-depth sampling is conducted on permanently established eelgrass sites in different parts of the lower Bay, Orth said.

But the implications could be significant because eelgrass is the dominant underwater grass in high-salinity areas of the Chesapeake Bay and provides crucial habitat for a host of other species, such as blue crabs.

If the beds do not bounce back next year, the loss would threaten the health of those species, too, because large areas would either have very sparse grass or be left barren.

Eelgrass is at the southern edge of its range in the Chesapeake Bay, and can be killed by warm temperatures. Murky water, which blocks sunlight critical for the plant's survival, increases its susceptibility to temperature stress.

How bad the situation becomes will in part be determined by the weather next summer, Orth said. Normally, eelgrass plants regrow from rhizomes buried in the sediment, similar to what happens on lawns in the spring. That won't happen if the rhizomes are dead.

The plants also produce seeds, and seeds produced before the dieback could result in plants next year. But these seedlings would not flower and produce seed until 2012. If conditions are bad next year, those plants could die before they flower, leaving even fewer seeds.

"If we have back to back years like this where the grass dies out, it will have devastating effects," Orth said. "There would be very little eelgrass in many areas. Its recovery rate would be really slow since the recovery does depend on seeds being carried in from adjacent eelgrass beds."

A large die-off occurred during the hot summer of 2005, but seeds produced that year helped to spur a slow recovery that began the following year and was continuing through 2010.

Orth said it was worrisome that two temperature-related diebacks have taken place within six years, when none had been observed since the annual monitoring was initiated in 1984.

"If this continues to happen, the likelihood of eelgrass remaining as a persistent member of our community could be really thwarted," he said.