Bay Journal

Massive eelgrass die-off may hurt blue crab recovery

  • By Karl Blankenship on December 01, 2005
Eelgrass is critical for blue crabs as it provides shelter for the young crabs returning to the Bay after spending months as larvae in coastal ocean areas.

 (Peter Bergstrom )

Scientists are worried that a massive die-off of eelgrass in the lower Bay could hurt the recovery of the fragile blue crab population and have catastrophic consequences for high-salinity areas of the Chesapeake if there is no rebound next year.

The die-off, which scientists began to notice in August, may have been caused by warmer than normal water temperatures in the summer, combined with poor water clarity earlier in the year, which blocked sunlight from reaching the plants.

“It’s possible that when you couple those things together, it was just a fatal blow to eelgrass,” said Bob Orth, a seagrass expert with the Virginia Institute of Marine Science. “We are really concerned about it.”

The die-off appears to be widespread, he said, affecting whole beds of grass throughout the lower Bay.

The decline is of particular concern because eelgrass is the dominant underwater grass in high-salinity areas of the Bay. If the beds do not bounce back next year, huge areas would be barren of any underwater vegetation, which provides crucial habitat for a host of other species.

Scientists say this year’s die-off is particularly worrisome because of its potential impact on blue crabs. Females spawn near the mouth of the Bay during the summer, and their larvae float into coastal ocean areas for several months before they re-enter the Bay as small crabs. Those crabs typically seek shelter in grass beds when they return to the Bay.

Rom Lipcius, a blue crab researcher at VIMS, said the lack of beds could make the crabs more vulnerable to predation. “We don’t know what it means for the blue crabs yet,” he said.

But Lipcius added that the die-off is of particular concern now because the crab population is already at a low level. “This may be a major environmental event that is going to cause further decline,” he said.

Eelgrass is a widespread species of underwater grass, although it prefers cooler temperatures and is near the southern limit of its range in the Bay. It is not unusual for the lush beds, after producing a crop of seeds in late spring, to shed their leaves during the heat of the summer, essentially becoming dormant.

But the rhizomes—the reproductive shoots from the plants—usually bounce back to life when temperatures cool in late summer and early fall. But in most areas, that did not happen this year, and the rhizomes that remained looked dead.

Orth said that examinations of some beds in November revealed only one to three reproducing shoots per square meter, compared with 1,000 shoots or more in a normal fall.

If the rhizomes are mostly dead, that would leave only the seeds produced this spring to grow into new plants. But those plants would not be able to produce new seeds until 2007, Orth said, meaning there could be a dramatic loss of production next year.

A similar die-off occurred in 1975, Orth said, but the grass beds later rebounded.

But the die-off raises worries about what next year’s growing conditions will be like, he said. If they are poor, not only could it finish off any rhizomes, but also the plants produced from this year’s seeds. “If this situation occurred again next year, it could be catastrophic,” he said.

The decline will not show up in the annual aerial survey conducted for the Bay Program because grass beds in the lower Bay were photographed before the die-off.

The poor status of grasses in the lower Bay is in stark contrast to parts of the mid and upper Bay, where freshwater grasses have rebounded this year. Grasses in the Susquehanna flats near the top of the Bay are at their highest levels in more than 30 years.

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About Karl Blankenship

Karl Blankenship is editor of the Bay Journal and Executive Director of Chesapeake Media Service. He has served as editor of the Bay Journal since its inception in 1991. .(JavaScript must be enabled to view this email address).

Read more articles by Karl Blankenship

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