UNDERWATER grasses make up the forest of the Chesapeake Bay. They nurture and shelter the estuary's creatures, hiding vulnerable young fish and crabs and feeding the geese and ducks. They are a powerful indicator of the health of the Chesapeake.

After years of slow progress in restoring these submerged plants, the latest survey shows a slight setback. That's understandable given the uncontrolled vagaries of nature, such as weather, that don't stay neatly on a trend line.

The worst news from the annual submerged grasses survey is that losses are occurring in places most important to blue crabs, notably Tangier Sound, the major nursery of juvenile crabs. Underwater vegetation there has declined by two-thirds since 1992; Tangier Sound embraces 25 percent of total grass cover in the estuary.

That's in contrast to the thriving underwater grasslands in Western Shore tributaries and in the Upper Bay. The Severn River showed a 20-percent increase, while recording similar water quality readings as Tangier Sound.

Finding the reasons for that difference will be critical to designing programs to restore the grass of the Chesapeake.