by Alicia Pimental
April 01, 2007
Bay-wide acreage of underwater bay grasses (SAV) decreased by 25 percent in 2006, dropping to 59,090 acres from 78,263 acres in 2005, according to data from scientists with the Bay Program. This loss marks the first setback for SAV after two consecutive years of moderate gains and the lowest total SAV acreage figure since 1989.
Bay grass acreage is broken down into three zones: Upper, middle and lower Bay.
- Grasses in the upper Bay fell 20 percent to 15,510 acres.
- Middle Bay grasses diminished by 23 percent to 30,659 acres.
- SAV in the lower Bay covered 12,922 acres, a loss of 33 percent.
Scientists are attributing acreage declines in the upper and middle Bay to:
- The very dry spring in 2006, which caused more saline water to penetrate into many of the Bay's upper reaches. The higher salinity levels are believed to have increased stress on and loss of SAV species used to fresher water.
- An abnormally large rain event in early June that “muddied” the upper and middle Bay for about a month. The massive amount of sediment that followed this event caused further stress on bay grasses and likely contributed to additional acreage losses.
The lower Bay is still experiencing the effects of a large eelgrass dieback that took place in late summer 2005 after a period of record high temperatures. Many of the areas affected by the dieback in 2005 did not produce grass at all in 2006, while the remaining SAV beds observed were very thin.
SAV losses in the lower Bay could be particularly hard on blue crabs, which use grass beds as nursery areas where they hide from predators until they grow large enough to migrate up the Bay and its tributaries. This additional habitat loss, among other factors, could contribute to the extended period of low blue crab abundance currently observed in the Bay.
Although SAV acreage decreased bay-wide, there were some bright spots in bay grass restoration in 2006.
Large, dense beds on the Susquehanna Flats area remained healthy and vibrant despite the deluge of sediment following the June rain event.
Widgeon grass spread throughout the lower Rappahannock River.
Hydrilla continued to do well in the Mattaponi, Pamunkey, Chickahominy and upper James rivers.
SAV beds remained very dense in the tidal freshwater areas of the Potomac from Broad Creek down to Aquia Creek.
Researchers on the St. Mary's River also witnessed healthy SAV populations.
SAV is critical to the Bay's ecosystem because the grasses provide habitat for fish and shellfish, help reduce shoreline erosion, absorb excess nutrients and trap sediment. SAV once covered an estimated 200,000 acres along the shallows and shorelines of the Bay.
Bay grasses can only grow if water is clear enough for sunlight to reach its underwater leaves. Since water clarity is reduced by excess nutrients and sediment from the land, the Bay Program looks at annual bay-wide SAV survey results as an indication of the Bay's response to pollution control efforts. Based on long-term trends, significant progress is still needed before the Bay is clean enough for SAV to recover to historic levels.
The health and density of bay grasses is just one indicator of the overall health of the Bay. The Bay Program's 2006 Chesapeake Bay Health and Restoration Assessment, which provides the most current scientific data and tracks restoration progress, is currently in production and will be made public on April 18.