WASHINGTON – A lack of rainfall in the Chesapeake region this year could be a mixed blessing for the bay in coming months, scientists said.
Baltimore received .18 inches of rain last month, a March record low in the city since 1870, when National Weather Service records begin. Rainfall this week did little to make up for low year-to-date regional totals, which has benefits and detriments to bay life.
Low precipitation reduces pollutant run-off from land surrounding the bay, but it can also increase salt levels, or salinity, in the bay over time, scientists said.
A textbook estuary, the Chesapeake Bay receives saltwater from the ocean at its Virginia south end, while freshwater feeds in from watershed runoff and the Susquehanna River at the bay’s north end, said Jenn Aiosa, a staff scientist with the Chesapeake Bay Foundation.
This delicate balance of fresh and salty waters is vital to bay species survival. The balance was off-kilter in March, with a 65 percent below average freshwater flow from streams and rivers into the bay, according to the U.S. Geological Survey. It was a March record low, and that may raise salinity levels.
High salinity enhances oyster growth and reproduction, but it also strengthens oyster diseases such as dermo and MSX, said Ken Paynter, associate professor at the University of Maryland’s Center for Environmental Studies.
“It’s a two-edged sword,” Paynter said.
Present salinity levels do not pose much danger to oysters, but if the dry spell continues, dermo will be especially virulent, Paynter said.
Benefits or detriments to life in the bay may be months away. Salinity changes take time because the bay is so large, Paynter said. Pennsylvania rainfall can make up for a Maryland dry streak by carrying freshwater down the Susquehanna River and into the bay.
NWS data, however, show Pennsylvania cities near the Susquehanna had a dry March as well. Harrisburg, the state capital situated on the river, had its second driest March on record with .68 inches of precipitation. Williamsport, located along the river’s west branch, had its sixth-driest March with 1.15 inches.
Dry weather means less nutrient runoff into the bay, which improves water clarity. This is a healthy advantage for underwater plants on which fish and crabs feed, Aiosa said. Other plants, however, especially those in upper, fresher waters, are adversely affected by the high salinity.
Less runoff, and thus less nitrogen, will benefit crab and fish because there is more dissolved oxygen available, said Scott Phillips, Chesapeake Bay coordinator with the U.S. Geological Survey.
It’s unclear whether the old adage of “April showers” will bring relief — the NWS forecast does not give any signal that precipitation will be above or below normal this month.
While the Chesapeake region may not be steeped in drought or see dramatic salinity changes now, the bay foundation is keeping close tabs on rainfall trends, Aiosa said.
“It’s sort of a little red flag,” she said. “It could have an indication of what the summer may look like.”