Drought raises salinity


Effects: Recent dry weather has translated into a saltier bay and a mixed bag of consequences.

May 10, 2002|By Tom Horton | Tom Horton,SUN STAFF

WE'RE FAMILIAR by now with the impacts of the drought, curtailing water use and threatening crops, across the four-fifths of Maryland that is land.

But dry weather also has big implications for the state's wet acreage - from boosting the annual crop of stinging sea nettles to raising the potential for toxic Pfiesteria in Middle River.

Drought translates into salt for the bay, as fresh water flowing down its tributary rivers shrinks, allowing ocean water to expand farther up the estuary.

Normally, the bay is a mix of about half ocean water, with 30 to 33 parts salt per thousand parts of water, and half river water, with zero salt.

But now it's about two-thirds ocean. Salt rules the bay. This week the salinity was 10 parts per thousand in Baltimore Harbor, higher than it's been there this time of year for at least a decade and a half.

On the Magothy, Severn, Potomac and rivers of the Eastern Shore, salinity in recent weeks and months has been running at or near its highest in more than a decade.

There have been times, of course, when the bay was a whole lot saltier - but that was during megadroughts of about 500 years ago.

Using growth rings from old trees, and chemical evidence from ancient fossils on the bay bottom, scientists have reconstructed 2,000-year patterns of drought and salinity.

These show the Chesapeake region was extremely dry and salty during the 1500s and 1600s, and quite wet and fresher during most of the past 150 years. A marked excursion occurred in the 1960s, one of the driest, saltiest periods in 300 years.

Whether the current drought and salt is good or bad for the bay is a mixed bag indeed. First, some likely winners:

Water quality. Less river inflow delivers to the bay tens of millions of pounds less polluting nitrogen and phosphorus a year; also tens of billions of pounds less sediment. This makes for clearer water, benefiting the growth of underwater grasses. Lower river flows in the spring also help reduce the bay's perennial problems with lack of oxygen in the water.

Hard clams, normally limited in their northward range in the bay by lack of salinity. They need about 18 parts per thousand to reproduce.

Oysters, which reproduce better in salty years.

Bay anchovies, important food for rockfish. Dry springs favor the growth of stinging nettles (putting swimmers among the losers, listed later). Nettles eat comb jellies, which prey on bay anchovy eggs and larvae.

Some probable losers include:

Swimmers (as mentioned above). It is going to be a banner year for jellyfish.

Some underwater grasses. Although clearer water in drought times helps these overall, certain species are harmed by too much salt.

Oysters. They reproduce better in saltier water, but the diseases that have troubled them in recent decades also flourish in drier times.

Farmers. This summer, farmers who draw irrigation water from tidal rivers may have to shut off the pumps as salty water creeps upstream. Corn can only stand 1 to 2 parts per thousand, and soybeans about three-tenths part per thousand.

Finally, there's a more speculative category:

Pfiesteria. Middle River in Baltimore County is loaded with the organism, which was implicated in fish kills and human memory loss on the Eastern Shore in 1997.

But so far it has not assumed its toxic form, quite possibly because Middle River is usually too fresh to attract menhaden, a baitfish whose large schools can trigger a Pfiesteria attack.

The river will likely be salty enough for menhaden this year. Another salt-loving toxic alga already has closed oyster beds to fishing when it moved farther up the Potomac River than usual.

Spawning fish. Rockfish, shad, herring and white perch all spawn in bay rivers at or above where the water becomes fresh. In dry years, salt extending upstream compresses those areas, though with so many other factors affecting spawning, it is hard to predict the impact.

Similarly, the areas where blue crabs mate will be shifted by higher salinity, but with uncertain impacts.

The bottom line on all this? On scales of months to years to centuries, the bay is a highly variable place, where the oceans constantly war with the rivers for dominance. Species that live there must be highly adaptable to change.

Get tickets to hear bay advocate in concert

Next week's column will feature Tom Wisner, who has dedicated the past four decades to evoking the bay in song and story. He will be in concert the evening of May 19 at the Avalon Theatre in Easton, an experience not to be missed. Tickets are $18. Information: 410-822-0345.

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