In recent years, most of the concern about flows into the Chesapeake have focused on a series of unusually high spring flows that flood the Bay not only with water, but also with high loads of sediment and algae-producing nutrients.

But severe low flows raise their own concerns.

In an estuary such as the Chesapeake Bay, freshwater from rivers mixes with saltwater from the ocean, creating a series of zones with different levels of salinity — areas near the mouth of the Chesapeake have high salinities, while areas up the Bay and its rivers gradually turn fresh.

This makes the Bay very productive. Some species that rely on certain levels of salt water live in different parts of the Bay, while other species, such as fish and crabs, may migrate from area to area during different stages of their lives in search of food.

Salinity levels in the Bay vary naturally from season to season, and year to year, based on the amount of freshwater that flows in from rivers. But declines in the amount of freshwater could change salinity patterns, particularly late in the year, allowing saltwater to more frequently — and for longer periods of time — enter areas that are predominantly fresh.

A report two years ago from the Susquehanna River Basin Commission warned that future reductions in flows from the Susquehanna could aggravate the decline of some Bay species, such as oysters. “The attainment of the goals of the [Chesapeake Bay Program] may depend on maintaining freshwater inflows from the Susquehanna River,” the report said.

It wasn’t the first report to raise such concerns. In 1984, the Army Corps of Engineers released what is considered to be the most comprehensive examination of the potential effects of future low flow conditions on the Bay.

Its report warned that by 2020 an “an alarming portion of the freshwater inflow to Chesapeake Bay will be lost through consumptive use of water.”

That could have a variety of far-reaching impacts, the report said, such as changing circulation patterns in the Chesapeake. But it focused mainly on the most obvious consequence of lower freshwater flows: increasing salinity.

By 2020, anywhere from 30 percent to 50 percent of all the freshwater flow that normally would go into the Bay in the late summer months could, under various drought conditions, be lost because of consumptive use. This could cause salinities to be 2 to 4 parts per thousands or more higher than their long-term average, the report said.

Saltwater would increasingly penetrate areas of the Bay that are almost always fresh, the report predicted. Fresh water areas of the Bay can be moved 5 to 20 miles upstream during droughts, but during future droughts they could move 40 miles in some places.

The consequences could be felt in a number of ways.

For example, swimmers would notice right away that higher salinity would mean more sea nettles. Not only would nettles inhabit more of the Bay, the report said, but their reproduction would likely improve. The result would more more nettles in more areas, and in greater densities, the report said.

Boat and marina owners could be affected by changes in the distribution of ship worms over time, the report said, as the number of boat slips susceptible to the wood-borers could increase by about 15 percent. Even though extreme low flow conditions may be temporary, the worms can move rapidly into new habitats, the report said, and areas not affected in the past, such as Annapolis, may become vulnerable.

More frequent, and more severe, low-flow events would affect species at all levels of the Bay’s food web, from tiny one-celled phytoplankton and zooplankton on up to fish and waterfowl, the report said. That would likely reduce the production of some fish species, including striped bass and shad, the report said, but it was difficult to quantify the extent of those losses.

More obvious impacts would affect species that can’t move. The report singled out the valuable soft-shell clam fishery, oysters and freshwater submerged aquatic vegetation as areas of particular concern.

Potential declines in oysters are “particularly disturbing,” the report said. Although oysters thrive in salt water, so do the parasites that have been devastating their population — MSX and dermo. Areas far up the Bay and its tidal tributaries, which increasingly have been used as havens from disease because of their fresher water, would become more vulnerable to disease pressure.

Impacts on underwater grass beds are of “particular significance,” the report said. Low salinity grass species are a favored food for many of the waterfowl, and that function cannot be replaced by grasses species that thrive in higher salinities. Many duck species which have already been dramatically reduced in the Bay — redheads, canvasbacks, widgeon and others — would further decline as a result, the report said. Canvasbacks, it said, could decline 30 percent to 50 percent in the Bay under future drought conditions.

While populations of some mobile or short-lived organisms, such as algae, can quickly recover from drought conditions, the report noted that the recovery of grass beds or colonies of bottom-dwelling benthic organisms, such as clams, could take a decade or longer.

The report suggested considering the possibility of establishing reservoirs in the headwaters of each major tributary for the purpose of offsetting consumptive use during low flows.

Before that could happen, though, more detailed studies of the low-flow impacts on biology in the Bay are needed to help determine what level of flows need to be sustained and planned for, said Donald Jackson, a retired SRBC scientist who in 1996 outlined a research strategy to examine the impacts of low flows on the Chesapeake.

“In general terms, I think there is some reasonable evidence to say that there is an effect of low flows on the Upper Bay,” he said. “However, it is not adequate to allow the commission to incorporate that into resource planning for the basin.”

His proposed research strategy led to a scientific workshop where participants agreed that, while it was difficult to justify any attempt to modify natural loads, there was justification to develop flow management plans for tributaries to offset human-caused changes to flows.

“That,” Jackson said, “is ‘controllable’ in the same sense that the Bay Program talks about reducing controllable nutrient loads to the Bay.”

Copies of Jackson’s research strategy, “Chesapeake Bay Low Flow Strategy Study,” are available from the SRBC by calling 717-238-0423.