Photo:  A ship carrying road salt pulls into the Port of Oswego, N.Y., on Lake Ontario. The lake’s water level has dropped three inches during this month alone.

OSWEGO, N.Y. — From his office at the port here, Jonathan Daniels stared at a watermark etched on the rocks that hug one of the commercial piers — a thick dark line several inches above the surface of Lake Ontario — and wondered how much lower the water would dip.

“What we need is some rain,” said Mr. Daniels, director of the Port of Oswego Authority, one of a dozen public port agencies on the United States side of the Great Lakes. “The more we lose water, the less cargo the ships that travel in the Great Lakes can carry, and each time that happens, shipping companies lose money,” he said. “Ultimately, it’s people like you and I who are going to pay the price.”

Water levels in the Great Lakes are falling; Lake Ontario, for example, is about seven inches below where it was a year ago. And for every inch of water that the lakes lose, the ships that ferry bulk materials across them must lighten their loads by 270 tons — or 540,000 pounds — or risk running aground, according to the Lake Carriers’ Association, a trade group for United States-flag cargo companies.

As a result, more ships are needed, adding millions of dollars to shipping companies’ operating costs, experts in maritime commerce estimate.

“When a ship leaves a dock, and it’s not filled to capacity, it’s the same as a plane leaving an airport with empty seats: It cuts into their earning capacity,” said Richard D. Stewart, a co-director of the Transportation and Logistics Research Center at the University of Wisconsin-Superior.

“Because it’s mostly raw materials we’re talking about, the average consumer may see an increase in pennies in the price they pay for, say, a new car or washing machine,” Dr. Stewart said. For major manufacturers or firms managing big projects, however, the increase in transportation costs “is much more significant,” he said.

The port of Oswego receives scraps of aluminum from Canada, which are rolled into sheets at a local plant and sent to car manufacturers; soy beans for a bio-diesel plant in nearby Fulton; and parts for windmills that are used to generate power on a farm south of Canandaigua Lake, near Rochester, said L. Michael Treadwell, director of Operation Oswego County, a nonprofit economic development agency. The windmill parts arrive from Brazil and Indonesia, in ships that enter Lake Ontario through the St. Lawrence Seaway, which connects the lake to the Atlantic Ocean.

The port also handles soy beans grown in central New York and sent to the Middle East, and it receives potash, a mineral used in fertilizer, and road salt, which are distributed by truck and rail to companies across the Eastern United States.

The water levels in all five Great Lakes — Superior, Michigan, Huron, Erie and Ontario — are below long-term averages and are likely to stay that way until at least March, according to the Army Corps of Engineers. (The same is true at Lake St. Clair, which straddles the border between the state of Michigan and the province of Ontario and is between Lake Huron and Lake Erie; it is not considered one of the Great Lakes, although it is part of the Great Lakes system.)

Most environmental researchers say that low precipitation, mild winters and high evaporation, due largely to a lack of heavy ice covers to shield cold lake waters from the warmer air above, are depleting the lakes. The Great Lakes follow a natural cycle, their levels rising in the spring, peaking in the summer and reaching a low in the winter, as the evaporation rate rises.

In the past two years, evaporation has been higher than average, and not enough rain and snow have fallen in the upper lakes — Superior, Michigan and Huron — which supply water to the lower lakes, to restore the system to its normal levels, said Keith Kompoltowicz, a meteorologist at the Corps of Engineers’ office in Detroit, which monitors water levels in the lakes. “Mother Nature is largely the driving force on what the water levels are, and it plays a large role in what we project water levels to be,” Mr. Kompoltowicz said.

The International Joint Commission, which advises the United States and Canada on water resources, is conducting a $17 million, five-year study to determine whether the shrinking of the Great Lakes is related to the seasonal rise-and-fall cycles or is a result of climate change, said Greg McGillis, a spokesman for the commission. A final report is expected in March 2012.

Lake Ontario’s water level can be regulated through releases from a dam on the United States-Canada border, which allowed the lake to maintain its normal levels until May, Mr. McGillis said. Then a drought hit, and the releases became less generous, said Robert O’Gorman, supervisor of the United States Geological Survey field station here. The drought and the lower inflows from the upper lakes, diminished Lake Ontario’s water level, he said.

Lake Ontario stood at 244.1 feet as of Wednesday — 3 inches below where it was at the beginning of the month, 5 inches below last month’s average and about a foot below last year’s average. The water, however, is still about 2 feet above the lake’s low of 242.19 feet, registered in 1934, according to the Corps of Engineers.

The picture is just as serious in the upper Great Lakes and is particularly grave in Lake Superior, where water levels have hovered below average since 1998 and, based on provisional data, set record lows in August and September. It is the longest stretch of below-average readings at Lake Superior since the Corps of Engineers started tracking the Great Lakes’ levels in 1918.

On average, 240 million tons of cargo travel across the Great Lakes every year. The United States fleet circulating in the Great Lakes has 63 ships, which have lost a total of 8,000 tons of cargo capacity for every inch of water the lakes have fallen below normal this year, said James H. I. Weakley, president of the carriers’ association. Those 8,000 tons, he said, correspond to enough iron ore to produce 6,000 cars, or enough coal to provide electricity to the Detroit area for three hours, or enough stone to build 24 houses.

Mark W. Barker, president of Interlake Steamship Company, said the nine ships his company operated made about 50 trips a year across the Great Lakes, and the larger ones have transported 1,800 tons less per trip this year compared with last year — the equivalent of losing an entire ship’s capacity over the length of a season.

“We get paid by the ton, so we’re losing a lot of revenue per trip, and we’re just going to have to reclaim that loss by increasing our rates,” said Mr. Barker, whose family has owned the company since 1987. “It’s either doing that or risk the business.”

The Great Lakes region is home to about 70 percent of the steel industry in North America and about half of the heavy manufacturing in the United States, Mr. Weakley said.

Here in Oswego, a city of 18,000 residents that is 40 miles north of Syracuse, the port has acquired renewed significance in the past two years, largely because of a budding renewable energy sector that depends in part on lake shipments. The area’s economy has struggled since the decline of its agricultural-based industries, like brewing, began in the 1970s.

Mr. Daniels, the port director, said that water transportation was still one of the most efficient alternatives for companies that rely on bulk cargo, and that Oswego was banking “on the water coming back to the lakes.”

“If the low levels in the Great Lakes are a result of global warming, I don’t know,” he said. “What I know is that we can’t control nature. All we can do is hope for rain.”