From the outdoor terrace bar of Hemingway's beloved Gritti Palace along the Grand Canal, eternal Venice seems the very picture of energy and prosperity.

Launches full of commuters and tourists churn up the waterways, barges laden with fresh vegetables and fruit wallow their way to market, and laundry scows heaped with stacks of white linen make deliveries to local hotels. Gondolas carrying lovers drift silently in the waves, while sleek water taxis tie up at the floating piers to let off chic patrons and their matching luggage at the opulent hotels along the famed canal that bisects this unique island city nestled in a lagoon.

To an observer feasting on this daily water spectacle, nothing seems amiss: Venice, La Serenissima (the most serene) appears its normal, bustling, confident, arrogant, colorful self.

Yet all is not well here in this jewel of a city. Despite all appearances -- the river traffic, the guided tours swarming around St. Mark's Square, the hoopla over a new Venice international art exhibition and next week's Feast of the Redeemer rowing regatta -- this is a rare summer of discontent.

UNLIKE IN THE PAST, it is not a new foreign invader, nor the Plague, nor the perceived indifference to the city by Rome, nor the high tides that have caused Venice's denizens to grumble, but the sudden decline in tourism, the city's principal industry.

Only last winter the crowds were so great that some here advocated closing the city off for anyone not in possession of a room reservation during the traditional, and recently revived, masked revelries of Carnival.

Now, in the wake of the European terrorist scare and lingering fears of the effects of the fallout from the Chernobyl nuclear accident, the problem has drastically changed. It isn't the excess of tourists that has Venetians grumbling -- but the lack of them, especially Americans, whose lavish spending habits have long fueled the local economy.

"We are facing a most serious economic recession because of the decline of tourism, especially American tourism, this summer," said Augusto Salvadori, the city's alderman in charge of tourism. "The drop in U.S. tourism this year is a great problem for Venice."

Venice earns about $1.5 billion a year from tourism.

Arrigo Cipriani, the owner of Harry's Bar, Venice's most prestigious -- and expensive -- restaurant, said that since April, the beginning of the American tourism season, his business has dropped steadily. In April it was down 20 percent, in May, 25 percent, and in June, 30 percent. "The situation is just short of disastrous for us," Cipriani said.

Other restaurant, hotel and shop owners are experiencing the same.

Edoardo Mario Masprone, president of the Venetian Hotel Owners' Association, said that business from U.S. tourism in the main hotel and restaurant district dropped almost 50 percent during the first six months of this year. U.S. spending this June is down 80 percent over June of last year.

VENETIANS, OF COURSE, are nothing if not resourceful. Over the 1,100 years of this unique city's history, they have faced, and survived, attacks by Lombards, Franks, Ottoman Turks, Genoese, Hapsburg Austrians, Napoleon, the bubonic plague in the 16th century and Nazi occupation in the 20th century.

Thus, faced with a new threat to their livelihood, the Venetians have begun to rally to confront what Salvadori and other Venetian officials insist are America's exaggerated fears about Venice, a city, they maintain with statistical evidence, that is infinitely safer than just about any major American city.

Venetians have thus decided to reassure their skittish U.S. clients with a special festival -- the Days of Venetian Friendship with America.

After much negotiation among the city's suffering businessmen, Salvadori this week flew to Rome for a meeting with U.S. Ambassador Maxwell Raab. Afterwards, he announced plans to hold the special four-day festival in September. According to Salvadori, more than 100 U.S. dignitaries -- city mayors, government officials, tourist industry leaders, film and theater personalities and media personages -- will be invited to Venice for the special festival. It will include performances, dinners in traditional Venetian palazzi, tours, and a special regatta in the lagoon. Beyond this, the city's businesses are considering issuing a special courtesy credit card to every American tourist. The card would guarantee a higher exchange rate for every dollar Americans spend in the city.