Like its partners in the Western industrial world, Canada now faces a period of slow economic growth, well below the forecasts and hopes of the past couple of years. Government officials (privately) and business economists alike agree that a 3 percent growth rate is more realistic for 1978 than the 5 percent projected. And 3 to 4 percent is the best growth range to be anticipated for the 1980s.

Despite the moderate success of the now-expiring Canadian anti-inflation program - which limited consumer nonfood prices to about a 6 percent level of increase, and wage-salary gains to 7.9 percent - the Canadian dollar has been weak.

The government here recently negotiated a $3 billion standby line of credit with a consortium of American banks led by Citibank.

But there is little real anxiety here about the Canadian dollar so long as it hovers around 88 to 90 U.S. cents. It dropped from an average of 100 U.S. cent in 1976 to a low of 85 cents this spring, but steadied recently around the 88-cent mark.

Some experts expect that the Canadian dollar will react a bit nervously as the time draws near for a referendum in Quebec on the sepatarist issue.

But the basic reason for the Canadian dollar's weakness is not sepatarist politics, but a sizeable current account deficit (on the order of $5 billion), the stagflation at home, and low levels of business investment.

An Organization for Economic Cooperation and Development study recently cited by Prime Minister William Davis of Ontario shows that Canada ranks last among 10 major countries in expenditures on research and development as a percentage of gross national product.

In fact, a lower Canadian dollar rate is welcomed in many business circles here. It makes imported manufactured goods more expensive, thus aiding a "Buy Canada" movement pushed by the government as one way of reducing the trade deficit. A cheaper Canadian dollar also gives exports a push - especially those competitive with the United States.

Of course, a cheaper Canadian dollar also means that some industries will have to pay more for imported raw materials but, as the Royal Bank of Canada pointed out in a recent analysis, "For a nation of raw material producers, such cost increases should be relatively insignificant."

Of more significance is the possibility that the deteriorating Canadian dollar will add to pressures for higher wages. Canada has just appointed Dr. Sylvia Ostry, head of the semi-official Economic Council, to undertake a special "monitoring" of wage-price developments. But only an "educational effort" (translate that as jaw-boning) is in prospect.

But more than anything else, the unique feature of the Canadian economy that wrinkles the brows of the planners here in the Trudau government is the wide in balance in the economic development of the country, which leads to strong regional differences.

In brief, there are two Canadian economies, with a prosperous heartland focused on the industrial Province of Ontario extending westward to provinces with abundant natural resources and in contrast, the Atlantic provinces, isolated areas of poverty and very high unemployment dependent on the federal governmetn.

Then there is Quebec, beset by problems not only stirred by the French sepatarist move, but by the concentration of textile and other industries that would rapidly decline in the Western World without extensive doses of protectionism.

"It would be a fool who'd suggest that the economy and national unity are not interrelated," says one of Trudeau's key advisers. "It becomes very difficult to run a national economic policy when one part of the country is running near a peak, and the other is far below potential."

Canada's West continues to be a major source of economic growth, with the Provinces of Alberta and British Columbia leading the way. This year and next, Quebec and Manitoba are likely to be the weakest provinces.

It is significant that the bulk of the nation's gross product is now produced west of Ontario, and the movement westward - into Ontario and beyond - is continuing. A major recent symbol was the announcement by the Sun Life insurance company that it was moving its headquarters from Monteal to Toronto, a fall-out of the squabble with the Levesque sepatarist party over French national and language rights.

A rough perspective on the shifting trends among the provinces can be gained from a look at the changes in per capita personal income. In 1973, Ontario led the way with $6,541, slightly ahead of British Columbia with $6,331 and Alberta with $5,909.

By 1977, Alberta had shot into the lead with per capita income of $11,037, followed by British Columbia at $10,123, and Ontario at $9,915. Saskatchewan, with $8,480 per capita, now exceeds Quebec, with $7,772.

According to the C.D. Howe Institute in Montreal, the income of the three western provinces increased $10 billion in the past five years from oil and gas alone. The bulk of the extra money has been produced in Alberta - which has been able thereby to keep provincial tax rates low.

All this explains why the Trudeau government has so much trouble selling economic unity. University of Toronto President John Evans said in a recent speech that the provinces consider "provacative the assumption that their natural resources such as oil and gas in the west and hydroelectric power in the east must be shared, without exclusive benefits to the provinces concerned."

Trudeau and his closest advisers nonetheless sound a generally optimistic note - predictable, perhaps, as they face an election this year or next. Their specific concerns at the moment seem less the internal threat to unity than Canada's vulnerability to international shocks.

Currently, their big worry is that the new multilateral tradenegotiations going on in Geneva will hurt Canada's industry - and U.S. Special Trade Representative Robert Strauss concedes there may be good reason for this concern.

The major nations are close to an agreement to cut tariffs by an average of about 40 percent, based on what is known as the "Swiss" formula, which slices high tariffs proportionately more than low tariffs. While this should benefit consumers over the world, it could hit Canada hard.

The reason is that there are substantial tariff walls in this highly protected country aiding the textile, shoe, furniture, and other labor-intensive industries. These tariff walls would come down sharply. But most of Canada's exported manufactured goods face only relatively low tariffs which wouldn't be eased much. So the trade-off for Canada might be negative.

"Whenever it looks like we can manage the domestic situation, then the international situation threatens," a Trudeau adviser sighed. "So the economic summit is going to be terribly important for us."