Seaway officials last week said overall cargo shipments headed west totaled 4.3 million metric tons, a 6.78% jump over August 2011.

Year-to-date shipments were 21.3 million tons (metric), rising 1.5% over the same period last year.

Leading the commodity types for the month were coal and iron ore.

Coal for power generation and steel production totaled 523Mt, a 45% spike year-on-year.

Year-to-date coal shipments rose 31% over last year to 2.7Mt.

Iron ore shipments, meanwhile, were up 1.4Mt, or 43% percent, in August. Year to date, the commodity totaled 6.6Mt, up 31%.

Grain shipments were the seaway’s worst performer, down for the fourth consecutive month due to the extreme drought conditions of the US.

In August, grain was down 32% versus 2011 and year-to-date total grain shipments were down 21% to 3.5Mt.

“With four months remaining in the 2012 navigation season for the Great Lakes-St. Lawrence seaway system, tonnage figures for the month of August remain on the positive side,” Saint Lawrence Seaway Development Corporation trade development director Rebecca Spruill told maritime trade publication MarineLink.

“The uptick in total cargo transported in August emphasized the strength of shipments of traditional commodities like iron ore and coal and was a sound indication of the value of the seaway system for North American exports to Europe and China.”

The Saint Lawrence Seaway Development Corporation, an agency of the US Department of Transportation, oversees the movement of marine traffic through US-owned and operated facilities of the Saint Lawrence Seaway.

It shares the system with Canadian counterpart Saint Lawrence Seaway Management Corporation.