With a north wind reddening his face and near-zero temperatures stiffening his fingers, Jim Sebastian leaned over the plunging bow of the Coast Guard cutter Mesquite, took aim at a buoy bobbing 15 feet away and emptied his shotgun.

Five charges of double-ought buckshot from the 12-gauge Remington pump-action riot gun slammed into a thick carapace of ice covering the 3-ton, flashing red beacon.

Officers and enlisted men, congealed by the cold, watched from the bridge and pilot house of this World War II-vintage ship as Sebastian reloaded and blasted away again. Other Coastguardsmen, bundled in full-length, one-piece parkas with protruding fur-lined hoods, were sheltered in the lee of the forecastle. The wind-chill factor was minus 20 degrees Farenheit and falling.

When he had freed the buoy, Sebastian waved in the direction of the bridge. Now the crew could snare the 20-foot-long beacon with boat hooks and stout lines, snub it alongside the ship and bring it aboard with a huge crane.

Mission accomplished, they would attach a small, unlit marker buoy to the spot and carry the expensive buoy with its automated light to land, safe from battering by a thickening ice pack that could sink it. Massive floes are beginning to choke inland America's coastline, ending major navigation until next spring.

Hauling 1,321 buoys from the five Great Lakes ahead of the crushing ice is a seasonal rite, part of the Midwest's obeisance to the weather as it battens down for winter 1985.

For much of the next hour, the Mesquite's young officers maneuvered their 180-foot ship close enough to lift the buoy, known on nautical charts as Calumet Northeast Shoal Light.

While the captain, Lt. Cdr. Robert Young, strode his cramped bridge and conversed quietly with officers, Lt. (j.g.) Monique Foster, a 1984 Coast Guard Academy graduate, called out a stream of commands.

"Right standard rudder! Right full rudder! Shift your rudder!" she ordered, as the helmsman repeated the commands. "Ease your rudder! Rudder amidships!"

Using the rudder, powerful stern engines and a sideways bow thruster propeller that gives the 1,026-ton Mesquite unusual maneuverability, Foster repeatedly positioned the flared, fat prow near the buoy. But each time, wind and waves forced the ship off.

When the crew finally passed a line around the buoy, stubborn Calumet Light thumped Mesquite's hull with echoing blows. At last, Young ordered reluctantly, "Drop the line. Don't bring it aboard."

Too many waves, too much wind, too much winter. The Mesquite turned away before tender or target were damaged.

"Now secure buoy-handling stations!" the quartermaster called over the ship's public-address system. Deck hands headed for the galley to shed foul-weather gear and warm up over coffee. The ship churned at 10 knots toward other navigational buoys clustered in the busy Chicago harbors area.

But by nightfall, the Mesquite had managed to pick up only three buoys, not the six Young had hoped for when the grueling day began 13 hours earlier and 90 miles to the north in Milwaukee harbor.

It has been a difficult year for the aging tender, which has spent most of its time in the Great Lakes where it was built in 1943 by the Marine Iron and Shipbuilding Co. of Duluth, Minn. The shipyard, founded in 1880, now specializes in steel fabrication, only occasionally repairing the ships it once built.

Mechanical problems and reassignment have removed two of the five vessels tending the Great Lakes buoys. The Mesquite, handsome in black hull, white deck house and tan smokestack, has worked seven-day weeks since October to pull out the 180 big buoys, double its usual load, and replace them with small markers that can ride out ice and storms.

The Coast Guard's 250 large vessels are divided between the "Black Fleet," tenders and tugs whose hulls are painted black, and the more glamorous white-hulled patrol ships of the "White Fleet." Any Coast Guard vessel larger than 65 feet, regardless of its mission, is called a cutter, a 15th century English seadog name for a government revenue-collecting ship.

For the Mesquite, about two dozen big beacons still remain afloat as time grows short. Winter's gathering fury soon will make further operations impossible, forcing the ship to shelter until spring in her home port of Charlevoix, Mich., on the east side of Lake Michigan.

His ship quietly docked for the night alongside Chicago's famed, but deserted and darkened Navy Pier, Young, a 1973 graduate of the Coast Guard Academy, began doggedly rejuggling his schedule, his enthusiasm undimmed.

"This is the most rewarding work I've ever done," the skipper said