Keeping an eye on the fjord

When the water at the bottom of Masfjorden was renewed in April 2021, eleven years had passed since this last happened. Renewal of the water is vital for securing oxygen for life in the fjord.

"The fjord is cut off from what is happening on the coast. When the water is not replaced, the basin stagnates," says Torunn Sandven Sagen, a PhD candidate at the Bjerknes Centre and the Geophysical Institute at the University of Bergen.

With colleagues from the university and the Norwegian Institute of Marine Research, she recently published a study of the event in 2021. The fjord researchers have investigated whether a computer model of the Norwegian coast can recreate what happened when the bottom water in Masfjorden was renewed.

A wall at the opening

Masfjorden is a branch of Fensfjorden, north of Bergen and south of Sognefjorden. Like most fjords in Western Norway, it has a shallow sill at its mouth, like a wall against the sea outside.

"Masfjorden is a classic fjord, almost a textbook example," says Torunn Sandven Sagen.

The fjord has a distinct sill with a deep basin inside. Water passes back and forth over the sill, but renewal of the bottom water requires special conditions. Outside of the sill, dense coastal water must well up, then slosh over the sill and sink to the bottom of the fjord.

Even though the water at depth is dense and stable, it is affected by tidal waves. Gradually, the deep water is mixed with the water above, making it less dense. This builds up the potential for a renewal once conditions outside the fjord allow it.

Winds of renewal

Together with observations, simulations with a coastal computer model showed how northerly winds created favorable conditions before the fjord water was replaced in 2021. The wind pushed the surface waters away from the coast, so that denser water welled up from the deep, all the way to the sill at the mouth of Masfjorden.

Masfjorden has not only one, but two, sills. To replace the water in the inner fjord basin, water from the outside must cross both sills. A complete renewal can take several weeks.

"The water does not reach the bottom immediately, but when water that is dense enough gets across the sill, it sinks straight down," says the fjord researcher.

In the spring of 2021, observations showed that such water crossed the outer sill. During the next ten days, the temperature in the deep water inside gradually increased. 

After 28 days a leap in temperature occurred in the innermost basin, near the bottom, 470 meters below the surface. Within a few hours, the water was 0.15 degrees warmer.

In stable bottom water such a change does not happen frequently. Dense water from outside the fjord had reached their instruments.

The bottom water of Masfjorden had been renewed for the first time in eleven years.

Smells like rotten eggs

Researchers from the Norwegian Institute of Marine Research and the University of Bergen regularly visit Masfjorden. The oldest, long data series go back to the 1970s.

During the cruises, water samples are also taken. The oxygen concentration in Masfjorden is low without being critical. 

However, in a branch of Masfjorden called Haugsværfjorden, the bottom sixty meters are devoid of oxygen.

"The water from the bottom of Haugsværfjorden smells like rotten eggs," says Torunn Sandven Sagen. "Taking water samples there is not fun."

She does not want to guess when the water in Masfjorden will be renewed again. 

"I believe it will take a while. But it would be interesting if it happened."

Those who want to study the renewal of fjord water, must be patient. Usually, eight to ten years will pass between each event.

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