Deep water renewal – finally!

It’s been almost ten years since the water in the deepest basin on Masfjorden was last renewed – but now it has finally happened! My colleague Lars Asplin from the Institute of Marine Research in Bergen do regular hydrographic surveys* of the fjords in western Norway– and the last profiles from Masfjorden show increased levels of temperature and oxygen at depth. Between April and August, the oxygen concentrations in the basin have increased from 2 ml/L to 4 ml/L, making life much, much easier for whoever lives down there!

The concentration of dissolved oxygen in Masfjorden in April (upper panel) and August (lower panel).The concentration in the MAsfjorden basin has increased from about 2 mL/L (dark blue) to about 4 mL/L (cyan). Figure courtesy: Lars Asplin, HI.

The oxygen concentrations at depth can only increase if new water enters the basin -i.e during a deepwater renewal. A deepwater renewal can take place if sufficiently dense water is lifted up to sill level outside the sill, for example, when northerly winds move the surface water off-shore** and denser water from below is pumped upward. The coastal records from Sognesjøen, suggest that water dense enough was present at sill level during the last part of April – so the deepwater renewal probably occurred around then.

The news about the renewal was waiting for me in the inbox when returning from holiday – along with an e-mail telling me that the UIB-biologists are heading to Masfjorden this week on a research cruise…. after a bit of frantic e-mail writing back and forth we managed to organize that they take a few extra samples of the new deep water for us!

We’ve got moorings on the Masfjorden sill and in the basin that are to be recovered by fjord oceanography students in February – they will tell us more about when and how the renewal happened!! The students will for sure have a lot of exciting data to write about in their report!

* i.e. they measure profiles of temperature, salinity, and oxygen with a CTD.

** due to the rotation of the Earth, the winds cause a transport of surface water to the right of the wind direction (in the northern hemisphere)