Last summer, we were excited to learn that the deep water of Masfjorden had finally been renewed – the deep fjord basin was replenished with new, oxygen-rich water, making it a much better place for fish to swim around in. We were equally excited when we headed up to the fjord last week (together with the paleo oceanographers), to recover the moorings that have been standing on the sill and in the deep basin for almost a year now. We were finally to learn more about when and how the renewal actually occurred!
The moorings were recovered without problems, and all looked good until Algot was to display the data on the screen. Were we expected to see the wiggly line displaying the current-meter record, revealing the strength and length of the renewal episode – there was only a straight line. Zero. A whole year of zeros, nothing but zeros. All the other sensors worked fine… but for some, still unknown reason, the velocity record was missing… the one record I had looked so much forward to seeing!
When it rains, it pours – especially if you are in Bergen. The second disappointment was waiting for a bit further into the fjord – one of the instruments that were supposed to record oxygen concentrations (and salinity and temperature) had leaked through a defect connector. The instrument (which is quite expensive) is ruined and the data is gone…
I guess we will just have to hang around another ten years or so for the next deep water renewal in Masfjorden 🙁
Snow, rain, and wind… the weather forecast was everything but promising when I left Bergen harbor together with a team of excited scientists last week onboard Kristine Bonnevie, our research ship. She headed northward towards the nearby fjords “Lurefjorden” and good old “Masfjorden.” The gear on deck was slightly different from what I am used to; the familiar mooring buoys and instruments boxes were accompanied by what I soon learned is called a “multi-corer” and a “gravity corer.”
These strange-looking creations are designed to collect mud – or sediments – from the ocean floor. As the sediment deposits chronologically, one layer at a time, they form an archive of the past. The deeper into the sediments you dive, the further back in time you go. Remnants of marine life deposited on the bottom – microscopic shells from foraminifera, for example – have incorporated information about the ocean properties at the time when they were living. Advanced bio-geochemical analyses can bring that information back. Or you can identify the shells – different species thrive in different conditions, some when there’s a lot of oxygen in the basin, others when there is little. If you know how to interpret the signals – you can turn what to most people looks just like mud into a historical record of fjord hydrography and learn how the oxygen concentration in the fjord basins has changed in time. To me, mud is mud, but luckily, we had Irina, Stijn, Agnes, and Dag-Inge, on board. They are paleo-oceanographers; they know how to turn mud into exciting science!
I almost forgot Mattia – our fresh PhD-student who arrived a couple of weeks ago to chilly and rainy Bergen from sunny Sicily. He will be working with the mud for the next three years.
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 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)