The fjords of western Norway are breathtakingly beautiful – but ever so often, the media reports on the situation beneath the glittering fjord surface and the image they are sharing from the fjord depths is less pretty. Under the combined pressure of anthropogenic activity and climate change, many of our fjords are suffering from declining oxygen concentrations, with profound consequences for the local ecosystem.
The deep fjord waters are isolated from the open ocean by a sill, and it is only renewed when water sufficiently dense is present above sill level outside the fjord. If the water outside the fjord for some reason becomes less dense, e.g as a consequence of climate change and altered atmospheric forcing, then the renewals will occur less frequently, and the biological activity in the fjord will use up the available oxygen. The available data suggest that the average period between two renewals is longer than it used to be.
In FJO2RD, we will use a combination of observations, numerical modeling, and paleo reconstructions to explore if, how, and why the renewal frequency has changed as a result of physical climate change and anthropogenic activity and how this is linked to the current deoxygenation.
The work will focus on two fjords in western Norway that have different sill depths and therefore are influenced by different coastal water masses. Analyses of sediment cores will provide the first historical oxygenation records from western Norwegian fjords for the last 400 years and provide a baseline against which the recent changes can be compared. Data from eight the hydrographical stations along the Norwegian coast goes back to the 1930s and will be combined with new, high-resolution mooring records and historical numerical simulations of the shelf circulation to learn about the large-scale oceanic and meteorological factors that control the density variability and trends on the continental shelf and hence how often the fjord water is renewed.
FJO2RD focuses on the physical factors – variability and trends in the hydrography on the continental shelf, mixing and diffusion within the basin – that determines when and how often the deep-water in a stagnant fjord basin is renewed with oxygen rich coastal water. Data from 6 out of 8 long term hydrographic stations along the Norwegian coast show that the densest water observed at a given depth is getting less dense and more variable. This suggests that the renewal frequency of many Norwegian fjords have decreased during the last 30 years. A lower renewal frequency contribute to decreasing oxygen concentrations and potentially hypoxia, with dire consequences for the ecosystems.
In FJO2RD, we will use a combination of observations, numerical modeling, and paleo reconstructions to explore if, how, and why the renewal frequency has changed as a result of physical climate change, its link to deoxygenation and the potential anthropogenic contribution.
The work will focus on two fjords in western Norway that have different sill depths and therefore are influenced by different coastal water masses. Benthic foraminifera assemblage and geochemical analyses of sediment cores, will give the first historical oxygenation records from western Norwegian fjords, and provide a baseline against which the recent changes can be compared. Records from the hydrogaphic stations will be combined results from ocean hind cast runs and atmospheric reanalysis to learn about the large-scale oceanic and meteorological factors that control the density variability and cause the observed trend. Moorings records and observation from the fjord sills and the basin will inform us on renewal time scales and dynamics and allow us to validate model simulations. We will evaluate the effect of a modified (by hydropower) and increased freshwater input on basin mixing and renewal frequency using high resolution modeling. The impact of hypoxia on biogeochemical processes in the basin will be assesed.