Author(s): Jarle Berntsen; Dag Aksnes; Elin Darelius

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Keywords: Fjords, oxygen, fresh water, water renewal, modelling

Abstract: In deep fjords the oxygen concentrations have been gradually reduced over the last years. Ocean warming and increased eutrophication due to discharges of sewage and wastes from the fish farming industry con- tribute to this decline. Masfjorden, Norway, is one such fjord which has a maximum depth of 480 m and the decline in oxygen in the period from 1977 to 2015 has been approximately 0.52 ml O2/l/year (see Aksnes et al., 2019). By submerging freshwater down to the bottom of the fjord inside the sill, a buoyant plume from the outlet will entrain denser water from the ambient and the density of water inside the fjord below the sill depth will gradually be reduced. This will facilitate that oxygen rich water from the coast replaces low oxygen water in the deep fjord. In the present study, water renewal in Masfjorden based on the use of submerged fresh water is investigated using the Bergen Ocean Model (BOM) which is a general circulation model, see org.uib.no/bom. From the conducted numerical experiments we conclude that the deep water masses inside the sill can be renewed with oxygen rich coastal water over approximately one year by using submerged fresh water runoffs with a volume flux of 0.05 m³/s. The volume of Masfjorden below the sill depth is approximately 3.8×10⁹ m³, OB is the oxygen concentration in the basin water and OS is the oxygen concentration in the coastal water outside. The density ρB of the basin water is approximately 1027.14 kg/m³ and the density of the coastal water outside ρS is approximately 1026.65 kg/m³. Masfjorden is at the west coast of Norway and the supply of freshwater to the fjord system is high. Some of this fresh water may be piped down to the bottom of the fjord forced by the pressure difference. Assuming that the submerged fresh water is well mixed with the ambient water as it enters the fjord, it can be estimated that it takes approximately one year to reduce ρB to the value of ρS. Reference: Aksnes DL, Aure J, Johansen P-O, Johnsen GH, Salvanes AGV (2019) Multi-decadal warming of Atlantic water and associated decline of dissolved oxygen in a deep fjord, Estuarine, Coastal and Shelf Science, 228, 106392.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022SS2090

Year: 2022