Vertical diffusivity and oxygen concentrations in a fjord basin - A case study of the deep basin in Masfjorden

Abstract

The basin layer in a sill fjord is isolated from the coast, and may become hypoxic or anoxic if the renewal frequency is low. Deep water renewals and vertical mixing are the physical processes which affects the oxygen concentration in the basin. The temporal evolution of density and oxygen concentrations in the basin layer in Masfjorden, a sill fjord in western Norway, have been studied based on observations of hydrographic data obtained during February 2019 - March 2021. The vertical diffusivity in the basin layer were estimated from the observations and model results from the hydrodynamic model NorFjords160 by the budget method presented in Stigebrandt and Aure (1989). The observed vertical diffusivity was used in further calculations to find the oxygen consumption rate, and in an attempt to see how a partial renewal affects the oxygen concentrations close to the bottom in the basin. The hydrographic data showed a region with oxygen stressed conditions in the basin layer with concentrations around 2.5 mL L^(-1). Two shallow partial renewals occurred in the upper parts of the basin layer during summer of 2019 and 2020. The the mean oxygen concentration in the basin declined during the observational period, while the oxygen concentration at 450 m increased during the same period. The comparison of the volume mean vertical diffusivity in the observations and in the model suggests a realistic, but lower density reduction rate, in the model.