Modelling of the Meromictic Fjord Hunnbunn (Norway) with an Oxygen Depletion Model (OxyDep)

A biogeochemical model OxyDep coupled with three-dimensional hydrodynamic model GETM was used to simulate the hydrophysical and biogeochemical regimes of the meromictic Fjord Hunnbunn over the summer period. The main goal was to parameterize the oxygen depletion processes resulting in formation of suboxic and anoxic conditions in the water column. OxyDep considered five state variables: dissolved oxygen, inorganic nutrient, dissolved organic matter, particulate organic matter, and biota. This model parameterized the main processes responsible for the changing of the water column oxygen conditions – i.e. organic matter (OM) synthesis; OM decay due to oxic mineralization and denitrification; consumption of oxygen from sulphur and metals oxidation; and the processes at the boundaries (air–water exchange and the sediment–water exchange). Results of numerical experiments have reproduced the main features of the observed structure and have allowed to reveal the main components responsible for the formation of biogeochemical structure of the meromictic water objects. With the hydrodynamical model block used it was impossible to reproduce the presence of a permanent pycnocline. We suppose that special attention must be paid when using terrain following vertical coordinates (i.e. GETM) to avoid spurious vertical mixing. The results also showed that an application of simplified biogeochemical model blocks can be used as a useful tool for analysing and forecasting oxygen and nutrient regime changes.