Basin water renewal in land-locked fjords with shallow threshold sills: impacts from natural and anthropogenic change

At higher latitudes there are many land-locked fjords with shallow threshold sills or channels and deep basins inside. The renewal of water within these deep, semi-enclosed, basins is essential to maintaining water quality (e.g. dissolved oxygen levels), particularly at depth. Deep water renewal events are episodic and controlled primarily by the density difference between water masses inside and outside the fjord basin, as well as the balance between external tidal forcing across the threshold sill and freshwater runoff into the fjordic basins. The magnitude and frequency of renewal events can also be affected by human activities such as dredging of channels or sills separating fjordic basins, bridge crossings, and other obstructions, and controlled freshwater releases from hydropower plants. In this paper, we focus specifically on one case study, the Bolstadfjord in western Norway, which has a shallow and narrow threshold sill-channel that controls water exchange into, and out of, the land-locked fjordic basin. A series of numerical 2DV model simulations, using the Bergen Ocean Model, are conducted to assess the importance of threshold sill depth in ensuring that connectivity between the semi-enclosed basin and the open sea is maintained, as required for the continued occurrence of deep water renewal events. These model simulations, based on benchmark river inflow records between 1977 and 1990 and tidal boundary forcing, indicate that increasing the threshold sill depth increases the density excess of bottom waters within the basin as a result of larger and more frequent water renewal events. This also reduces the occurrence of hypoxic or anoxic conditions and the production of hydrogen sulphide within the basin at depth. Model simulations also reveal how renewal events are affected by climate change impacts from increasing sea temperatures and reducing salinity of intruding waters, leading to a slight reduction in density excess in the basin and a small increase in the volume of anoxic basin waters. When a 25% increase in river flows (from increased freshwater runoff) is also included, the density excess in the basin is further diminished due to a reduction in the strength and frequency of deep water renewal events. Anthropogenic changes to river inflows from controlled hydropower releases into the fjord are also assessed, with a model simulation of a continuous, regulated release flow shown to prevent deep water renewal in the basin entirely. This highlights the importance of natural variability in river freshwater inflows, and especially extended periods of low flow, as necessary conditions for deep water renewal and, hence, maintenance or improvement of water quality at depth within the basin. Finally, comparisons between measured temperature, salinity and DO profiles obtained in Bolstadfjord in 1932 and 2024 indicate slight warming and lower salinity at depth within the basin, potentially indicate a reduction in full deep water renewal events due to climate change impacts.