Assessment of possible measures to improve the water quality in Hunnebunn

Summary

Hunnebunn is an enclosed fjord in Eastern Norway. The surface area is around 1 km2 and at its deepest water depth is 11 m. Hunnebunn is connected to Tosekilen and the Glomma estuaries by a 1.8 km long, 1-3 m deep and approx. 50 m wide channel. The sill depth in this channel, which is called Talbergsundet, is approx. 1.5 m and the water mass inside the fjord which is deeper than this have no free connection to the outside areas. There is strong tidal flow in the channel which creates some circulation in the surface water, but the water mass under the pycnocline, which is often around 3 m depth, is very stationary. As a result, the residence time of the bottom water is long, and the oxygen is exhausted by decomposing organic matter. The water below approx. 6 m depth is usually anoxic and large amounts of hydrogen sulphide are formed. All higher life forms die in the anoxic bottom water and only anaerobic bacteria are able to survive. Therefore, the concentration of nutrients becomes very high, and throughout the season this provides a steady upward flux of nutrients to phytoplankton 1-2 m higher up in the body of water, at 4-5 m depth. The concentration of planktonic algae in these blooms has never been documented, but presence of very high oxygen supersaturation, indicate that the concentration is high. In parts of the season the pycnocline situated at around 3 m, becomes much weaker. During these periods, nutrient concentrations in the surface becomes high, which can result in blooms of benthic algae on the beaches around Hunnebunn. This is a phenomenon that also has been reported in the past. But in recent years, there has been a development towards more, and denser blooms of benthic algae on the beaches, something which has a very negative effect on the recreational value of the area. Sporadic investigations have been made of both hydrography and aquatic plants in and along Hunnebunn since the 1950s. Thorough investigations of the hydrography were done in 1952-1953 and in 1999, where oxygen was absent on all occasions below approx. 6 m, and in 1999 very high concentrations of nutrients were recorded. NIVA has made occasional investigations in the period 2007-2017 of hydrography and marine chemistry. Profiles of nutrient, iron-, manganese- and mercury compounds, dissolved organic matter, salt ions and hydrogen sulphide have been measured. High concentrations of toxic methylmercury in the bottom water were recorded. The NIVA Fjord model manages to simulate the dynamics of Hunnebunn, with very high oxygen concentrations above the anoxic bottom layer, and very high concentrations of nutrients and hydrogen sulphide in the bottom layer. The model confirms that oxygen saturation is due to algae blooms. Using the model, the impact of several measures to improve water quality in the Hunnebunn has been assessed. The measure that has the clearest positive effect on the water quality in Hunnebunn is to create artificial vertical mixing in the basin. It is therefore recommended that discharge of air bubbles in the deep water in Hunnebunn is authorized by the municipality. But measures to increase the vertical mixing artificially should not be implemented without monitoring the water masses. Based on the observations made, especially the thorough surveys in 1999, the ecological status of Hunnebunn should probably be classified as very poor based on Veileder 02:2018. It is recommended that the water mass of Hunnebunn is monitored, and not just the bathing water quality in the surface layer.