Monitoring of the outer Oslo fjord - inputs and surveys in the watermasses in 2018. Technical report

Summary

Discharges Due to the routines related to data reporting and data processing, only 2017 data available for this report are available. Agriculture is the largest single source for supplies of both man-made phosphorus and nitrogen. Imports from the population (wastewater treatment plants) and natural drainage are about the same. Industrial emissions of phosphorus have gone down considerably in recent years. Emissions from the population seem to have had a slight increase for both phosphorus and nitrogen in recent years. The four largest rivers (Glomma, Drammenselva, Numedalslågen and Skienselva) represent almost 90% of freshwater supplies to the Outer Oslofjord including Inner Oslofjord. For the period 1990- 2016 there is an increasing and statistically significant trend in water flow. A significant increase in nitrogen and phosphorus supplies from three of the rivers (Glomma, Drammenselva, Numedalslågen) is also measured in the same period. This is related to the increased water flow. Water masses The stations close to the main fjord have annual replacement of the bottom water and thus relatively good oxygen conditions in the bottom water. For Drammensfjorden and Iddefjorden, no ex-change was recorded in the bottom water and the conditions in these fjords are still poor. In Frierfjorden there was a replacement of bottom water in the spring of 2018 which improved the conditions in the fjord. At a number of stations, we registered higher oxygen minimum in the autumn of 2018 than in the same time in 2017. This indicates lower organic load to the bottom water in these areas in 2018 compared to 2017. Generally, there was somewhat higher nitrate + nitrite concentration in the winter period. But summer concentrations were lower at most stations in 2018 than in 2017. For phosphate, concentrations in 2018 were about the same as in previous years, or somewhat lower. The exception was the Iddefjord, where high concentrations were recorded in July, due to incorporation of phosphate-rich intermediate water up to the surface layer.