Monitoring of Outer Oslofjorden 2019-2023 – inputs and surveys in the water masses in 2019. Technical report

Summary

Nutrient inputs calculations Due to routines related to data reporting and processing of data, only data from 2018 are available to this report. Agriculture is the largest single source for inputs of both man-made phosphorus and nitrogen. Inputs from the human population (wastewater treatment plants) and natural runoff are about the same size. Industrial emissions of phosphorus have decreased significantly in recent years, while emissions from the population appear to have had a slight increase for nitrogen in recent years. The four largest waterways (Glomma, Drammenselva, Numedalslågen and Skienselva) represent close to 90% of the freshwater inputs to the outer areas of Oslofjorden, including Inner Oslofjord. The long-term trends show increasing inputs of nitrogen and phosphorus. This can be largely linked to increased water flow. Drammenselva shows an increasing supply of organic- and particulate matter, but other rivers do not have sufficient data to show a similar trend. However, there is a trend across Scandinavia with browning of fresh water. Water masses: The program includes 17 water mass stations, stretching from Frierfjorden in the west to Iddefjorden in the east, as well as all the way into Drammensfjorden. All stations were sampled seven times (in January, February, June, July, August, September and November). In addition, three stations in the Hvaler-area (Ramsø, Leira and Haslau) were sampled three extra times (March, May and October) on behalf of Borregaard AS. All samples were collected without deviation, other than what is mentioned in the report below and continuously reported to the Council throughout the year. The stations that are close to the main fjord all have relatively good oxygen conditions, as there will be an annual replacement of the bottom water. However, fjords that are within a threshold will easily develop anoxic conditions, as the bottom water will be replaced less frequently. Typically, Iddefjorden, Frierfjorden and Drammensfjorden are anoxic below a certain depth. During the winter period in 2019 there was a complete replacement of bottom water in both Iddefjorden, Frierfjorden, which provides better oxygen conditions for a period of time. The anoxic conditions were back in both fjords before the first summer sampling in June. There appears to be a close correlation between freshwater influence and nitrogen levels at the stations surveyed. Drammensfjorden, Iddefjorden and Frierfjorden are clearly freshwater influenced in the surface layer and high nitrogen levels were measured. Dissolved organic carbon (DOC) was measured in the surface layer for the first time in 2019. The stations in Iddefjorden and Ringdalsfjorden stand out from the rest with high levels of both DOC and chlorophyll ɑ. There are also high values of DOC at the Drammensfjorden station, compared to the other stations.