Monitoring of the Outer Oslofjord 2014-2018. Bottom surveys in 2018. Technical report

Summary

Samples for analysis of soft bottom fauna were taken at seven stations in Outer Oslofjord in 2018. The stations in the Tønsbergfjord, Skjebergkilen, Slevikkilen, Ringdalsfjorden, Breiangen and Haslau all had good condition for the soft bottom fauna. The total number of species varied somewhat between the stations, with S9 (Haslau) being the most species rich with 81 species, and SL-1 (Slevikkilen) and OF-5 (Breiangen) as the least species rich with 25 and 31 species respectively. Station OF-5 also had low numbers of individuals. It was otherwise a general feature of all the stations that there were low numbers of larger bioturbators as sea mice. Such animals help keep the sediments oxygenated, and the absence of the animals can be an indication of a somewhat disturbed environment. The station Drammensfjorden was completely lifeless with very bad conditions for the soft bottom fauna. This is probably due to poor oxygen conditions in the bottom water in the fjord. There was little correlation between the state of soft bottom fauna and organic content in sediment in this study. Station DD-1 in the Drammensfjord was lifeless and had a very poor condition for soft bottom fauna, but good condition for organic content in the sediment. Conversely, station R-5 and SL-1 had a very poor organic content condition, but good condition for soft-bottom fauna. Station 915 had a moderate state of organic content, and a good condition for soft bottom fauna. The other stations had a good condition for both soft bottom fauna and organic content in the sediment. The SPI (Sediment Profile Imaging) surveys of bottom sediments in the Outer Oslofjord in 2018 showed that the benthic habitat quality (BHQ) at 21 of the 27 stations was either good or less good, and that 13 stations had at least good BHQ. Only one station, S-9 Haslau in the Singlefjord, achieved very good BHQ.