Salinity and oxygen conditions in Drammensfjorden

Summary

Loggers for measuring salinity have been installed on board the trawler Luna, and these have measured continuously while trawling for salmonids in Drammensfjorden and Sandebukta has taken place during the period 18th to 27th of September 2017. While trawling in Sandebukta and in the outer Drammensfjord outside Svelvik on September 18th, the highest salinity was 14,7 psu. When trawling inside Svelvik, the salinity was not higher than 1 psu. CTD measurements taken September 25th, 2017 show a freshwater layer of thickness 6,7 ± 0,5 m. The freshwater layer thickness has been defined as the depth where the salinity was equal to 7 psu. The water discharge from the river Drammenselva during the past 10 days was 583 m3/s prior to September 25th. When historical measurements of discharge from the river and salinity in Drammensfjorden are compared, it shows that there is a clear co-variation, and the thickness of the freshwater layer can be written as a function of water discharge alone. This relationship gives an estimated thickness of the freshwater layer of 7,7 ± 0,9 m, which corresponds well with the observed thickness. Just below the freshwater layer in the inner part of Drammensfjorden, a layer with low oxygen saturation was observed, below 50 % saturation. This layer is in the depth range 10-20 m near the river outlet. Further out in the fjord this layer becomes thinner and with somewhat higher oxygen saturation. At Selvikstranda south in the inner Drammenfjord, the layer is in the depth range 10 to 14 m and the oxygen saturation was about 60 %. Hypoxic or oxygen-poor water has here been defined as water with oxygen concentration lower than 3 mg O2/L. In this report, a conversion factor has been used to convert to volume concentration equal to 0,7, so this is the same as a concentration of 2,1 ml O2/L. This corresponds to an oxygen saturation of about 30 %. In Drammensfjorden in September 2017 it was observed hypoxic water below 45-63 m deep throughout the fjord within Svelvik. Anoxic water, that is water with zero oxygen concentration, was found below 94.5 m south in inner Drammenfjord near Svelvik. Further north in the fjord, approximately where the Norgips plant is located, anoxic water was found below 82.5 m. Anoxic water was also found further into the fjord, in the middle of the fjord between Solumstrand and Lahellbukta, so most likely the bottom of the fjord is anoxic to that point. Oxygen conditions like this were observed in the bottom water of the fjord since mid-2016. Before this, it has not been observed as low oxygen concentrations since mid-2008. In the river Drammenselva there is an area just west of the island Holmen where the maximum water depth is not more than approximately 3 m. This shallow area prevents saltwater to intrude further up the river. In the southernmost of the two outlets of Drammenselva, Strømsøløpet, south of Holmen, it is a deeper area with depths of 16-17 m. This deeper area has a sill of about 10 m that separates it from the fjord. Here, saltwater was found below 8 m and anoxic water under 13 m. When there is deep water renewal in this small basin, it will probably be renewed with water that has relatively low in oxygen saturation, which is found just below the freshwater layer outside the river outlet. Because of this, the water in this basin do not need to be stagnant for a long period, before anoxic conditions are developed.