Sammendrag
This report presents results from monitoring performed in the Inner Oslo fjord in 2013 for the
municipalities surrounding the fjord. A shorter version of the report in Norwegian can be
found in NIVA report no. 6697. The monitoring was conducted by Norwegian Institute for
Water Research (NIVA) in cooperation with the University of Oslo (UiO). The monitoring
includes physical, chemical and biological conditions in the fjord.
The Inner Oslofjord is a relatively enclosed area. Discharges to the fjord will potentially cause
eutrophication and elevated levels of contaminants. Such influences are still relevant because
the area around the Oslofjord is expected to have a significant population increase in the
future with the increased inputs that follows. The capacity of existing plants for treating
municipal wastewater is also near their upper limit, and requires major expansion to meet
future wastewater challenges.
The local nutrient inputs to the Oslofjord has been significantly reduced since the mid -1970s
to 2003, but has since increased until 2007. Subsequently, the values remained virtually
unchanged.
The environment conditions in the fjord has been constantly improving as a consequence of
wastewater remediation, reduced discharges and measures to reduce discharges of untreated
sewage from overflows. The reduced discharges have over time resulted in lower nutrient
concentrations in the fjord. Concentrations of phosphorus and nitrogen in the surface layer
showed a decrease from the beginning of the 1980s. For phosphorus, there was a minimum in
1999, after which the concentration increased slightly until 2003. From 2003 until 2013 the
concentration has decreased slightly, but with concentrations higher than in 1999. For
nitrogen a minimum was observed in 2011, but the concentration increased slightly the two
subsequent years.
A significant deep water renewal took place in winter 2012/2013. This stopped the
development of hydrogen sulfide in the bottom water in Bunnefjorden. Overall,
approximately 106% of the water between 20 m and the bottom was renewed in the Inner
Oslofjord between August 2012 and April 2013.
The oxygen conditions in the Bunnefjorden has improved since 2001, roughly coinciding in
time with when the discharge point from the Bekkelaget treatment plant was moved from 20
to 50 m depth. An improvement in the oxygen situation has also been observed in the West fjord since 2001 at depths greater than 20m. Although it is too early to conclude, it seems as if
the deep water discharge in in the basin at Bekkelaget has also led to an improvement in parts
of the water column in the Bunnefjord. Changes in oxygen conditions in the deeper parts of
Bunnefjorden is however still primarily a result of natural variations in the length of time that
passes between 2 consecutive deep water renewals.
Mean secchi-depth in 2013 was at the same level as the mean for the years 2002-2010. The
results show that the surface layer in the fjord has become successively less murky during the
past four decades. The total algal biomass integrated over the year was in 2013 approx. 10 %
lower than the previous two years, while the amount of chlorophyll in 2013 was about the
same as the average for the period 2002-2010.
Many shrimps where observed in the Inner Oslofjord in 2013 except for one station
(Vesthullet). Shrimp was also observed in Bunnefjorden at Hellviktangen as a result of a
significant deep water renewal in the area during the winter 2013 that secured favorable
conditions for shrimp. There was also an unusual high number of species compared to earlier
at Hellvik in the Bunnefjord and in the Lysakerfjord. The dominant shrimp species in 2013
was Crangon allmanni.
Overall, there have been major changes in seaweed flora in the area since the late 1970s.
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