Monitoring the pollution status of the inner Oslofjord 2010. Rapport nr.109, june 2010, Summary
Author: John Arthur Berge, Rita Amundsen, Birger Bjerkeng, Gunhild Borgersen, Erik Bjerknes,Janne Kim Gitmark, Jakob Gjøsæter, Merete Grung, Hege Gundersen, Tor Fredrik Holth, Ketil
Hylland, Torbjørn Johnsen, Halvor Knutsen, Anna Birgitta Ledang, Evy R. Lømsland, Jan Magnusson, Inger Lise Nerland, Esben M. Olsen, Øystein Paulsen, Thomas Rohrlack, Kai Sørensen,Mats Walday
Source: Norwegian Institute for Water Research, ISBN No 978-82-577-5916-2
The Inner Oslofjord monitoring programme is designed to cover the pollution status of the fjord, with
main focus on discharge of nutrients (eutrophication). The programme also includes studies related to
contaminants, although these are mainly monitored through other investigations/programs like the
national monitoring programme CEMP (Coordinated Environmental Monitoring Programme).
The Inner Oslofjord monitoring comply with a 10 year’s program with analysis of basic elements like:
the deep water renewal, oxygen conditions, hydrochemical properties, nutrients, turbidity, salinity,
temperature, phytoplankton, blue-green algae, the hyperbentic fauna (benthic sledge) and shallow
water fish species (beach seine). Every second year biomarkers in fish are investigated. Once during
the 10 year period the soft bottom communities and the observations of flora and fauna on rocky
shores and sub tidal benthic communities are investigated. Special investigations, such as mapping of
flora and fauna in rocky shore and sub tidal benthic communities as well as other smaller elements,
complete the programme. The monitoring also includes automatic observations from a system on
board a ferry (“Color Fantasy”) between Norway and Kiel in Germany (FerryBox).
Conclusions:
Pollution load
The discharge of nutrients to the Inner Oslofjord was at its maximum around 1970. Sewage from
750 000 persons in the Oslo area are today treated by the 3 main sewage treatment plants, established
for chemical treatment of phosphorous between 1970 and 1990. Removal of nitrogen started in
1995/96 and was completed in 2001, when the latest sewage treatment plant became operative. The
total load from industry and municipal sewage in 2009 is estimated to be 58 tons phosphorous and
1912 tons nitrogen. The discharges were at a minimum in 2003-2004, but has later increased slightly,
probably caused by increase in the human population in the local catchment area, but has been stable
over the two last years. The total amounts of contaminants discharged to the Inner Oslofjord is not
known.
Deep water renewal and oxygen concentrations
There were good conditions for a deep water renewal during the winter 2010. From October 2009 until
March 2010 northerly winds dominated. The first deep water renewal occurred between December and
January, and the major renewal took place during January and February 2010. The deep water was
stagnant from April to October. The water renewal in 2010 was larger than the previous four years.
The improvement of the oxygen conditions in the main basins of the fjord is caused by the
establishment of the sewage treatment plants. The oxygen condition in the Bunnefjord (intermittently
anoxic) has had a slight positive trend since 2000. Due to a large deep water renewal, the oxygen
conditions were good in Bunnefjorden in 2010 representing a high environmental quality. In the
Bekkelaget basin the oxygen conditions improved – from intermittently anoxic to oxic - since the
establishment of the new sewage treatment plant in 2001.The oxygen levels in the Drøbak Sound are
lower than in the 1950’s. This may affect the oxygen concentration levels in the inner basin after a
deep water renewal due to a lower start concentration in the incoming water.
Hydrochemical conditions – nutrients
The effect of the establishment of sewage treatment plants around Inner Oslofjord is clearly affecting
the water chemistry in a positive way and the conditions related to nutrients have improved since the
1970ies. Still, an increase in ammonium can be seen in 2010 compared to the period 2002-2009, but
the fjord is still within a state defined as very good.
Surface observations summer 2010 – visibility (Secchi depth) and phytoplankton
During the summer 2010 the inner part of the inner fjord had the poorest conditions in the surface
layer (high chlorophyll-a concentrations and low Secchi depths), while the remaining stations further
out had better conditions. Compared with previous years, there is a clear positive development with
the best conditions observed in the period 2002-2010 and the worst conditions in 1973-1982.
Phytoplankton
From the fluorescence observations (from the FerryBox on Color Lines ferry between Oslo and Kiel)
the spring bloom started in the end of January, which is extremely early. The measurements integrated
over the year gave lower plankton biomass than previous years but not as low as the Algal cell-carbon
integrated over the period April-December in 2010 was, which was 50% lower than the previous year.
Chlorophyll-a fluorescence data indicates that the spring bloom started at the end of January. The
highest level of algal cell-carbon was observed in the first weeks of June and the diatom Dactyliosolen
fragilissimus dominated. Also Chaetoceros, Pseudo-nitzschia and Skeletonema were prominent
among the diatom species in June. The dinoflagelates with the Dinophysis norvegica as the most
dominant and the coccolithophorid Emiliania huxleyi also showed maximum abundance during this
period. A weak bloom was observed during the last half of September with Cerataulina pelagica as
the most dominant species followed by Dactyliosolen fragilissimus and Pseudo-nitzschia. The algae
Dinophysis norvegica and D. acuminata was the only species that occurred in concentrations above
the action limit related to human toxicity.
Blue-green algae
Dense blooms of blue-green algae occur annually in Lake Årungen. Observations from 2007 showed a
transport of these algae by River Årungselva to Bunnefjorden. This may cause reduction in bathing
water quality in the southern part of Bunnefjorden. A sensor was therefore placed in Årungselva in
order to continuously monitor transport of blue-green algae to the fjord. As in previous years a dense
blooms of blue-greens were observed in Lake Årungen in 2010, but the blooms were smaller than in
2008 and 2009. The blooms started, however, late in the year 2010 and there was minimal production
before August. The blooms in Lake Årungen in 2010 did not consist of toxic algae and the transport to
the Bunnefjorden did therefore not represent a health problem.
Observations of hyperbenthos (shrimps)
Sampling was performed by towing a benthic sledge along the bottom in 7 areas in the Inner
Oslofjord. The sledge is equipped with a net that mainly collects animals on and just above the
sediment surface. The main focus in the investigation is the abundance of the different shrimp species
in the deeper parts of the fjord. In recent years, we have only occasionally recorded a few shrimps at
Hellvikstangen and none at Svartskog in the Bunnfjord. Further out in the Inner Oslofjord several
shrimp species and a larger number of individuals are usually observed. In the sample from
Hellvikstangen (ca. 80-90 m) north in the Bunnefjorden area, a relatively large amount of small
shrimps were observed in 2010. Similar good catches of shrimp were made in the Lysakerfjord.
An important prerequisite for a sustainable fishery for shrimps in the deeper part of the Oslofjord is
that the oxygen concentrations in the bottom water are above 2,5 ml/L. In 2010 the oxygen conditions
at the bottom at Hellvikstangen were about 2.4 ml / L and at the bottom in Lysakerfjorden about 3.3
ml / L. This is probably an important contributing factor to the increased number of shrimp in this part
of the fjord in 2010.
The dominant shrimp species in the Lysakerfjord in 2010 was Pandalina profunda. This is also the
species that is first observed when conditions improved. Compared with the period 2000-2008 there
was an almost normal occurrence of shrimps at Steilene in Vestfjorden in 2010. Observations this year
showed a low number of individuals of shrimp in the outer part of Oslofjord (Gråøyrenna) and
Drøbaksundet (Elle).
Soft bottom communities
The number of species and individual of benthic soft bottom organisms increased significantly from
1993 to 2009 and must be perceived as a clear environmental improvement. Overall the diversity
expressed as Shannon-Wieners index H and Hurlers index ES100 did, however, not change
significantly from 1993 to 2009. Neither did the indices NQI1 (Norwegian Quality Index, version 1)
or ISI (Indicator Species Index) indicate that the ecological condition on the average had changed
from 1993 to 2009. The results show that good ecological status is far from achieved in the fjord and
that there is still a potential for improvement.
Beach seine
The number of cod caught by beach seine in the Skagerrak area is at a historically low level following
a reduction over the last 20 years. The total number of fish species caught by beach seine is lower in
the Bunnefjord than in the Vestfjord and the number of species caught in the Inner Oslofjord is lower
than in the area outside.
Mapping of marine biotopes
The work in 2010 has focused on modelling the distribution of habitat types in Bunnefjorden based on
recordings made by underwater video camera in the period 2005-2008. Nature types are classified
according to the international "European Nature Information System” (EUNIS) and the Norwegian
"Naturtyper i Norge" (NiN)-systems. The results are used to model the distribution of habitat types.
The method used was analysis of classification (MLC) in ArcGIS, where one looks at typical
characteristics (depth, wave exposure, etc.) for all habitat types that are registered and from this
predict maps which also includes areas of the seafloor not investigated originally. In the investigated
area there was a total of 15 different NiN habitats, and the most common was "Naken løs eufotisk saltvannsbunn" (M15-2). The second most common was "Eufotisk normal svak
energi saltvannsfastbunn" (M11-2). Via the EUNIS system a total of 23 different habitats were
identified. Especially vulnerable habitats (sea grass and oyster beds) were also identified. The maps
are intended as an aid for planning within the municipality, but can also be a good starting point when
planning environmental research.
Effects of contaminants on cod from the Inner Oslofjord
It was observed an influence of pyrene in cod from the inner fjord compared with cod collected in the
outer fjord. The same was not observed for two other PAH-compounds (phenanthrene and benzo(a)-
pyrene). No significant impact of the metals zinc, copper and lead was observed based on the data
from 2009. The results of the investigation suggest that the conditions in the Inner Oslofjord has been
gradually improving in recent years, particularly considering the effects of lead.
Recommendations:
Special attention should be paid to:
- Effects of the population increase on the discharge of nutrients and environmental status of the
Inner Oslofjord
- Several of the sewage treatments plants are or will soon be run at maximum capacity. In order
to not aggravate the environmental conditions in the fjord a considerable effort is needed to
improve sewage treatment capacity and performance in the future
- A budget for the overall discharge of contaminants to the Inner Oslofjord has not been made.
Such a budget should be prepared and should include all types of sources.
- Future climatic change and the population increase around the Inner Oslofjord is a challenge
in relation to deep water renewal, oxygen conditions in the bottom water and goals of
improving the environmental situation in the fjord.
- Increasing the frequency of water renewal in the Bunnefjorden by pumping freshwater into the
deeper parts of the fjord is still relevant in order to improve the oxygen conditions in the fjord.
- The water quality in the Outer Oslofjord (The Drøbak Sound) has been reduced without
reaching the same levels as in the Inner Oslofjord, but further improvement of the Inner
Oslofjord seems to be increasingly dependent on the quality of this water. The decrease in the
oxygen concentrations in the Drøbak sound water may reduce oxygen transport to the Inner
Oslofjord.
- "Uncontrolled" discharges through river and free overflows from the sewage system.
- The historical record of contaminate discharges have been investigate by analyzing
contaminants in 210Pb-dated sediment cores from a local basin (Bærumsbassenget) in the Inner
Oslofjord. The results indicate that the discharges have been reduced during recent years and
that natural sedimentation may be a relevant process for sediment remediation in anoxic
basins. Analogous investigations should be performed also in other parts of the fjord.
- The cause of the bad conditions in the bottom in a fjord basin at 70 m depth north of Steilene
should be investigated in more detail.
- Ocean acidification is expected to be one of the most serious consequences of future climate
change. Most forecasts for future acidification are based on models for large sea areas. We do
however know relatively little about the present state of acidification in coastal areas and
fjords. Investigations should therefore be initiated to obtain updated information on
acidification-related parameters in the seawater in the Inner Oslofjord.
|
