Water circulation is high in both Hardangerfjorden and Boknafjorden, thanks to tides and density-driven currents which transport large quantities of nutrients into, and out of, the fjord systems. Neither winter nor summer concentrations of chemical nutrients were measured to be elevated in the two fjords and the emissions of the current fish production appears to be too low to cause overfertilisation. After assessing the available data from the two fjords, the expert group found no evidence of any increase in the phytoplankton biomass. Eutrophication is here defined as a 50 % increase in phytoplankton biomass (OSPAR).
In Hardangerfjorden, recent studies of macroalgae have been compared with equivalent studies carried out in the 1950s. The findings show that the number of species present in the fjord has increased, and also that species with warm water affinity have become more common. The abundance of habitat-forming species, such as brown algae, is unchanged since the 1950s. There is no evidence of any change in the lower depth limit of sugar kelp in either of the two fjords. In Hardangerfjorden, this limit is often determined by sea urchin grazing. In various places in Boknafjorden, sugar kelp grows at depths of up to 20-25 metres.
Some local impacts
Although no increase in nutrient levels has been detected in the pelagic zone in areas with intensive fish farming (except in the immediate vicinity of cages), there is a risk of local eutrophication in areas with poor water circulation. There may be impacts on the shore in the approximate zone up to 500-1000 metres from farms, although only minimal impacts were observed around farms sited in areas with good water circulation. Some impact on the shore communities can be seen in the local area around farms in the inner part of fjords, particularly if the farm is close to the shore.
Seaweeds are often used an as indicator when assessing eutrophication in coastal waters. One of the characteristics of healthy brown algae is that they are not overgrown with fast-growing, annual algae. However, at certain times of the year there were large quantities of these algae in both of the fjords studied. The expert group considers that it is hard to determine why brown algae are at times heavily overgrown with annual algae, as the phenomenon has also been observed in open coastal areas without any chemical nutrient emissions.
Monitoring programme is proposed
Given that Norwegian production of farmed fish is constantly increasing, the group argues that the situation in the fjords should be monitored closely.They therefore propose establishing a comprehensive monitoring network in both Hardangerfjorden and Boknafjorden. In combination with an extensive network of study sites, this will ensure that routine samples can be taken frequently. Establishing such a monitoring programme will allow us to detect possible future changes to the ecosystems in the fjords.