THE PARADOX OF NUTRIENT EMISSIONS IN NORWEGIAN FJORDS: THE PROBLEM IS TOMORROW, NOT TODAY

Background

In the Hardangerfjord at the western coast of Norway, annual aquaculture production is over 100,000 tons of salmon and is the largest anthropogenic source for excess nutrients. Environmental impacts of aquaculture are a major public concern in Norway, despite monitoring tells a different story: in 2024, seven of eight dissolved nutrients monitoring stations and all seven chlorophyll-a monitoring stations were classified as having very good ecological status (Figure 1). A management question arises: does good ecological status indicate remaining environmental capacity, or does it merely describe current conditions?

Figure Figure 1. Summary of overall assessment of the supporting parameters Left: Dissolved nNutrients salts (phosphate, nitrogen, total nitrogen and phosphate and ammonium). Right: Chl-A in side fjords from inner to outer Hardangerfjord. Blue indicates very good environmental status and green indicates good environmental status. Source: Husa et al (2025) and references therein.

Results and discussion

Remaining spare capacity for aquaculture production in the Hardangerfjord was assessed. Spatial varying annual primary production responses due to excess nutrients release were calculated with the ecosystem model NORWECOM.E2E (Hjøllo et al 2025). We compared a reference simulation without excess nutrients from aquaculture to 3 simulations using i) current aquaculture production (Figure 2 A), ii) current + planned increase in production (Figure 2B) and iii) doubled production. Corresponding nutrients were released at sites reporting production in 2024. Modelled responses (Figure 2A) show good ecological status today, with no widespread nutrient enrichment observed, although spatial hotspots emerge. Increased phytoplankton production under scenarios ii and iii indicated increasing risks for eutrophication in both the main fjord and the sidefjords.

Figure 2. Modeled response (percentage increase) in phytoplankton production due to discharge of dissolved nutrients from aquaculture with A) current aquaculture production B) current aquaculture production plus planned increase and C) doubled current aquaculture production. Phytoplankton production is calculated as the sum over the period 1 March–30 October 2024, based on data from ecosystem model NORWECOM.E2E (Hjøllo et al 2025). A 50% increase in phytoplankton production is a measure of possible eutrophy. Source: Husa et al 2025

In summary, the Hardangerfjord monitoring says: "Everything looks fine", while modelling says: "Not for much longer". Environmental monitoring is designed to detect impacts, not to quantify remaining carrying capacity. Recently, the management authority for aquaculture production (County Governor of Vestland) challenged indicator-based regulation of aquaculture nutrient emissions. All applications for further production increase in the Hardangerfjord were turned down based on negative trends in some monitored environmental parameters (e.g., macroalgae and oxygen) (Pedersen et al 2025), which together with the expected continuation of climate change impacts, is likely to reduce the Hardangerfjord's capacity to receive and assimilate additional emissions. The County Governor therefore concluded that a rise in aquaculture production is not sustainable. In many areas of environmental management, including climate adaptation and natural hazard management, decisions are routinely based on projected future risk rather than waiting for observable ecological deterioration. Should aquaculture management take the same pre-cautionary approach?

References

Hjøllo SS, et al (2025) Investigating cumulative impacts: coastal primary production in a warmer, darker, and nutrient-enriched future. Aquac Environ Interact. Husa V, et al (2025) Hardangerfjorden under pressure. Report from Institute of Marine Research. Pedersen, T, et al (2025): The Hardangerfjorden can't tolerate anymore. Note from the County Governor of Vestland (in Norwegian).