Department of
Biological Sciences

From terrestrial greening to coastal darkening

Northern Europe is getting greener and has so for more than a century due to land use change, reduced grazing, and climate change. Degrading plant materials dissolved in lakes and rivers creates a brown colour. More plants, leads to more colour, which we call browning. Since freshwater eventually ends up in the ocean, a common assumption has been that the dilution will render the effect of browning insignificant for marine life. However, coastal water masses do not mix continuosly with the ocean, and the fresh brown water is retained, as is easiliy seen in the brackish Baltic Sea, but also observable as a darkening effect in the much saltier Norwegian coastal waters. Darker waters is assumed to have consequence for all marine life directly or indirectly dependent on vision or photosynthesis.

The RCN funded project A green-blue link made browner: how terrestrial climate change affects marine ecology will try to disentangle the chain of events from atmospheric processes to terretrial greening to freshwater browning and eventually coastal ecology. The project is led by Anders Frugård Opdal at TEG, and involves partners from the AKVA group and Department of Geosciences at the University of Oslo, in addition to Umeå University in Sweden and the Finnish Environment Institute.

In 2009, Dag L. Aksnes (TEG) and collegues published the paper Coastal water darkening and implications for mesopelagic regime shifts in Norwegian fjords - one of the first publications to suggest a long term trend of decreasing coastal water clarity and its influence on marine ecology.

Prior to this, it was fairly well documented that freshwaters all over northern Europe was getting browner (Monteith et al 2007), although the drivers are still debated (Kritzberg 2017, 2020; Monteith, 2007).

The link between freshwater browning and coastal darkening was not obvious, perhaps due to a communication gap between the two research fields. We hope our project can be a step towards closing that gap.

Project objective
We know that freshwater is getting browner and coastal water is getting darker. What we don't know is 1) how closely these two independent observations are linked, 2) the ultimate drivers and 3) the ecological consequences.

Project outcomes and news

Connecting Baltic forests to the Barents Sea

In a newly published paper in Limnology and Oceanography Letters, Anders F. Opdal, Christian Lindemann, and Dag L. Aksnes demonstrate how a centennial increase in forest cover in Northern Europe has ramifications for coastal water clarity, all the way from the Baltic to the Barents Sea.

20 March 2023

Together with Tom Andersen and Dag O. Hessen at the UiO, Anders, Chris, and Dag have demonstrated a novel ecosystem connectivity along the land-ocean continuum spanning several thousand kilometres.

Due to changes in land use, climate and afforestation the last 120 years, many boreal areas have been experiencing increased terrestrial primary production.

This greening on land contributes to increased exports of natural organic matter that have made inland waters become browner. Although most freshwater ultimately drains to coastal waters, the link between freshwater browning and coastal water darkening is poorly understood.

In their study, Opdal and colleagues explore this relationship through a combination of centennial records of forest cover and coastal water clarity, contemporary optical measurements in lakes and coastal waters, as well as an ocean drift model.

They suggest a link between forest cover in Northern Europe and coastal water clarity in the Baltic, Kattegat, and Skagerrak Sea and show how brown-coloured freshwater from Northern European catchments can dictate coastal water clarity across thousands of kilometres, from the Baltic lakes to the Barents Sea.

Opdal AF, Andersen T, Hessen DO, Lindemann C, Aksnes DL. 2023.
Tracking freshwater browning and coastal water darkening from boreal forests to the Arctic Ocean
Limnology and Oceanography Letters. 8: 611-619. [ doi:10.1002/lol2.10320 ] [ open access ]

Sorge wegen brauner Seen und dunkler Küsten

This week, the ASLO 2021 Aquatic Sciences Meeting is taking place in virtual Mallorca. Anne Deininger (NIVA), Anders F. Opdal (TEG), Dag O. Hessen (UiO), and Helene Frigstad (NIVA) chaired session SS47 Freshwater browning and coastal darkening.

24 June 2021

Das Wasser einiger Fjorde an der Westküste Norwegens wird immer brauner (Credit: picture alliance / zb | Andreas Franke)

Prior to the conference, Anne and Anders were interviewed by the German radio braodcaster Deutsclandfunk, and their daily science program Forschung aktuell. Finally, we made it to German radio! Dubbed to German, natürlich. Listen to the whole interview here.

Seit einigen Jahren werden die Seen und Flüsse in Skandinavien immer dunkler. Inzwischen sind auch die Küstengewässer betroffen. Grund dafür ist nicht Umweltverschmutzung, sondern im Gegenteil: Ein Faktor ist sauberere Luft. Die dunkle Farbe bringt jedoch ganze Ökosysteme durcheinander.

Hakai Magazine published a feature article on coastal water darkening

Hakai Magazine published a feature article on coastal water darkening by Doug Johnson: The environmental threat you’ve never heard of.

10 February 2021

Brown freshwater draining from rivers to the coastal waters.( Photo: LWM/NASA/LANDSAT/Alamy Stock Photo.)

The article features interviews with Anders F. Opdal from the Theoretical Ecolgoy Group, in addition to Maren Striebel and Oliver Zielinski in The Coastal Water Darkening Project, and Amanda Poste from NINA. The story revolves around findings from a recent paper by Poste, Maeve McGovern, and colleagues at NIVA and University of Oslo. The magazine article is a bit doom-and-gloom, but the researchers did manage to get through a few moderating points.

All in all, it’s difficult to pin down any specific consequences of coastal darkening, says Opdal. [...] It’s a tricky thing to study, with many moving parts.

You can read the whole article here:
Hakai Magazine: The environmental threat you’ve never heard of

NIVA publishes a national review on coastal darkening

This is the first extensive review on the subject, and includes several findings from the current project From terrestrial greening to coastal darkening as well as earlier work by Dag L. Aksnes.

10 November 2020

Our colleagues at NIVA recently published a national review on coastal darkening research:
Increased light attenuation in Norwegian coastal waters - A literature review.

The report, lead authored by Helene Frigstad, was also transformed into a research article published in Frontiers in Marine Science:
Influence of Riverine Input on Norwegian Coastal Systems.

The report and article concludes that human induced changes on land are the primary driver of coastal darkening.

From the report cover. (Photo: Anne Deininger, NIVA)

Coastal water darkening in the national news

The Norwegian national broadcasting company (NRK) wrote a short news story about our project Nytt miljøproblem i Norge: Klimaendringer gjør havet mørkere [In Norwegian]. Anders F. Opdal and Anders F. Opdal at TEG were interviewed.

31 October 2020

Heavily brown coloured freshwater in a Swedish river.(Photo: Øyvind Fiksen.)

Despite the title, neither Opdal nor Aksnes stated that coastal darkening was an environmental problem.

Mørkere hav kan få konsekvenser for småfisken, ifølge professor. Nå skal forskere bruke de neste fire årene på å finne ut om det er en krise for økosystemet.

The article was followed up by a radio interview on the local news program Hordaland i dag. (Scroll to the header Kvifor blir sjøen mørkare or 1:09:45 into the timeline.)

Coastal water darkening in science news on the radio

Dag O. Hessen of University of Oslo and Dag L. Aksnes from TEG appear on the Norwegian radio program Ekko discussing the causes and consequences of darker coastal waters.

11 September 2020

The full radio program [In Norwegian] is available here:
(select the header Havet blir mørkere or scroll to 27:02 on the timeline).

Hva i all verden er det som skjer, undrer jeg. Hva er det som slukker lyset i fjordene og havet? [...]. Det viser seg at Dag Hessen er på saken.

Coastal Water Darkening at

The coastal water has become darker over the last century. In an interview at, Anders F. Opdal explains the phenomenon and its causes and consequences.

23 September 2019

A phytoplankton bloom in the North Sea in spring 2018. (Photo: NASA.)

Read the story at here: Mørkere vann forsinker algenes årlige vårfest i Nordsjøen

The magazine article, by Øystein Rygg Haanes, focuses on the potential effect of darkening on the annual phytoplankton spring bloom.

Påstår bestefaren din at vannet langs kysten var klarere da han var ung? Det trenger slett ikke være gubbete glorifisering av gode, gamle dager. Ny forskning viser at Nordsjøen var klarere for hundre år siden. En av årsakene til formørkningen er økt plantevekst på land, som fører til at mer fargestoff – nedbrutt organisk materiale – finner veien ut i havet.

The interview is based on this recent paper:

Opdal AF, Lindemann C, Aksnes DL. 2019.
Centennial decline in North Sea water clarity causes strong delay in phytoplankton bloom timing
Global Change Biology. 25: 3946-3953. [ doi:10.1111/gcb.14810 ] [ open access ] [ pdf ]

Phytoplankton Bloom Timing Influenced by Terrestrial Runoff

In their newly accepted paper in Global Change Biology), Anders F. Opdal, Christian Lindemann, and Dag L. Aksnes provide evidence suggesting that the centennial decrease in North sea water clarity may have delayed the phytoplankton spring bloom by up to 3 weeks compared to the early 1900s. This delay stands in contrast to the earlier bloom onset typically expected from global warming.

16 August 2019

Predicted phytoplankton response to increased non-chlorophyll light attenuation.

The study utilized a 100-year time series of Secchi disk depth along with various proxies for phytoplankton concentrations. The authors conclude that factors other than phytoplankton, such as increased dissolved organic matter of terrestrial origin, also known as browning, has caused the observed reduction in North Sea water clarity. Through several numerical simulations of water columns, the authors find that the observed coastal water darkening would have delayed the spring bloom by up to three weeks since the early 1900s. In the future, climate warming is expected to further increase browning in lakes and rivers due to increases in terrestrial greening, ultimately reducing water clarity in coastal areas where freshwater drains. However, increased ocean temperature is also suggested to advance the spring phytoplankton bloom due to earlier shoaling of the mixed layer. These contrasting responses highlight the importance of including water transparency in analyses of phytoplankton phenology and primary production.

Opdal AF, Lindemann C, Aksnes DL. 2019.
Centennial decline in North Sea water clarity causes strong delay in phytoplankton bloom timing
Global Change Biology. 25: 3946-3953. [ doi:10.1111/gcb.14810 ] [ open access ] [ pdf ]
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