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

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

February 10, 2021
Haiku Magazine and The Atlantic published a feature article on coastal water darkening by Doug Johnson. The environmental threat you’ve never heard of . The article features interviews with Anders F. Opdal from our own project, in addition to Maren Striebel and Oliver Zielinski from the The Coastal water darkening project, and Amanda Poste from NINA, presenting findings from a recent paper by Poste, Maeve McGovern and colleagues at NIVA and UiO. The magazine article is a bit doom-and-gloom, but we 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.

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

November 10, 2020
Our colleagues at NIVA recently published a national review report on coastal darkening research: Increased light attenuation in Norwegian coastal waters - A literature review. This is the first extensive review on the subject, and includes several findings from the current project as well as earler work by Dag L. Aksnes (TEG). 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 mainly on land, is the primary driver of coastal darkening.

Heavily brown coloured freshwater in a Swedish river. Foto:Øyvind Fiksen

October 31, 2020
The Norwegian national broadcasting company (NRK) wrote a short news story about our project Nytt miljøproblem i Norge: Klimaendringer gjør havet mørkere . Dag L. Aksnes and Anders F. Opdal at TEG where interviewed. 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". The full radio interview can be found here, under the Tidspunkter-menu "Kvifor blir sjøen mørkare", or scroll 1:09:45 into the program

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

September 21, 2019
Our paper in Global Change Biology was featured in the Norwegian online popular science magazine, Mørkere vann forsinker algenes årlige vårfest i Nordsjøen. The magazine article, by Øystein Rygg Haanes, focus 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.

September 11, 2020
Dag O. Hessen and Dag L. Aksnes appear on the Norwegian radio program Ekko discussing the causes and consequences of darker coastal waters. The full radio program is available here under the Tidspunkter-menu "Havet blir mørkere", or scroll 27:02 into the program.

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 delays spring bloom (Opdal et al, 2019, Global Change Biology 25: 3946-3953))

August 16, 2019
First scientific article in the project was published in Global Change Biology! The study Centennial decline in North Sea water clarity causes strong delay in phytoplankton bloom timing 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.
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