Coastal Darkening Threatens Ocean Food Chains

by Vins
Published: Last Updated on

In 2019, Maeve McGovern and fellow researchers at the Institute for Chemistry and Biology of the Marine Environment at the University Oldenburg in Germany began studying a new phenomenon in the world’s oceans. As pollution and runoff alter the color and clarity of coastal waters, the blocking of light leads to darker waters, or coastal darkening. Oliver Zielinski, who directs the Coastal Ocean Darkening project at the University of Oldenburg in Germany, told Hakai Magazine that these “changes in the physics will lead to biological changes.” Ultimately, coastal darkening has “the potential to cause huge problems for the ocean and its inhabitants,” Hakai Magazine reported in February 2021.

Coastal Darkening is usually the result of introducing organic matter, known as Terrestrially derived Organic Materials (or TOM), into an aquatic environment. TOM can be added naturally by heavy rain stirring up organic matter. Other TOMs are introduced as a result of human interference, such as using fertilizer or boating. When fertilizer is washed away and ends up in a large body of water it causes an algal bloom in that area. These algae works in the same way as the organic matter stirred up by heavy rain and creates a similar light-blocking layer. Boating across a body of water causes silt to be kicked up. This silt gathers and creates yet another light blocking layer. Light-blocking layers caused by humans are the most significant causes of coastal darkening.

Coastal Darkening affects sea life in two ways. First, coastal darkening prevents many types of sea life from gathering food. These impacts begin with plankton that feed off the light and extends up the food chain to impact other species that feed on plankton, and so on. On a large scale, these changes in ocean ecosystems could eventually lead to shortages of fish and other aquatic life not only humans, but other ocean predators. The second problem with coastal darkening is that sunlight normally serves to break down certain toxic chemicals in the water. Decreased light compromises this process, resulting in higher contamination levels, with impacts on oceanic food chains and consequences for humans as well.

As documented by researchers at the Institute for Chemistry and Biology of the Marine Environment, the long-term impacts of coastal darkening on the health of ocean life are enormous and complex. As mentioned above, a lack of light creates multiple problems for the coastal food web. A lack of light means a lack of food for small fish and other low-level consumers, which in turn means these low-level consumers have a reduced level of nutritional value for the organisms that feed on them.  This drop in nutritional value in low-level consumers leads to reduced food quality and trophic efficiency—or efficiency of energy transfer– in coastal food webs. At the same time, a lack of light directly prevents other larger fish and other predators from hunting their prey. Yet, this lack of light actually benefits other organisms (like jelly fish) in the same environment who do not rely on light to hunt.

Another effect of coastal darkening is an increase in contaminant transport. While small traces of contaminants are normal in coastal environments, the addition of unnatural contaminants creates an effect that makes it easier for contaminants to enter. The flux of inorganic sediments and TOM facilitates the transport of contaminants from catchment soils to rivers, fjords, and coastlines, directly influencing contaminant concentrations in surface waters and sediments. This effect ends, once again, with the food web weakening as a consequence of coastal darkening.

As of February 23, 2021, there has been no coverage at all of coastal darkening by corporate media. While scientific research on this phenomenon has been conducted since June 2019, the first journalistic accounts did not appear until February 2021. Those articles were printed by Hakai Magazine, an outlet focused specifically on coastal science and societies, The Atlantic, and EcoWatch. Notably, both the Atlantic and EcoWatch coverage was based on the Hakai Magazine report, so there has really only been a single unique media report of any kind about this topic.

Source: Doug Johnson, “The Environmental Threat You’ve Never Heard Of,” Hakai Magazine, February 10 2021,

Student Researcher: Victor Rodriguez (North Central College)

Faculty Evaluator: Steve Macek (North Central College)