Hakai Magazine

underwater view of kelp in New Zealand
Coastal darkening reduces the amount of light that penetrates into coastal waters with a range of consequences for local ecosystems and, potentially, the world. Photo by Kim Westerskov/Alamy Stock Photo

Kelp’s Carbon Sink Potential Could Be Blocked by Coastal Darkening

Coastal darkening, an environmental threat researchers are only beginning to study, is found to dramatically reduce the productivity of kelp.

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by Doug Johnson

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In New Zealand’s Hauraki Gulf, waves crash against cliffs and pull dirt into the ocean, while boats and storms stir up silt from the seafloor. Rivers carry fertilizer from the mainland that causes light-blocking algal blooms, which mingle with pollution from nearby Auckland. Together, they cloud the coastal ocean, depriving organisms living deeper in the water column of their main source of energy—sunlight.

As an environmental threat, this phenomenon, called coastal darkening, is relatively understudied. There is a growing body of work trying to understand how coastal darkening occurs and what it could mean for the ocean and the life within it. A paper published in 2020, for instance, suggests that coastal darkening could stunt and shift the relative abundance of different phytoplankton populations. Another from 2019 noted that coastal darkening might delay the timing of phytoplankton blooms—with potential consequences for the organisms that rely on them. And, as new research shows, coastal darkening may also amplify the effects of climate change.

Caitlin Blain, a marine ecologist at the University of Auckland, says that coastal darkening can severely hinder the growth of kelp, reducing its productivity by up to 95 percent. This drop in kelp’s productivity could have a range of consequences for the fish and other organisms that use the kelp for food or shelter. It could also upset kelp’s ability to sequester carbon, with consequences for the global climate.

To make this discovery, Blain and her team ventured out into the Hauraki Gulf to study seven kelp forests, which are mostly composed of Ecklonia radiata. At each site they set up two light loggers, one at the surface and one 10 meters down among the kelp, to measure the availability of sunlight.

Each of the seven kelp forests was mired by varying levels of particulates in the water. The sites closer to urban areas like Auckland, or to rivers that run through agricultural land, tended to be more obscured than those farther from the terrestrial inputs of particulate pollution.

Over the course of a year, the team returned to the sites four times to measure the growth of 20 sample kelps. Both in the wild and in the lab, the team also encased specimens in photorespirometry chambers to gauge how much oxygen each produced with different amounts of light. According to Blain, the amount of oxygen that kelp produces is roughly equal to the amount of carbon it uses to grow and, thus, the amount of carbon it sequesters.

The scientists found that because of the sunlight-blocking effect of particulate pollution, the darkest site received 63 percent less sunlight than the lightest one. The dearth of light meant that at the darkest site, the kelp’s primary productivity—the rate at which it converts energy from the sun into organic matter—was 95 percent lower. The kelps growing there accumulated two times less biomass. Overall, the team found that coastal darkening caused the kelp forests to fix up to 4.7 times less carbon.

Research from 2016 suggests that the world’s kelp forests sequester as much as 200 million tonnes of carbon each year. However, the extent to which kelp forests act as a sink in the global carbon cycle is still unclear, says Blain by email: “We are learning that kelp forests are some of the most productive ecosystems on the planet and are likely important contributors to carbon sequestration. However, their contribution is highly species and location specific, and is ultimately degraded by human impacts such as coastal darkening and climate-driven shifts in temperature.”

Oliver Zielinski, who ran the now-defunct Coastal Ocean Darkening project at the University of Oldenburg in Germany, says that although researchers are beginning to understand most of the causes behind the phenomenon, there is still much to learn about its broader impacts on aquatic life and the ocean at large. “It needs much more thorough investigation,” he says.

Coastal darkening is complex. It is the culmination of myriad processes on land and in the ocean, and the precise causes vary from coast to coast. One cause, for instance, involves plant matter from trees falling into rivers, dissolving into a brown slurry, and flowing out into the ocean to block sunlight. In cases like this, the effect depends on the types of trees nearby, as their leaves and twigs will dissolve into different compounds with varying effects on the light. In Norway, concerted tree-planting efforts have, somewhat ironically, caused an increase in coastal darkening. Learning to mitigate coastal darkening, says Therese Harvey, a marine ecologist and bio-optician at the Norwegian Institute for Water Research who was not involved in the new study, will require scientists to tackle it from a broad, interdisciplinary perspective.

Minimizing further anthropogenic warming, however, is a clear step toward mitigating coastal darkening, Harvey says. Climate change is set to cause some parts of the world to get more rain. This could, in turn, mean more detritus, organic material, and fertilizer reaching the ocean. But Blain’s research suggests that learning how to combat coastal darkening can also help us confront climate change.

Blain also notes that unlike other human-made climate problems, such as rising global temperatures, coastal darkening can be tackled at a local level because each coast experiences it differently. Further, there are steps, such as prohibiting development near some bodies of water, or fighting coastal erosion, that countries can take to see swift results.

Despite the layers of complexity, the threat posed by coastal darkening is, at its core, incredibly simple: “It affects light, and light is affecting all life in the sea,” says Harvey.