In New Zealand’s Hauraki Gulf, waves crash against cliffs and drag debris into the sea, while boats and storms stir mud from the ocean floor. Rivers carry fertilizer from the mainland, which causes light-blocking algal blooms to mix with pollution from nearby Auckland. Together they cloud the coastal ocean and deprive organisms that live deeper in the water column of their main source of energy – sunlight.
As an environmental threat, this phenomenon, known as coastal darkening, has received relatively little research. There is a growing body of work attempting to understand how coastal darkening occurs and what it could mean for the ocean and life in it. For example, a paper published in 2020 suggests that coastline darkening could slow and shift the relative abundance of different phytoplankton populations. Another in 2019 found that coastline darkening could delay the timing of phytoplankton bloom – with possible consequences for the organisms that depend on them. And, as new research shows, coastline darkening can also increase the effects of climate change.
Caitlin Blain, marine ecologist at the University of Auckland, says shoreline darkening can severely stunt the growth of seaweed and reduce its productivity by as much as 95 percent. This decline in seaweed productivity could have a number of consequences for the fish and other organisms that use the seaweed as food or shelter. It could also affect the ability of seaweed to store carbon, with consequences for the global climate.
To make this discovery, Blain and her team ventured into the Hauraki Gulf to examine seven kelp forests, mostly from Ecklonia radiata. At each location, they set up two light loggers, one on the surface and one 10 meters deep in the seaweed, to measure the availability of sunlight.
Each of the seven kelp forests was clogged with different particles in the water. The locations closer to urban areas like Auckland or rivers flowing through agricultural land tended to be more obscure than those further away from terrestrial inputs of particulate matter.
Over the course of a year, the team returned to the sites four times to measure the growth of 20 samples of seaweed. Both in the wild and in the laboratory, the team also wrapped the samples in photorespirometry chambers to measure how much oxygen each one produces with different amounts of light. According to Blain, the amount of oxygen seaweed produces is roughly the same as the amount of carbon it uses to grow, and thus the amount of carbon it binds.
The scientists found that the darkest spot received 63 percent less sunlight than the brightest due to the sunlight-blocking effect of fine dust pollution. The lack of light meant that in the darkest spot, the seaweed’s primary productivity – the speed at which it converts solar energy into organic matter – was 95 percent lower. The seaweed growing there accumulated twice less biomass. Overall, the team found that the coast darkening caused the kelp forests to sequester up to 4.7 times less carbon.
Research from 2016 suggests that the world’s kelp forests store up to 200 million tons of carbon each year. To what extent kelp forests act as a sink in the global carbon cycle, however, is still unclear, Blain says via email: “We are learning that kelp forests are among the most productive ecosystems in the world and are likely to make an important contribution to carbon seizure. However, their contribution is very species and location-specific and is ultimately impaired by human influences such as the darkening of the coast and climate-related temperature shifts. “
Oliver Zielinski, who led the now-discontinued Coastal Ocean Darkening project at the University of Oldenburg, says that while researchers are beginning to understand most of the causes behind the phenomenon, much remains to be learned about its broader effects on aquatic life and the ocean in general. “It needs a much more thorough investigation,” he says.
Coastal blackout is complex. It is the culmination of innumerable processes on land and in the sea, and the exact causes vary from coast to coast. For example, one cause is that plant matter falls from trees into rivers, dissolves into a brown slurry, and flows into the ocean to block sunlight. In such cases, the effect will depend on the tree species nearby as their leaves and branches dissolve into different compounds with different effects on light. Ironically, in Norway, concerted tree-planting campaigns have led to an increase in coastal blackout. To learn how 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, scientists need to approach it from a broad, interdisciplinary perspective.
However, minimizing further anthropogenic warming is a clear step towards mitigating coastal darkening, says Harvey. Climate change will lead to more rain in some parts of the world. This, in turn, could mean more debris, organic matter, and fertilizer entering the ocean. But Blain’s research suggests that learning to tackle coastal blackout can also help us address climate change.
Blain also notes that unlike other man-made climate issues like rising global temperatures, coastline darkening can be addressed at the local level, as each coast experiences it differently. In addition, countries can take steps to achieve quick results, such as banning development near some bodies of water or tackling coastal erosion.
Despite the layers of complexity, the core threat of coastal blackout is incredibly simple: “It affects light, and light affects all marine life,” says Harvey.
This story originally appeared in Hakai magazine and is part of Cover climate now, a global journalistic collaboration that strengthens reporting on climate history.