In Hauraki Bay, New Zealand, waves hit cliffs and pulled mud into the sea. Ships and storms stirred silt from the seabed. The river carries fertilizer from the mainland, which causes algal blooms blocking light and mixes with pollution in nearby Auckland. Together, they envelop the coastal ocean, depriving creatures living deep in the water column of their main source of energy – sunlight.
As an environmental threat, this phenomenon known as coastal darkening has not been studied enough. More and more work is trying to understand how coastal darkening occurs and what it means to the ocean and life in it. For example, a paper published in 2020 pointed out that coastal darkening may hinder and change the relative abundance of different phytoplankton populations. Another researcher in 2019 pointed out that the darkening of the coast may delay the time of phytoplankton bloom, resulting in potential consequences for phytoplankton dependent organisms. Moreover, as the new study shows, darkening the coast may also amplify the impact of climate change.
Caitlin blain, a marine ecologist at the University of Auckland, said that coastal blackening would seriously hinder the growth of kelp and reduce the productivity of algae by 95%. The decline in kelp productivity may have a series of consequences for fish and other organisms that use kelp as food or shelter. It may also undermine the ability of kelp to absorb carbon and have an impact on the global climate.
To make this discovery, Brian and her team ventured into Hauraki Bay and studied seven kelp forests, which are mainly composed of radiant kelp. At each location, they set up two daylights, one on the surface and one 10 meters below the kelp, to measure the utilization of sunlight.
Each of the seven kelp forests is plagued by varying degrees of particles in the water. Locations close to urban areas such as Auckland or rivers flowing through agricultural land are often more blurred than those away from land inputs of particulate pollution.
In a year, the research team returned to the site four times to measure the growth of 20 kelp samples. Whether in the field or in the laboratory, the research team also wrapped the samples in the photorespiration measurement room to measure the amount of oxygen produced by each sample under different amounts of light. According to Brian, the amount of oxygen produced by kelp is roughly equal to the amount of carbon it uses to grow. Therefore, kelp absorbs the amount of carbon.
Scientists found that due to the sun blocking effect of particle pollution, the darkest places received 63% less sunlight than the brightest places. The lack of light means that kelp’s primary productivity – the rate at which solar energy is converted into organic matter – is 95% lower in the darkest places. The kelp biomass growing there has been tripled. Overall, the team found that the darkening of the coast reduced the carbon content of kelp forests by 4.7 times.
Research in 2016 shows that the world’s kelp forests absorb up to 200 million tons of carbon every year. However, blain said by e-mail that the role of kelp forests as sinks in the global carbon cycle is unclear: “We understand that kelp forests are one of the most productive ecosystems on earth and may be an important contributor to carbon sequestration. However, their contributions are highly species and location specific and eventually degraded by human impacts, such as coastal darkening and climate driven temperature change.”
Oliver Zielinski, who runs the now abandoned coastal marine blackening project at the University of Oldenburg in Germany, said that although researchers have begun to understand most of the reasons behind this phenomenon, there is still a lot to learn about its broader impact on aquatic organisms and the ocean. “This needs more thorough investigation,” he said.
The darkening of the coast is complex. It is the pinnacle of countless processes on land and sea, and the exact reasons vary from coast to coast. For example, one reason is that plant matter from trees falls into rivers, dissolves into brown mud, and then flows into the ocean to block sunlight. In this case, the effect depends on the type of trees nearby, because their leaves and branches will dissolve into different compounds and have different effects on light. In Norway, it is ironic that coordinated tree planting efforts have led to an increase in coastal blackening. Therese Harvey, a marine ecologist and biological glasses expert at the Norwegian Institute for water research, said that to learn to alleviate the darkness in coastal areas, scientists need to solve this problem from a broad interdisciplinary perspective.
However, Harvey said that minimizing further anthropogenic warming is a clear step towards reducing coastal blackening. Climate change will lead to increased rainfall in some parts of the world. This in turn means that more debris, organic matter and fertilizer enter the ocean. But Brian’s research shows that learning how to fight coastal blackening can also help us deal with climate change.
Brian also pointed out that, unlike other man-made climate problems (such as global temperature rise), coastal darkening can be solved at the local level because each coast has a different experience. In addition, countries can take some measures, such as prohibiting the development near some water bodies or preventing coastal erosion, so as to achieve rapid results.
Despite the layers of complexity, the threat posed by coastal darkening is incredibly simple at its core: “it affects light, and light affects all creatures in the ocean,” Harvey said.