How Coastal Darkening Is Harming Kelp Forests

This article comes from Hakai Magazine. It is an online publication that covers science and society in coastal ecosystems. You can find more stories like these at hakaimagazine.com
The Hauraki Gulf in New Zealand is where waves smash against the cliffs, bringing dirt into the ocean. Meanwhile, storms and boats stir up silt on the seafloor. Fluvial fertilizer is carried from the mainland, which causes light-blocking algae blooms. They cloud the coast ocean and deprive organisms living in deeper water columns of their primary source of energy, sunlight.

This phenomenon, known as coastal darkening (or coastal darkening), is a serious environmental threat. However, it has been relatively poorly studied. A growing body of research is trying to figure out how coastal darkening happens and what it might mean for the oceans and all life in them. For example, a paper published in 2020 suggests that coastal darkening might reduce or shift the relative abundances of different phytoplankton species. Another 2019 paper noted that coastal darkness could delay phytoplankton blooms, which could have potential consequences for organisms that depend on them. New research has shown that coastal darkening can also increase the impacts of climate change.

Caitlin Blain is a marine ecologist from the University of Auckland. She says coastal darkening can seriously hinder the growth of the kelp and reduce its productivity by as much as 95 percent. The drop in kelp productivity could have many consequences for fish and other organisms that rely on the kelp as food or shelter. This could have negative consequences for the global climate by affecting kelp's ability carbon sequestering capacity.

Blain and her colleagues explored the Hauraki Gulf in search of seven kelp forests that were mostly Ecklonia radiata. To measure sunlight availability, they set up two light logs at each site, one at the top and one 10 m below the kelp.

The water quality in each of the seven kelp forest forests was different. Sites closer to urban areas, such as Auckland, or rivers that run through agriculturall land, were more difficult to see than sites further away from terrestrial particulate pollution.

The team visited the site four times over the course of one year to monitor the growth of 20 sample Kelps. The team used photorespirometry chambers for both the laboratory and the wild to measure the amount of oxygen that each specimen produced under different light conditions. Blain says that the amount kelp can produce is approximately equal to the carbon it needs to grow, and thus the carbon it stores.

Scientists discovered that the particulate pollution blockade caused by sunlight blocking effects made it receive 63 percent less sunlight than its lightest counterpart. Because of the lack of sunlight, the primary productivity (the rate at which kelp converts solar energy into organic matter) was 95 percent lower at the darkest location. There, the kelps accumulated twice as much biomass. The team discovered that the coastal darkening had caused the kelp forests in the area to store up to 4.7 times more carbon.

According to research from 2016, kelp forests in the world sequester up to 200 million tonnes of carbon annually. Blain via email says that it is not clear how much kelp forests contribute to the global carbon cycle. "We have learned that kelp forest ecosystems are among the most productive on the planet, and they are likely to be important contributors to carbon storage." Their contribution to the global carbon cycle is dependent on where it is located and highly specific by species. They are also subject to degradation due climate-driven temperature shifts and coastal darkening.

Oliver Zielinski was the former director of the now-defunct Coastal Ocean Darkening Project at the University of Oldenburg, Germany. He said that while researchers have begun to understand the cause of the phenomenon, there is much more to be learned about the wider impacts it has on the ocean. He says that the phenomenon requires more detailed investigation.

Coastal darkening is complex. It is the result of many processes both on land and in water, and the exact causes can vary from coast-to-coast. For example, plant matter is a result of trees falling into rivers. This causes a brown slurry to dissolve and then flows out into the ocean to block sunlight. The effect of this is dependent on the type of trees surrounding the tree. Their leaves and twigs can dissolve into different compounds that have different effects on the light. Ironically, the increase in coastal darkness has been caused by concerted tree planting efforts in Norway. Scientists will need to look at coastal darkening from multiple perspectives, according to Therese Harvey (a marine ecologist, bio-optician, and member of the Norwegian Institute for Water Research).

Harvey states that reducing further anthropogenic global warming is an important step towards reducing coastal darkening. Some parts of the globe will see more rainfall due to climate change. This could lead to more organic material and fertilizer reaching oceans. Blain's research shows that we can also combat coastal darkening by learning how to do so.

Blain also points out that, unlike other human-made climate issues, such as rising temperatures, coastal darkness can be addressed at a local level, since each coast experiences it differently. To see quick results, countries can also take steps like preventing development near water bodies or fighting coastal erosion.

Harvey says that despite the complexity of the problem, coastal darkening poses a threat to all life on the coast.

This article comes from Hakai Magazine. It is an online publication that covers science and society in coastal ecosystems. You can find more stories like these at hakaimagazine.com.

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