In New Zealand\u2019s 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\u2014sunlight.<\/p>\n
As an environmental threat, this phenomenon, called coastal darkening<\/a>, 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<\/a>, for instance, suggests that coastal darkening could stunt and shift the relative abundance of different phytoplankton populations. Another<\/a> from 2019 noted that coastal darkening might delay the timing of phytoplankton blooms\u2014with potential consequences for the organisms that rely on them. And, as new research shows<\/a>, coastal darkening may also amplify the effects of climate change.<\/p>\n 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\u2019s 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\u2019s ability to sequester carbon, with consequences for the global climate.<\/p>\n 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<\/em>. 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n 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\u2019s primary productivity\u2014the rate at which it converts energy from the sun into organic matter\u2014was 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.<\/p>\n