The Weird Way That Human Waste Is Killing Corals

“Even during a severe heatwave in 2015, not only did 20 percent of the reefs that were in clean water with herbivorous fish survive the heat, but some did too improvedsays Arizona State University ecologist Greg Asner, co-author of the study and leader of the study All Coral Atlas reef mapping project. “All over the planet there are large areas with the same problem. So this means that while we are overly focused on the impacts of climate change on coral reefs – which we should be, and don’t get me wrong, it’s crucial – the other factor killing reefs is coastal runoff and coastal pollution. It is certainly a global problem.”

This study examined 120 miles of Hawaii’s coastline between 2003 and 2019. Asner and his colleagues collected data on ocean temperatures and studied reefs — calculating, for example, how much biomass fish represent. On land, they calculated the amount of urban runoff in a given area — all the dirt washed off the streets, including motor oil and other harmful chemicals. They also calculated the sewage runoff and therefore how much nitrogen might end up in the sea. “The biggest problem we have of any land problem is human waste going into the sea,” says Asner. “We have a ridiculous amount of wastewater pollution caused by individual households.”

(You haven’t counted the multitude of drugs that pass through the human body and enter sewers via sewers. Scientists have only just begun to study which of these drugs might have potentially harmful effects on corals, Asner says, so that’s the aspect requires further research.)

The increasing heat is dealing a severe blow to a system already strained by pollution and overfishing. “In 2015, we received a tremendous wake-up call: the first and largest ocean heat wave to date hit and blanketed our corals for more than 12 weeks. We lost up to 50 percent in some areas, and more than 25 percent across the board,” says Asner. “The three factors together – the pollution, the low percentage of herbivorous fish and the heat – don’t add up, they just add up multiplicative. In these heatwaves, this leads to a very severe deterioration in the health of the reef.”