Pollution from land can greatly harm fragile coral reefs

Photo: Pexels/Kammeran Gonzalez-Keola

Coral reefs around the tropics are feeling the brunt of climate change, but other manmade environmental impacts such as land-based pollution are similarly hazardous to their health.

This is according to researchers who studied the health of coral reefs in Hawaii for two decades.

Their research highlights the need to mitigate “both local land and sea-based human impacts, especially in terms of pollutants and over-fishing, [which] provides coral reef ecosystems with the best opportunity to persist under climate change,” they stress.

The scientists have found that near highly populated areas on the shorelines of Hawaii, wastewater pollution and urban runoff put “immense stress” on local coral reefs already suffering from increasing water temperatures.

“There is a very strong perception that declining reef health is mainly driven by climate change, which is true in the long term. However, what we are putting in our waters from our shores, as well as the amount of fishing we are doing, are huge drivers that are more immediately actionable,” notes Greg Asner, director of Arizona State University’s Center for Global Discovery and Conservation Science who was one of the researchers behind the study.

These human-caused threats can greatly worsen the effects of climate change. This was observed in 2015, when the Hawaiian Islands experienced the worst marine heatwave on record for well over a century, according to the researchers.

Because of this unprecedented heatwave, “many reefs in our study region experienced severe bleaching and extensive coral death,” says Jamison Gove, a research oceanographer with the Atmospheric Administration’s Pacific Islands Fisheries Science Center.

“But to our surprise, we found that coral cover was unchanged at nearly 20% of reefs in our study. Which begs the question: why did some reefs fare better than others despite experiencing similarly extreme levels of heat stress?” Gove notes.

The difference, it turned out, lay in local land-sea conditions. The reefs that lost less coral during the heatwave were those with the lowest levels of land-based stressors.

This is not entirely surprising as wastewater contains high levels of nutrients, toxins and pathogens that can harm corals. The contents of septic tanks and cesspools released into the sea are especially harmful to reefs, the scientists say. That is why it is vital to eliminate such sources of pollution from land.

“Integrated land-sea management has been the guiding paradigm of coral reef conservation for decades, but the proof of its efficacy above managing land or sea in isolation has been wanting and difficult to test,” says Gareth Williams, an associate professor at Bangor University.

“Through the 20-year data set of local human impacts and environmental factors, we are able to see that only by adopting coupled land-sea policy measures, alongside global reductions in greenhouse gas emissions, will coral reef ecosystems and the human communities they support have the best opportunity for persistence in our changing climate,” Williams emphasizes.

This study’s findings are in line with others highlighting human-caused threats to reefs beyond climate change. These threats include the use of sunscreen and discarded fishing nets, both of which can damage reefs and their ecosystems.