Rain threatens coral reefs
Runoff harming Hawaii’s ecosystem, economy
HONOLULU – As muddy rainwater surged from Hawaii’s steep seaside mountains and inundated residential communities last month, the damage caused by flooding was obvious – houses were destroyed and businesses swamped, landslides covered highways, and raging rivers and streams were clogged with debris.
But extreme rain events predicted to become more common with alleged human-caused global warming not only wreak havoc on land – the runoff from these increasingly severe storms also threatens Hawaii’s coral reefs.
“These big events are the ones that have the greatest damage because they are the ones that put the most sediment and nutrients out onto the reef,” said C. Mark Eakin, senior coral adviser to the National Oceanic and Atmospheric Administration and the former director of the agency’s Coral Reef Watch program.
A warmer climate tends to amplify existing weather patterns, said Hawaii’s state climatologist, Pao-shin Chu, noting the islands have an overall wet climate and that powerful storms are expected to become more frequent.
“Given this climate change or global warming, as we have seen over the last hundred years, the atmospheric water vapor pressure is increasing,” Chu said. “We have some evidence showing that we already have some increasing, very intense rain.”
Coral reefs make up much of Hawaii’s nearshore ocean ecosystem and are critical to the state’s economy.
Hawaii’s reefs protect populated shorelines from massive ocean swells and storm surges from tropical storms – a benefit the U.S. Geological Survey valued at more than $860 million a year.
Adding tourism, fishing, cultural value and other factors, the state’s reefs are worth more than $33 billion, according to an Noaa-funded study.
March’s flooding was caused by a weather system that stalled over the islands and brought two weeks of rain, much of it heavy.
On Oahu’s north shore, “a very large flood wave” rushed down from the mountains and flooded the town of Haleiwa, said National Weather Service hydrologist Kevin Kodama.
“That’s a big challenge in Hawaii, where we have small, steep watersheds,” he said. “Most of the basins in the state will produce flash flooding.”
The runoff problem is multifaceted. Deforestation and grading on construction sites and farms lead to increased runoff.
Feral animals such as goats, pigs and deer clear vegetation, causing erosion and excessive sedimentation on reefs. And constant, low-level runoff carries gasoline and oil from roadways, household chemicals, trash and pesticides into the ocean.
Any significant change in ocean conditions, such as an influx of fresh water alone, can harm coral health. Contaminants and soil from land accumulate on reefs and can smother and kill the coral. Scientists say suspension of material in the water can also block sunlight coral needs to survive.
March’s floods were not the first of their kind.
A 2018 rainstorm on Kauai caused widespread flooding that cut off a community for weeks.
Ku’ulei Rodgers, a coral reef ecologist at the University of Hawaii’s Institute of Marine Biology, studied that 2018 flood as well as a 2002 flood in the same area. The 2002 rains swept earth from a construction site into the sea and “killed almost an entire reef,” Rodgers said.
When making policy decisions about how to safeguard reefs, Rodgers said, it’s important to understand that land and oceans are intertwined.
“(Native) Hawaiians knew there was a connection between the two because whatever they did upland would affect their fishing downstream,” she said. “The better the watershed, the better the reef and vice versa.”