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Are hurricanes to blame for fish dying and 'dead zones' appearing on NC's coast?

More than two decades of hurricanes have ravaged North Carolina's shores, leaving lasting negative effects beyond the devastation of the storms themselves.

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The storm names themselves evoke memories of boarded up windows, swamped bridges, flooded homes and what seemed at times a never-ending debris cleanup. People displaced by 2016's Hurricane Matthew are still waiting for aid.

Now, North Carolina researchers are linking the storms to long-term effects that have caused massive fish kills, unsafe water quality and more.

From 1996 to 2017, North Carolina saw hurricanes Fran (1996), Bertha (1996), Bonnie (1998), Floyd (1999), Dennis (1999), Isobel (2003), Frances (2004), Ophelia (2005), Ernesto (2006), Earl (2010), Arthur (2014), Joaquin (2015), Hermine (2016) Matthew (2016) Harvey (2017), Irma (2017) and Maria (2017), along with many more tropical cyclones.

Scientists at N.C. State University and the University of North Carolina at Chapel Hill have studied those storms and how they have shaped the Tar Heel state's coastal ecology.

Those include:

An increase in potentially deadly algae blooms, which can cause fish kills, water quality unsafe for humans and the closure of fisheries;

▪ More fish and shellfish kills, the localized death of fish populations;

▪ "Dead zones" or areas with excessive nutrient pollution that depletes oxygen needed for marine organisms to live, according to the National Oceanic and Atmospheric Association.

"In a span of 20 years, coastal North Carolina has been impacted by a major rise in the number of hurricanes, as well as two 500-year flood events: Hurricanes Floyd in 1999 and Matthew in 2016," said Hans Paerl, a UNC scientist. "But the trend isn't limited to North Carolina. It extends throughout the U.S. North Atlantic and Pacific Ocean basins."

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The major hurricanes over the past 20 years could show an overall trend of an increase in Category 2 and stronger storms, according to Paerl and his colleagues' research published in "Biogeochemistry."

"One manifestation of climate change may be a higher frequency of storms," said David Garrison, a program director in the National Science Foundation's Division of Ocean Sciences. "This study provides valuable insights into how coastal estuaries will be affected."

The second-largest estuary — the area where a large river meets the ocean — in the United States is in North Carolina.

The Neuse River, which flows into the Pamlico Sound, has been a subject of Paerl and his team's research into how hurricanes have impacted the coastal ecosystem.

"Understanding how an increase in extreme events such as hurricanes affects coastal ecosystems is critical to preparing for a stormier future," Paerl said.

Paerl, NCSU scientist Christopher Osburn and their team used data to find how hurricanes affected the flow of nutrients into the Neuse estuary and the sound, since excessive nutrients can pollute waters and damage ecosystems by killing off fish and other organisms.

Using satellite images from before and after storms, along with water quality tests (including sensors on ferries in the sound), scientists observed flooding and erosion and huge inputs of nutrients and organic matter from wetlands and marshes swept down to the estuary. That organic matter included lawn clippings, leaves, corn stalks, straw, manure, wood and food-processing waste.

Once that organic matter washed down to the estuary, it was converted into carbon dioxide by bacteria, then was released into the atmosphere.

"Hurricanes lead to large discharges of nutrients and organic matter from watersheds," Paerl said. "These nutrients are then shunted to coastal waters, changing the biogeochemistry of coastal ecosystems."

Large amounts of floodwater and organic matter can lead to dead zones in the estuary, where fish and other creatures can't live.

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"In the discharges after hurricanes Floyd and Matthew, these dead zones grew large enough to affect shellfish and finfish habitats for miles," Paerl said. "Fish kills lasted for months."

Understanding how these storms affect the coast "is important at a time when such extreme weather events are becoming more frequent and intense," Paerl said, and the impacts have been "mostly unknown" so far.

North Carolina must learn to manage the flow of nutrients and organic matter into estuaries and coastal waters, Osburn said, with a plan "to retain and process nutrients and organic matter on land, rather than allow them to be discharged downstream," which is needed "as we encounter stormier, increasingly extreme climate conditions."

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