Seagrass and the Ecosystem | When Sharks Attack

In 2019, the North Carolina coastline is in the grip of a disturbing string of shark attacks. Two different species, bowls and black tips, have attacked four swimmers in little more than six weeks. The cause is a mystery. The final piece of the puzzle might be found not in the waters of North Carolina, but in the skies above.

Just months before the spike, the Tar Heel State was struck by one of the most destructive storms on record—Hurricane Florence. On September 13th, 2018, 100 mile-per-hour winds toppled trees and caused widespread power outages. Upwards of 30 inches of rain unleashed catastrophic flooding. So how might this devastating storm connect to the upwelling of shark attacks in 2019?

According to coastal habitat expert Taylor Moretti, it could come down to one of the state's most unique geographical features: a 3,500 square mile network of estuaries that's wedged between the barrier islands and the mainland. Dozens of freshwater rivers and streams flow into the salty sea. It's a complex ecosystem full of life and it's built upon a tiny, unassuming plant—seagrass.

Seagrass is actually not grass at all; it's a lily that is capable of living in salt environments. It has a root system, it flowers. Seagrass provides vital shelter for all manner of creatures. "This right here is a tiny little blue crab, very very teeny." The animals that make seagrasses their home range from small microscopic creatures like arthropods to larger creatures like crabs and smaller fish.

These small marine animals attract larger species like sea turtles and manatees, which in turn draw in sharks. Atlantic sharpnose, bowls, black tips, and even the occasional large tiger come from the oceans into these estuaries to look for prey. It's this food chain that provides North Carolina with its rich array of marine life.

But during the storm, this chain gets broken. Intense tropical storms can have an impact on the shark fauna of these estuaries, and it can be long-lasting. When tropical storms pass through, they often leave a lot of fresh water in their wake. When a storm hits, it dumps fresh water across the region, both along the coast and inland.

The water that originates on land travels to the estuaries via rivers and streams. But along the way, it picks up some unwanted hitchhikers: sediment and pollution. The contaminants form a toxic stew that's deadly for seagrass. Studies have shown that flood sediment can bury entire meadows. Other areas are decimated when suspended debris blocks out crucial sunlight.

It can take up to five years for the plants to grow back, and when the seagrass vanishes, so does the shark's prey, and that reverberates up the entire food chain. So sharks typically will leave the system. This chain reaction can have dire consequences for beachgoers. If the sharks are not in the estuary, we know they're not on land, so they're out near shore.