Grad student cataloging threatened anchialine pools

Over the past three years, Lisa Marrack has hiked along the 175-mile Ala Kahakai National Historic Trail, diligently surveying the features of 450 anchialine pools.

Using Light Detection and Ranging, or LiDAR, data, Marrack, a University of California, Berkeley graduate student, is producing high-resolution models of the trail’s coastal corridor to visualize the potential impacts of sea level rise on these brackish coastal systems.

“Global sea levels are expected to rise 0.5 meters by the year 2050 and between 1 to 1.9 meters by 2100,” she said Thursday evening during a ReefTalk at Puukohola Heiau National Historic Site. “Low-lying coastal zones are at risk of becoming inundated by ocean water and will be more susceptible to high surf events and erosion.”

In an attempt to protect anchialine pools, Marrack is determining which ones could potentially be lost because of the inundation. She is also indicating future locations best suited for new pool habitats, which “will emerge if open space exists.” In addition, she is figuring out which pools, when inundated, may cause the unwanted dispersal of introduced species and should be considered for removal efforts.

Her work is funded by the National Park Service and is part of its Climate Change Response Program. By next year, Marrack plans to share the models and results online. The website will also have an application people can use to create their own maps with sea level scenarios and zoom into areas of interest.

Marrack has already begun sharing some of the information with county planners, the National Park Service and community groups. The predictions are conservative, because many variables and uncertainty still exist. For example, no one knows exactly how carbon dioxide emissions might evolve over the next century, she said.

Still, Marrack hopes her findings will inspire land owners, developers, government officials and others to take up conservation actions and embrace coastal planning opportunities that protect these ecosystems for future generations.

Born and raised on the Big Island, Marrack was a marine ecologist at Kaloko-Honokohau National Historical Park, where she worked on various national resource projects, including coral reef assessments, water quality monitoring and anchialine pool investigations.

She said anchialine pools, particularly the ones in Puako, were her childhood playground.

The island’s arid west coast contains the highest concentrations of anchialine pools in Hawaii. They have historically been used for drinking water, bathing, watering crops and opelu fishing, she added.

Anchialine pools occur in porous substrate, such as lava, where marine and groundwater mix.

They have no surface connection to the sea, but rise and fall with the tides. They are habitats for numerous endemic species, including the quarter-inch-long red shrimp known as opaeula or Halocaridina ruba, Marrack said.

These endemic shrimp depend on this habitat to retreat into the groundwater at low tides. The shrimp can move between pools in the groundwater, but once they are established in a water table, they tend to stay. Opaeula larvae, on the other hand, can move between islands via ocean currents — something that doesn’t happen frequently. However, detailed genetic mapping has found the West Hawaii species came from Oahu, Marrack said.

While most of the anchialine pools mapped are in relatively undeveloped areas and are in good shape, some have been degraded. Common culprits are introduced species like tilapia that prey on the grazing shrimp, causing algae to accumulate, Marrack said. She spoke about the various fish removal techniques that exist and are being investigated, as well as how invasive fish removal efforts should target pools most likely to be sources of introduction for future habitat.

For more information about this ReefTalk, email Pelika Bertelmann of the University of Hawaii Sea Grants College Program at pelikaok@hawaii.edu.