Anchialine Pools of Hawaiʻi Island

The term "anchialine" derives from the Greek term ankhialos, meaning “near the sea."

Anchialine pools are brackish groundwater-dependent ecosystems fed by wai (freshwater) and kai (seawater), with no surface connection to the ocean. These rare ecosystems require a subterranean connection to the sea, provided by porous karst or volcanic rock. As a result, unlike tidepools, it is hard for species that cannot navigate through the underground network (hypogeal habitat) to establish on the surface of an anchialine pool (epigeal habitat).

An anchialine pool receives more sunlight than its underground habitat, so it is a focal point of high primary productivity. The pools' relatively high amount of algae and plants feeds all sorts of groundwater ecosystem organisms. In fact, the abundance of food catalyzes population expansion!

The Hawaiian Archipelago hosts the only anchialine pools in the United States, and 80% of the known anchialine pools in the world are on Hawaiʻi Island. Over 600 of the region's anchialine pools occur along the Kona coast of Hawaiʻi Island, although anchialine pools can also be found on Oʻahu, Maui, Molokaʻi, and Kahoʻolawe. These numbers are extremely conservative estimates: East Hawaiʻi contains vast and relatively unexplored anchialine systems, where there are hundreds of undocumented pools that could ultimately be overlooked and developed over or misused in some way. David Chai, an anchialine pool expert, estimates that there could be nearly as many pools in East Hawaiʻi as the Kona side.

As the authors of Hawaiian Anchialine Pools: Windows to a Hidden World noted, “Hawaiian anchialine pools contain some of the rarest aquatic animals on the planet.” 

Yet, each anchialine pool is unique—scientific research has proven that the water flow, water composition and quality, geography, nutrients, temperature levels, biota, and more vary from pool to pool. One pool cannot replace any other pool, or host the same community of organisms.

Many pools are endangered by development, groundwater use, and invasive species, which could destroy their valuable, inimitable resources. 

Moreover, anchialine pools are culturally important in Hawaiʻi. The pools' ʻōpae ʻula (endemic shrimp) served as bait to catch ʻopelu (mackerel scad, Decapterus marcellus). Along the Kona coast where there are no streams, anchialine pools were critical sources of drinking water. As noted by Veronica Gibson, Leah Bremer, Kimberly Burnett, Nicole Keaka Lui, and Celia Smith in " Biocultural values of groundwater dependent ecosystems in Kona, Hawaiʻi ," "there are a variety of Hawaiian names" for anchialine pools, "recognizing the hydrologic and biological diversity of these ecosystems" and their "distinct social uses and values."

These names include "loko wai kai (mixing fresh and saltwater pond), wai ʻōpae (waters containing ʻōpae shrimp), loko wai (freshwater pond), ana wai (water cave), hāpuna (source water, spring, or pool), kiʻo wai (pool of water), kumu wai (source or spring water), luawai (well), māpuna (spring water), kāheka (tide pools with groundwater influence), and wai puna (spring water)."

To care for anchialine pools, then, is to care for place, for kin, for community, and for oneself.

Anchialine pools and their organisms are resilient and dynamic when it comes to natural disasters like tsunamis and swells. But they are also fragile. Human activities easily unbalance them.

Humans have introduced invasive species, both unwittingly and intentionally. Tilapia, mosquito fish, guppies, and koi compete with ʻōpae ʻula for resources, and directly prey on them.

All pools eventually lose their groundwater connection and become marshes, as a result of sediment accumulation (this process is called senescence)—but invasive fish accelerate senescence. Their excrement accumulates on the pool bottom, changing groundwater circulation. Eventually, anchialine pools stop being anchialine pools because of invasive fish. 

Invasive species distort the pools' delicate ecological equilibrium—and often ruin them without human intervention. Unlike native fishes, invasives can reproduce and lay eggs in anchialine pools. Then more invasives are born, alter the pool, and reproduce. Ecological destruction can continue indefinitely. (On the other hand, if a native marine fish ends up in an anchialine pool, it will only disrupt the pool ecosystem for the duration of its lifetime. After it eventually dies, the pool can return to its native ecology.)

Invasive plants grow along the edges of anchialine pools, hiding hunting cats, rats, and mongooses from native water and sea birds.

Development and overpopulation cause groundwater pollution and surface water run-off, modifying or even destroying anchialine pool habitats. 

Some people swim in anchialine pools, and, unfortunately, the chemicals in many personal care products such as sunscreen, body lotion, and shampoo adversely alter the pools' water chemistry. 

Sea level rise estimations indicate that many pools will be inundated. In Kaloko-Honokōhau National Historical Park, it is estimated that 53% of the pools will be under water if there is 3.3 feet of sea level rise, and 97% would be inundated with 6.2 feet of sea level rise (Explore the Ecosystem Effects of Sea Level Change in this embedded app.)

This suite of human impacts on anchialine pools covers the spectrum of immediate- to long-term impacts. Anchialine pools are declining, and we are losing their associated resources and cultural practices. This is why awareness of, research on, and work to mālama (to tend and care for) the anchialine pools is critical.

Scientists have been working to understand how sea level rise will affect anchialine pools: if waters rise, many pools will become tidepools, making it easy for invasive fish to invade them. And, new pools will form. A team of scientists led by The Nature Conservancy published  a sea level rise projection app . With this information, resource managers can understand  how rising seas will affect their communities , planning development and conservation accordingly. 

Scientists are still working to understand the genetic make-up of anchialine pool inhabitants, which provide key insights into how related or unique they are—a crucial insight for determining rarity, endangerment, and conservation priority. Dr. Scott Santos at Auburn University is a leading expert on the population genetics of ʻōpae ʻula.  His work at the Santos Lab  has found  13 distinct genetic groups . If one pool’s shrimp are wiped out by invasive tilapia, a substitute metapopulation cannot necessarily fulfill the previous population’s niche. 

Anchialine pools are dynamic: destroyed from tsunamis, large swells, and lava flows, their organisms travel through the subterranean lava tubes to reach other anchialine pools. A 2018 lava flow in Manukā created new anchialine pools. These new pools have elicited many questions, only some of which have been answered. Troy Sakihara and his team at the Hilo Division of Aquatic Resources are working to identify, map, and survey the pools, and have found rare shrimp like the Hawaiana procaris and the Calliasmata pholidota. They've also found a strange white-ish covering on the pool floor.

Although dynamic, anchialine pools are sensitive to human activity. Swimming in anchialine pools can transfer chemicals from lotions and skin products, as well as physically alter the environment: one footprint changes the topography of the pool floor. These are big changes for the small creatures in the pool—changes that they might not be able to survive. 

Humans can also affect the environment by unintentionally carrying invasive species, or by intentionally placing them in the pools. Many invasive guppies and tilapia have been able to colonize a pool because someone has put them there. 

Humans also sometimes leave rubbish behind. 

The greatest lesson about visiting the pools is to come educated, come invited, and come respectfully.

Many organizations welcome the public during community workdays. For example, the Hawaiʻi Wildlife Fund team hosts quarterly "wetland workdays" with community volunteers, student groups, and partner agencies to maintain the seashore paspalum along the periphery of the northernmost pool, and was working with state partners (and regulators) to move forward with the plan to use rotenone (plant-based piscicide) to eradicate the southernmost pool of Mozambique tilapia.  

Inspired by the statewide Hui Mālama Loko Iʻa Network, the Hui Loko Network was established in 2014 as a space for loko iʻa and wai ʻōpae caretakers to gather in community. It is facilitated by The Nature Conservancy.

As a group of more than 23 sites worldwide, these cultural practitioners, coastal managers, and landowners are working together to rejuvenate these systems so that they can once again be healthy, productive, and abundant. The Hui Loko Network is guided by the knowledge that a‘ohe hana nui ke alu ‘ia (no task is too big when done together by all). They embrace the spirit of ka‘ana ‘ike and laulima (equally sharing knowledge and cooperation) to learn from and support each other’s efforts to restore and preserve these treasured places for Hawai‘i’s people and the native plants and animals that depend upon them.

The Hui Loko Network collectively cares for more than 200 acres of loko iʻa and wai ʻōpae, hosting volunteer workdays often.

Swimmerets - also known as pleopods, these are the leg-like abdominal structures on crustaceans like shrimp and lobsters

Karst - a geological landscape with caves and sinkholes that forms as limestone from ancient reefs dissolves

Time lags/advances - the salinity or water levels of an anchialine pool vary due to tides, but they do not change at the same time as the tides

Hypogeal - underground anchialine habitat

Epigeal - aboveground anchialine habitat

Macrophyte - aquatic plants that aren't microscopic

Naiads - the aquatic larval stage of insects like damselflies, which have gills and live in the water, then go through a metamorphosis to become a winged adult

 https://www.nature.org/en-us/get-involved/how-to-help/places-we-protect/kiholo-preserve/ 

 https://www.nps.gov/media/video/view.htm?id=3A0BD53F-EE9A-30F9-ACFC0CE94917A8C4 

 https://dlnr.hawaii.gov/blog/2022/11/03/nr22-166/