Anchialine Pond, South Maui

An anchialine system (, from Greek ''ankhialos'', "near the sea") is a landlocked body of water with a subterranean connection to the ocean. Depending on its formation, these systems can exist in one of two primary forms: pools or caves. The primary differentiating characteristics between pools and caves is the availability of light; cave systems are generally aphotic while pools are euphotic. The difference in light availability has a large influence on the biology of a given system. Anchialine systems are a feature of coastal aquifers which are density stratified, with water near the surface being fresh or brackish, and saline water intruding from the coast at depth. Depending on the site, it is sometimes possible to access the deeper saline water directly in the anchialine pool, or sometimes it may be accessible by cave diving.

Anchialine systems are extremely common worldwide especially along neotropical coastlines where the geology and aquifer systems are relatively young, and there is minimal soil development. Such conditions occur notably where the bedrock is limestone or recently formed volcanic lava. Many anchialine systems are found on the coastlines of the island of Hawaii, the Yucatán Peninsula, South Australia, the Canary Islands, Christmas Island, and other karst and volcanic systems.

Geology

Karst landscape formation

Anchialine systems may occur in karst landscapes, regions with bedrock composed of soluble sedimentary rock, such as limestone, dolomite, marble, gypsum, or halite. Subterranean voids form in karst landscapes through the dissolution of bedrock by rainwater, which becomes mildly acidic by equilibrating with carbon dioxide from the atmosphere and soil as it percolates, resulting in carbonic acid, a weak acid. The acidic water reacts with the soluble sedimentary rock causing the rock to dissolve and create voids. Over time, these voids widen and deepen, resulting in caves, sinkholes, subterranean pools, and springs. The processes to form these karst morphological features occur on long geological timescales; caverns can be several hundred thousand to millions of years old. Since the caverns which house karst anchialine systems form through the dissolution of bedrock via water percolation, current karst anchialine systems developed around the last glacial maximum, approximately 20,000 years ago when the sea level was ~120 meters lower than today. Evidence of this can be seen in speleothems (stalactites and stalagmites), a terrestrial cave formation observed at 24 meters water depth in anchialine pools in Bermuda and 122 meters water depth in a blue hole in Belize. The marine transgression after the last glacial maximum caused saline groundwater to intrude into karst caverns resulting in anchialine systems. In some anchialine systems, lenses of freshwater overlay the saltwater environment. This is caused by the accumulation of freshwater from meteoric or phreatic sources above the intruded saltwater or the vertical displacement of freshwater from intruding saltwater. Horizontal white “bathtub ring” stains are observed in submerged sections of Green Bay Cave, Bermuda, indicating paleo-transition zones between freshwater and saltwater at a lower sea level.

Volcanic formation

Anchialine systems are also commonly found in coastal mafic volcanic environments such as the Canary Islands, Galapagos Islands, Samoa, and Hawaii. Lava tubes are the primary mechanism that creates anchialine systems in these volcanic environments. Lava tubes occur during eruptions of fluid-flowing basaltic pahoehoe lava. As lava flows downhill, the atmosphere and cooler surfaces come in contact with the exterior of the flow, causing it to solidify and create a conduit through which the interior liquid lava continues flowing. If the solid conduit empties of liquid lava, the result is a lava tube. Lava tubes flow towards lower elevations and typically stop upon reaching the ocean; however, lava tubes can extend along the seafloor or form from submarine eruptions creating anchialine habitats. Saltwater intruded into many coastal lava tubes during the marine transgression after the last glacial maximum creating many volcanic anchialine pools observed today. Volcanic anchialine systems typically can develop more rapidly than karst systems; on the order of thousands to tens of thousands of years due to their rapid formation at or near the Earth's surface, making them vulnerable to erosional processes.

Tectonic faulting formation

Tectonic faulting in coastal areas is a less common formation process for anchialine systems. In volcanic and seismically activity areas, faults in coastal environments can be intruded by saline groundwater resulting in anchialine systems. Submerged coastal tectonic faults caused by volcanic activity are observed in Iceland and in the Galapagos Islands, where they are known as “grietas,” which translates to “cracks.” Faulted anchialine systems can also form from tectonic uplift processes in coastal regions. The Ras Muhammad Crack area in Israel is an anchialine pool created by an earthquake in 1968 from the uplift of a fossil reef. The earthquake resulted in a fault opening approximately 150 meters from the coastline, which filled with saline groundwater creating an anchialine pool with water depths of up to 14 meters. Deep anchialine pools created by faulting from the uplift of a reef limestone block are also seen on the island of Niue in the Central Pacific.

Hydrology process

Hydrological processes can describe how the water moves between the pool and the surrounding environment. Collectively, these processes change the salinity and the vertical density profile, which sets the conditions for the ecological communities to develop. Although each anchialine system is unique, a box model simplifies the hydrology processes included in each system.

Box model

To predict mean salinity of an anchialine pool, the pool can be treated as a well-mixed box. Various sources (sinks) add (remove) water and alter the salinity. Below lists several important saline sources and sinks of the pool.

* The seawater seepage into the pool (SE): The barrier between a pool and the ocean controls how much seawater intrudes into a pool. If there are many caves in the barrier or the soil has high porosity, the pool is easier to exchange with the seawater. For example, pools near Kona's coast are saltier than inland pools.
* Evaporation (E): Evaporation removes water from the pool, increasing the salinity. The salinity may be higher than the ocean water under solid evaporation. In a shallow pool without significant seawater flushing, weather events, like a hurricane passing through, cause a significant salinity fluctuation.
* Pool water reflux into the substrate (RE): The reflux is similar to the seawater seepage but in a different direction. The substrate soaks up the dense bottom water and reduces the total salt in the pool.
* Evaporative pumping by the pool brine (EP): The pumping effect buffers evaporation. Under extreme evaporation, the salinity is much higher than water in mud. The salinity difference reverses the osmotic pressure and releases the low salinity water (freshwater or seawater) into the brine. Thus, it slows the rate of salinization.
* The influx of freshwater (F): The freshwater is from surface runoff and groundwater. For example, after considerable rain, lots of freshwater on the surface flows into the pool and dilutes salt water.
* Surface-to-depth relation of the pool water body (S/D): The relationship describes a ratio of evaporation and total water volume. Evaporation is in proportion to the surface area. In a vast and shallow pool, evaporation concentrates brine faster.

The ratio between the evaporation and water exchange with the surrounding, $PS$, implies if the box reach an equilibrium state or not.

$PS = \frac + \frac+\frac.$

For example, when the evaporation (E or S/D) removes freshwater faster than the influx, the salinity get higher than the ambient ocean. If $PS \sim 1$, salinity is close to open ocean salinity because the salt inflow balances the evaporation. If $2>PS>1$, the pool is metahaline (~40 psu). If $PS >2$, the pool is hypersaline (60~80 psu).

Stratification

The box model gives an estimate of the saline environment but does not imply the strength of the halocline. The depth of the seawater intake should be considered for the vertical salinity structure. In a pool containing fresh or brackish water, if the denser seawater flushes near the surface, it reduces stratification. However, in the same scenario in a polyhaline pool, the seawater forms a freshwater lens at the top, reinforcing the stratification and potentially creating a hypoxic environment depending on oxygen reaction rates.

Biogeochemistry

Water chemistry of anchialine systems are directly related to the amount of connectivity to the adjacent marine and freshwater inputs, and evaporative losses. Major nutrient compositions ( carbon, nitrate, phosphate, and silicate) from the ocean and groundwater sources determine the biogeochemical cycles in an anchialine system. These cycles are affected by the hydrological processes of anchialine systems which vary based on the type, size, and relative inputs of marine and freshwater into the system. Deeper anchialine systems, such as larger pool that resemble lakes, may become highly salinity stratified with depth. The surface consists of brackish oxygen-rich waters followed by a distinct pycnocline and chemocline, below which water has higher salinity and decreased dissolved oxygen ( anoxic) concentrations. This stratification and available nutrient resources establishes redox gradients with depth which can support a variety of stratified communities of micro-organisms and biogeochemical cycles.

Redox conditions

In deeper stratified systems water below the chemocline can be associated with an increase in dissolved hydrogen sulfide, phosphate, and ammonium, and a decrease in particulate organic carbon. The physical and chemical stratification determines which microbial metabolic pathways can occur and creates a vertical stratification of redox processes as oxygen decreases with depth. Oxygen-rich surface waters have a positive reduction potential (Eh), meaning there are oxidizing conditions for aerobic respiration. The chemocline layer has a negative Eh (reducing conditions) and low nutrient availability from the respiration above, so chemosynthetic bacteria reduce nitrate or sulfate for respiration. The productivity in the surface and chemocline layer creates turbid water, below which both oxygen and light levels are low but dissolved inorganic nutrient levels are high creating communities of other reducing microorganisms.

Physical nutrient cycling

Highly stratified anchialine systems, by definition, have little turbid mixing from wind or water movements. Instead it is suggested that advection of nutrients back into the surface water is caused by the rain of particulate matter below the chemocline displacing water upwards and by the vertical movement of mobile organisms. Introduction of nutrients and organic matter from terrestrial runoff into the surface waters also adds to the nutrient cycling in anchialine systems.

Biology

Ecology

Anchialine systems have a highly specialized collection of organisms with distinctive adaptations. The species that occupy a given system are strongly determined by the presence or absence of light (pools or caves). A broad diversity of algae and bacteria can be found in anchialine systems, however only few species dominate a given habitat at a time. Systems closer to the coastline tend to have more influence from marine phytoplankton and zooplankton as they are advected in through the groundwater. Systems further inland are more dominated by freshwater algae and terrestrial deposits but exhibit increasingly restricted diversity within algal communities. Due to the ephemeral nature of many anchialine systems and their limited distribution across the planet, many of their inhabitants are either well adapted to tolerate a broad range of salinity and hypoxic conditions or are introduced through tides from neighboring marine habitats. Species that occupy these habitats are generalists or opportunistic as they exploit conditions intolerable for most other species.

Crustaceans

Crustaceans are by far the most abundant taxa in anchialine systems. Crustacean biodiversity includes Copepoda, Amphipoda, Decapoda, Ascothoracida, and a variety of water fleas.

Non-crustacean invertebrates

Dominant non-crustacean invertebrates groups within anchialine systems include sponges and other filter feeders (most common in Blue Holes), which thrive in moderate flow systems where the structure acts in a way to compress the water and make particulate organic matter less dilute, improving filter feeding efficacy. This is often seen in the hydrodynamic 'pumping' of Blue Holes by Tubellaria (flatworms), and Gastropoda (snails and other mollusks). There are also other smaller non-crustacean inverts including chaetognaths (voracious zooplankton).

Anchialine pools

Hypogeal shrimps have been observed to have high population densities in anchialine ponds upwards of hundreds of individuals per square meter.Brock, R. E., & Kam, A. K. (1997). Biological and water quality characteristics of anchialine resources in Kaloko-Honokohau National Historical Park.
/ref> Many of the shrimp species present in these systems migrate into and out of pools with the tide through the connection at the water table. It is hypothesized they enter pools during flood tides to feed and retreat to cover with ebb tides. There are a range of fish species that can be found in anchialine pools and their presence usually indicates lower populations of hypogeal shrimp and an absence of epigeal shrimp. In Hawaii, the pools are home to the ʻōpaeʻula (Hawaiian shrimp, ''Halocaridina rubra'').

Anchialine pools are considered an ecosystem that combines elements from brackish surface water bodies, subterranean systems, and terrestrial landscapes and are usually wet lit. Algal primary producers inhabit the water column and benthos

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